Site-directed spin labeling measurements of nanometer distances in nucleic acids using a sequence-independent nitroxide probe

Exploring combined spin-labeling approach for structural studies of mRNA in the human ribosome

In this study, we investigated the structural variability of mRNA in the human ribosome by comparing two spin-labeling strategies: one involving an mRNA analog bearing two spin labels attached to the ribose-phosphate backbone and the other placing labels at the nucleotide bases. The use of two strategies of spin labeling of mRNAs allowed us to study for the first time the effect of the structure and location of spin labels on the measured interspin distances in human ribosome complexes. Experiments using dipolar EPR spectroscopy, supported by molecular dynamics calculations, demonstrated that labels introduced at nucleotide bases provide a higher resolution between mRNA conformations in the ribosome mRNA channel, compared to labels introduced at the ribose-phosphate backbone. Although ribose-phosphate labeling turned out to be less informative on its own for studying mRNA conformations in the ribosome than the previously used base labeling, it can find application in other complex studies of the structure of RNAs and their complexes.

Structure and Internal Dynamics of Short RNA Duplexes Determined by a Combination of Pulsed EPR Methods and MD Simulations

We used EPR spectroscopy to characterize the structure of RNA duplexes and their internal twist, stretch and bending motions. We prepared eight 20-base-pair-long RNA duplexes containing the rigid spin-label Çm, a cytidine analogue, at two positions and acquired orientation-selective PELDOR/DEER data. By using different frequency bands (X-, Q-, G-band), detailed information about the distance and orientation of the labels was obtained and provided insights into the global conformational dynamics of the RNA duplex. We used 19F Mims ENDOR experiments on three singly Çm- and singly fluorine-labeled RNA duplexes to determine the exact position of the Çm spin label in the helix. In a quantitative comparison to MD simulations of RNA with and without Çm spin labels, we found that state-of-the-art force fields with explicit parameterization of the spin label were able to describe the conformational ensemble present in our experiments. The MD simulations further confirmed that the Çm spin labels are excellent mimics of cytidine inducing only small local changes in the RNA structure. Çm spin labels are thus ideally suited for high-precision EPR experiments to probe the structure and, in conjunction with MD simulations, motions of RNA.

Revealing the DNA Binding Modes of CsoR by EPR Spectroscopy

In pathogens, a unique class of metalloregulator proteins, called gene regulatory proteins, sense specific metal ions that initiate gene transcription of proteins that export metal ions from the cell, thereby preventing toxicity and cell death. CsoR is a metalloregulator protein found in various bacterial systems that "sense" Cu(I) ions with high affinity. Upon copper binding, CsoR dissociates from the DNA promoter region, resulting in initiation of gene transcription. Crystal structures of CsoR in the presence and absence of Cu(I) from various bacterial systems have been reported, suggesting either a dimeric or tetrameric structure of these helical proteins. However, structural information about the CsoR-DNA complex is missing. Here, we applied electron paramagnetic resonance (EPR) spectroscopy to follow the conformational and dynamical changes that Mycobacterium tuberculosis CsoR undergoes upon DNA binding in solution. We showed that the quaternary structure is predominantly dimeric in solution, and only minor conformational and dynamical changes occur in the DNA bound state. Also, labeling of the unresolved C- terminus revealed no significant change in dynamics upon DNA binding. These observations are unique, since for other bacterial copper metalloregulators, such as the MerR and CopY families, major conformational changes were observed upon DNA binding, indicating a different mode of action for this protein family.

Spectroscopically Orthogonal Labelling to Disentangle Site-Specific Nitroxide Label Distributions

Biomolecular applications of pulse dipolar electron paramagnetic resonance spectroscopy (PDS) are becoming increasingly valuable in structural biology. Site-directed spin labelling of proteins is routinely performed using nitroxides, with paramagnetic metal ions and other organic radicals gaining popularity as alternative spin centres. Spectroscopically orthogonal spin labelling using different types of labels potentially increases the information content available from a single sample. When analysing experimental distance distributions between two nitroxide spin labels, the site-specific rotamer information has been projected into the distance and is not readily available, and the contributions of individual labelling sites to the width of the distance distribution are not obvious from the PDS data. Here, we exploit the exquisite precision of labelling double-histidine (dHis) motifs with CuII chelate complexes. The contribution of this label to the distance distribution widths in model protein GB1 has been shown to be negligible. By combining a dHis CuII labelling site with cysteine-specific nitroxide labelling, we gather insights on the label rotamers at two distinct sites, comparing their contributions to distance distributions based on different in silico modelling approaches and structural models. From this study, it seems advisable to consider discrepancies between different in silico modelling approaches when selecting labelling sites for PDS studies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00723-023-01611-1.

A DNA unwinding equilibrium serves as a checkpoint for CRISPR-Cas12a target discrimination

CRISPR-associated proteins such as Cas9 and Cas12a are programable RNA-guided nucleases that have emerged as powerful tools for genome manipulation and molecular diagnostics. However, these enzymes are prone to cleaving off-target sequences that contain mismatches between the RNA guide and DNA protospacer. In comparison to Cas9, Cas12a has demonstrated distinct sensitivity to protospacer-adjacent-motif (PAM) distal mismatches, and the molecular basis of Cas12a's enhanced target discrimination is of great interest. In this study, we investigated the mechanism of Cas12a target recognition using a combination of site-directed spin labeling, fluorescent spectroscopy, and enzyme kinetics. With a fully matched RNA guide, the data revealed an inherent equilibrium between a DNA unwound state and a DNA-paired duplex-like state. Experiments with off-target RNA guides and pre-nicked DNA substrates identified the PAM-distal DNA unwinding equilibrium as a mismatch sensing checkpoint prior to the first step of DNA cleavage. The finding sheds light on the distinct targeting mechanism of Cas12a and may better inform CRISPR based biotechnology developments.

Stabilizing Nitroxide Spin Labels for Structural and Conformational Studies of Biomolecules by Maleimide Treatment

Nitroxide (NO) spin radicals are effective in characterizing structures, interactions and dynamics of biomolecules. The EPR applications in cell lysates or intracellular milieu require stable spin labels, but NO radicals are unstable in such conditions. We showed that the destabilization of NO radicals in cell lysates or even in cells is caused by NADPH/NADH related enzymes, but not by the commonly believed reducing reagents such as GSH. Maleimide stabilizes the NO radicals in the cell lysates by consumption of the NADPH/NADH that are essential for the enzymes involved in destabilizing NO radicals, instead of serving as the solo thiol scavenger. The maleimide treatment retains the crowding properties of the intracellular components and allows to perform long-time EPR measurements of NO labeled biomolecules close to the intracellular conditions. The strategy of maleimide treatment on cell lysates for the EPR applications has been demonstrated on double electron-electron resonance (DEER) measurements on a number of NO labeled protein samples. The method opens a broad application range for the NO labeled biomolecules by EPR in conditions that resemble the intracellular milieu.

A DNA Unwinding Equilibrium Serves as a Checkpoint for CRISPR-Cas12a Target Discrimination

CRISPR-associated proteins such as Cas9 and Cas12a are programable RNA-guided nucleases that have emerged as powerful tools for genome manipulation and molecular diagnostics. However, these enzymes are prone to cleaving off-target sequences that contain mismatches between the RNA guide and DNA protospacer. In comparison to Cas9, Cas12a has demonstrated distinct sensitivity to protospacer-adjacent-motif (PAM) distal mismatches, and the molecular basis of Cas12a's enhanced target discrimination is of great interest. In this study, we investigated the mechanism of Cas12a target recognition using a combination of site-directed spin labeling, fluorescent spectroscopy, and enzyme kinetics. With a fully matched RNA guide, the data revealed an inherent equilibrium between a DNA unwound state and a DNA-paired duplex-like state. Experiments with off-target RNA guides and pre-nicked DNA substrates identified the PAM-distal DNA unwinding equilibrium as a mismatch sensing checkpoint prior to the first step of DNA cleavage. The data sheds light on the distinct targeting mechanism of Cas12a and may better inform CRISPR based biotechnology developments.

"Store-bought is fine": Sensitivity considerations using shaped pulses for DEER measurements on Cu(II) labels

The narrow excitation bandwidth of monochromic pulses is a sensitivity limitation for pulsed dipolar spectroscopy on Cu(II)-based measurements. In response, frequency-swept pulses with large excitation bandwidths have been adopted to probe a greater range of the EPR spectrum. However, much of the work utilizing frequency-swept pulses in Cu(II) distance measurements has been carried out on home-built spectrometers and equipment. Herein, we carry out systematic Cu(II) based distance measurements to demonstrate the capability of chirp pulses on commercial instrumentation. More importantly we delineate sensitivity considerations under acquisition schemes that are necessary for robust distance measurements using Cu(II) labels for proteins. We show that a 200 MHz sweeping bandwidth chirp pulse can improve the sensitivity of long-range distance measurements by factors of three to four. The sensitivity of short-range distances only increases slightly due to special considerations for the chirp pulse duration relative to the period length of the modulated dipolar signal. Enhancements in sensitivity also dramatically reduce measurement collection times enabling rapid collection of orientationally averaged Cu(II) distance measurements in under two hours.

Conformations and Local Dynamics of the CopY Metal Sensor Revealed by EPR Spectroscopy

Metal transcription factors regulate metal concentrations in eukaryotic and prokaryotic cells. Copper is a metal ion that is being tightly regulated, owing to its dual nature. Whereas copper is an essential nutrient for bacteria, it is also toxic at high concentrations. CopY is a metal-sensitive transcription factor belonging to the copper-responsive repressor family found in Gram-positive bacteria. CopY represses transcription in the presence of Zn(II) ions and initiates transcription in the presence of Cu(I) ions. The complete crystal structure of CopY has not been reported yet, therefore most of the structural information on this protein is based on its similarity to the well-studied MecI protein. In this study, electron paramagnetic resonance (EPR) spectroscopy was used to characterize structural and local dynamical changes in Streptococcus pneumoniae CopY as a function of Zn(II), Cu(I), and DNA binding. We detected different conformations and changes in local dynamics when CopY bound Zn(II), as opposed to Cu(I) ions. Furthermore, we explored the effects of metal ions and DNA on CopY conformation. Our results revealed the sensitivity and selectivity of CopY towards metal ions and provide new insight into the structural mechanism of the CopY transcription factor.

Dynamical interplay between the human high-affinity copper transporter hCtr1 and its cognate metal ion

Abnormal cellular copper levels have been clearly implicated in genetic diseases, cancer, and neurodegeneration. Ctr1, a high-affinity copper transporter, is a homotrimeric integral membrane protein that provides the main route for cellular copper uptake. Together with a sophisticated copper transport system, Ctr1 regulates Cu(I) metabolism in eukaryotes. Despite its pivotal role in normal cell function, the molecular mechanism of copper uptake and transport via Ctr1 remains elusive. In this study, electron paramagnetic resonance (EPR), UV-visible spectroscopy, and all-atom simulations were employed to explore Cu(I) binding to full-length human Ctr1 (hCtr1), thereby elucidating how metal binding at multiple distinct sites affects the hCtr1 conformational dynamics. We demonstrate that each hCtr1 monomer binds up to five Cu(I) ions and that progressive Cu(I) binding triggers a marked structural rearrangement in the hCtr1 C-terminal region. The observed Cu(I)-induced conformational remodeling suggests that the C-terminal region may play a dual role, serving both as a channel gate and as a shuttle mediating the delivery of copper ions from the extracellular hCtr1 selectivity filter to intracellular metallochaperones. Our findings thus contribute to a more complete understanding of the mechanism of hCtr1-mediated Cu(I) uptake and provide a conceptual basis for developing mechanism-based therapeutics for treating pathological conditions linked to de-regulated copper metabolism.

Site-Specific Labeling Reveals Cas9 Induces Partial Unwinding Without RNA/DNA Pairing in Sequences Distal to the PAM

CRISPR-Cas9 is an RNA-guided nuclease that has been widely adapted for genome engineering. A key determinant in Cas9 target selection is DNA duplex unwinding to form an R-loop, in which the single-stranded RNA guide hybridizes with one of the DNA strands. To advance understanding on DNA unwinding by Cas9, we combined two types of spectroscopic label, 2-aminopurine and nitroxide spin-label, to investigate unwinding at a specific DNA base pair induced by Streptococcus pyogenes Cas9. Data obtained with RNA guide lengths varying from 13 to 20 nucleotide revealed that the DNA segment distal to the protospacer adjacent motif can adopt a "partial unwinding" state, in which a mixture of DNA-paired and DNA-unwound populations exist in equilibrium. Significant unwinding can occur at positions not supported by RNA/DNA pairing, and the degree of unwinding depends on RNA guide length and modulates DNA cleavage activity. The results shed light on Cas9 target selection and may inform developments of genome-engineering strategies.

Orientation and dynamics of Cu2+ based DNA labels from force field parameterized MD elucidates the relationship between EPR distance constraints and DNA backbone distances

Pulsed electron paramagnetic resonance (EPR) based distance measurements using the recently developed Cu2+-DPA label present a promising strategy for measuring DNA backbone distance constraints. Herein we develop force field parameters for Cu2+-DPA in order to understand the features of this label at an atomic level. We perform molecular dynamics (MD) simulations using the force field parameters of Cu2+-DPA on four different DNA duplexes. The distance between the Cu2+ centers, extracted from the 2 μs MD trajectories, agrees well with the experimental distance for all the duplexes. Further analyses of the trajectory provide insight into the orientation of the Cu2+-DPA inside the duplex that leads to such agreement with experiments. The MD results also illustrate the ability of the Cu2+-DPA to report on the DNA backbone distance constraints. Furthermore, measurement of fluctuations of individual residues showed that the flexibility of Cu2+-DPA in a DNA depends on the position of the label in the duplex, and a 2 μs MD simulation is not sufficient to fully capture the experimental distribution in some cases. Finally, the MD trajectories were utilized to understand the key aspects of the double electron electron resonance (DEER) results. The lack of orientational selectivity effects of the Cu2+-DPA at Q-band frequency is rationalized in terms of fluctuations in the Cu2+ coordination environment and rotameric fluctuations of the label linker. Overall, a combination of EPR and MD simulations based on the Cu2+-DPA labelling strategy can contribute towards understanding changes in DNA backbone conformations during protein-DNA interactions.

Cu2+-based distance measurements by pulsed EPR provide distance constraints for DNA backbone conformations in solution

Electron paramagnetic resonance (EPR) has become an important tool to probe conformational changes in nucleic acids. An array of EPR labels for nucleic acids are available, but they often come at the cost of long tethers, are dependent on the presence of a particular nucleotide or can be placed only at the termini. Site directed incorporation of Cu2+-chelated to a ligand, 2,2'dipicolylamine (DPA) is potentially an attractive strategy for site-specific, nucleotide independent Cu2+-labelling in DNA. To fully understand the potential of this label, we undertook a systematic and detailed analysis of the Cu2+-DPA motif using EPR and molecular dynamics (MD) simulations. We used continuous wave EPR experiments to characterize Cu2+ binding to DPA as well as optimize Cu2+ loading conditions. We performed double electron-electron resonance (DEER) experiments at two frequencies to elucidate orientational selectivity effects. Furthermore, comparison of DEER and MD simulated distance distributions reveal a remarkable agreement in the most probable distances. The results illustrate the efficacy of the Cu2+-DPA in reporting on DNA backbone conformations for sufficiently long base pair separations. This labelling strategy can serve as an important tool for probing conformational changes in DNA upon interaction with other macromolecules.

Posttranscriptional site-directed spin labeling of large RNAs with an unnatural base pair system under non-denaturing conditions

Site-directed spin labeling (SDSL) of large RNAs for electron paramagnetic resonance (EPR) spectroscopy has remained challenging to date.

Site-directed spin labeling (SDSL) of large RNAs for electron paramagnetic resonance (EPR) spectroscopy has remained challenging to date. We here demonstrate an efficient and generally applicable posttranscriptional SDSL method for large RNAs using an expanded genetic alphabet containing the NaM-TPT3 unnatural base pair (UBP). An alkyne-modified TPT3 ribonucleotide triphosphate (rTPT3^COTP) is synthesized and site-specifically incorporated into large RNAs by in vitro transcription, which allows attachment of the azide-containing nitroxide through click chemistry. We validate this strategy by SDSL of a 419-nucleotide ribonuclease P (RNase P) RNA from Bacillus stearothermophilus under non-denaturing conditions. The effects of site-directed UBP incorporation and subsequent spin labeling on the global structure and function of RNase P are marginal as evaluated by Circular Dichroism spectroscopy, Small Angle X-ray Scattering, Sedimentation Velocity Analytical Ultracentrifugation and enzymatic assay. Continuous-Wave EPR analyses reveal that the labeling reaction is efficient and specific, and Pulsed Electron–Electron Double Resonance measurements yield an inter-spin distance distribution that agrees with the crystal structure. The labeling strategy as presented overcomes the size constraint of RNA labeling, opening new avenues of spin labeling and EPR spectroscopy for investigating the structure and dynamics of large RNAs.

An EPR Study on the Interaction between the Cu(I) Metal Binding Domains of ATP7B and the Atox1 Metallochaperone

Copper’s essentiality and toxicity mean it requires a sophisticated regulation system for its acquisition, cellular distribution and excretion, which until now has remained elusive. Herein, we applied continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopy in solution to resolve the copper trafficking mechanism in humans, by considering the route travelled by Cu(I) from the metallochaperone Atox1 to the metal binding domains of ATP7B. Our study revealed that Cu(I) is most likely mediated by the binding of the Atox1 monomer to metal binding domain 1 (MBD1) and MBD4 of ATP7B in the final part of its extraction pathway, while the other MBDs mediate this interaction and participate in copper transfer between the various MBDs to the ATP7B membrane domain. This research also proposes that MBD1-3 and MBD4-6 act as two independent units.

Cu(I) Controls Conformational States in Human Atox1 Metallochaperone: An EPR and Multiscale Simulation Study

Atox1 is a human copper metallochaperone that is responsible for transferring copper ions from the main human copper transporter, hCtr1, to ATP7A/B in the Golgi apparatus. Atox1 interacts with the Ctr1 C-terminal domain as a dimer, although it transfers the copper ions to ATP7A/B in a monomeric form. The copper binding site in the Atox1 dimer involves Cys12 and Cys15, while Lys60 was also suggested to play a role in the copper binding. We recently showed that Atox1 can adopt various conformational states, depending on the interacting protein. In the current study, we apply EPR experiments together with hybrid quantum mechanics–molecular mechanics molecular dynamics simulations using a recently developed semiempirical density functional theory approach, to better understand the effect of Atox1’s conformational states on copper coordination. We propose that the flexibility of Atox1 occurs owing to protonation of one or more of the cysteine residues, and that Cys15 is an important residue for Atox1 dimerization, while Cys12 is a critical residue for Cu(I) binding. We also show that Lys60 electrostatically stabilizes the Cu(I)–Atox1 dimer.

Structural rearrangements in mRNA upon its binding to human 80S ribosomes revealed by EPR spectroscopy

The model mRNA (MR), 11-mer RNA containing two nitroxide spin labels at the 5′- and 3′-terminal nucleotides and prone to form a stable homodimer (MR)₂, was used for Electron Paramagnetic Resonance study of structural rearrangements in mRNA occurring upon its binding to human 80S ribosomes. The formation of two different types of ribosomal complexes with MR was observed. First, there were stable complexes where MR was fixed in the ribosomal mRNA-binding channel by the codon-anticodon interaction(s) with cognate tRNA(s). Second, we for the first time detected complexes assembled without tRNA due to the binding of MR most likely to an exposed peptide of ribosomal protein uS3 away from the mRNA channel. The analysis of interspin distances allowed the conclusion that 80S ribosomes facilitate dissociation of the duplex (MR)₂: the equilibrium between the duplex and the single-stranded MR shifts to MR due to its efficient binding with ribosomes. Furthermore, we observed a significant influence of tRNA bound at the ribosomal exit (E) and/or aminoacyl (A) sites on the stability of ribosomal complexes. Our findings showed that a part of mRNA bound in the ribosome channel, which is not involved in codon-anticodon interactions, has more degrees of freedom than that interacting with tRNAs.

Unraveling the Impact of Cysteine-to-Serine Mutations on the Structural and Functional Properties of Cu(I)-Binding Proteins

Appropriate maintenance of Cu(I) homeostasis is an essential requirement for proper cell function because its misregulation induces the onset of major human diseases and mortality. For this reason, several research efforts have been devoted to dissecting the inner working mechanism of Cu(I)-binding proteins and transporters. A commonly adopted strategy relies on mutations of cysteine residues, for which Cu(I) has an exquisite complementarity, to serines. Nevertheless, in spite of the similarity between these two amino acids, the structural and functional impact of serine mutations on Cu(I)-binding biomolecules remains unclear. Here, we applied various biochemical and biophysical methods, together with all-atom simulations, to investigate the effect of these mutations on the stability, structure, and aggregation propensity of Cu(I)-binding proteins, as well as their interaction with specific partner proteins. Among Cu(I)-binding biomolecules, we focused on the eukaryotic Atox1-ATP7B system, and the prokaryotic CueR metalloregulator. Our results reveal that proteins containing cysteine-to-serine mutations can still bind Cu(I) ions; however, this alters their stability and aggregation propensity. These results contribute to deciphering the critical biological principles underlying the regulatory mechanism of the in-cell Cu(I) concentration, and provide a basis for interpreting future studies that will take advantage of cysteine-to-serine mutations in Cu(I)-binding systems.

Experimental Validation of the ALLNOX Program for Studying Protein-Nucleic Acid Complexes

Measurement of distances between spectroscopic labels (e.g., spin labels, fluorophores) attached to specific sites of biomolecules is an important method for studying biomolecular complexes. ALLNOX (Addition of Labels and Linkers) has been developed as a program to model interlabel distances based on an input macromolecule structure. Here, we report validation of ALLNOX using measured distances between nitroxide spin labels attached to specific sites of a protein-DNA complex. The results demonstrate that ALLNOX predicts average interspin distances that matched with values measured with pairs of labels attached at the protein and/or DNA. This establishes a solid foundation for using spin labeling in conjunction with ALLNOX to investigate complexes without high-resolution structures. With its high degree of flexibility for the label or the target biomolecule, ALLNOX provides a useful tool for investigating the structure-function relationship in a large variety of biological molecules.

A Cytidine Phosphoramidite with Protected Nitroxide Spin Label: Synthesis of a Full-Length TAR RNA and Investigation by In-Line Probing and EPR Spectroscopy

EPR studies on RNA are complicated by three major obstacles related to the chemical nature of nitroxide spin labels: Decomposition while oligonucleotides are chemically synthesized, further decay during enzymatic strand ligation, and undetected changes in conformational equilibria due to the steric demand of the label. Herein possible solutions for all three problems are presented: A 2-nitrobenzyloxymethyl protective group for nitroxides that is stable under all conditions of chemical RNA synthesis and can be removed photochemically. By careful selection of ligation sites and splint oligonucleotides, high yields were achieved in the assembly of a full-length HIV-1 TAR RNA labeled with two protected nitroxide groups. PELDOR measurements on spin-labeled TAR in the absence and presence of arginine amide indicated arrest of interhelical motions on ligand binding. Finally, even minor changes in conformation due to the presence of spin labels are detected with high sensitivity by in-line probing.

Spin labelling for integrative structure modelling: a case study of the polypyrimidine-tract binding protein 1 domains in complexes with short RNAs

A combined method, employing NMR and EPR spectroscopies, has demonstrated its strength in solving structures of protein/RNA and other types of biomolecular complexes. This method works particularly well when the large biomolecular complex consists of a limited number of rigid building blocks, such as RNA-binding protein domains (RBDs). A variety of spin labels is available for such studies, allowing for conventional as well as spectroscopically orthogonal double electron-electron resonance (DEER) measurements in EPR. In this work, we compare different types of nitroxide-based and Gd(iii)-based spin labels attached to isolated RBDs of the polypyrimidine-tract binding protein 1 (PTBP1) and to short RNA fragments. In particular, we demonstrate experiments on spectroscopically orthogonal labelled RBD/RNA complexes. For all experiments we analyse spin labelling, DEER method performance, resulting distance distributions, and their consistency with the predictions from the spin label rotamers analysis. This work provides a set of intra-domain calibration DEER data, which can serve as a basis to start structure determination of the full length PTBP1 complex with an RNA derived from encephalomycarditis virus (EMCV) internal ribosomal entry site (IRES). For a series of tested labelling sites, we discuss their particular advantages and drawbacks in such a structure determination approach.

Interaction of triarylmethyl radicals with DNA termini revealed by orientation-selective W-band double electron-electron resonance spectroscopy

Spin labels selectively attached to biomolecules allow high-accuracy nanoscale distance measurements using pulsed electron paramagnetic resonance (EPR), in many cases providing the only access to the structure of complex biosystems. Triarylmethyl (TAM) radicals have recently emerged as a new class of spin labels expanding the applicability of the method to physiological temperatures. Along with other factors, the accuracy of the obtained distances crucially relies on the understanding of interactions between biomolecules and spin labels. In this work, we consider such crucial interactions and their impact on pulsed EPR distance measurements in TAM-labeled DNAs. Using orientation-selective high-frequency (94 GHz) double electron-electron resonance (DEER) we demonstrate strong specific interactions between DNA termini and TAM labels, leading to a significant restriction of their conformational mobility. An understanding of such interactions guides the way to select optimum TAM-labeling strategies, thus refining nanoscale EPR distance measurements in nucleic acids and their complexes under physiological conditions.

Determination of helix orientations in a flexible DNA by multi-frequency EPR spectroscopy

Distance measurements are performed between a pair of spin labels attached to nucleic acids using Pulsed Electron-Electron Double Resonance (PELDOR, also called DEER) spectroscopy which is a complementary tool to other structure determination methods in structural biology. The rigid spin label Ç, when incorporated pairwise into two helical parts of a nucleic acid molecule, allows the determination of both the mutual orientation and the distance between those labels, since Ç moves rigidly with the helix to which it is attached. We have developed a two-step protocol to investigate the conformational flexibility of flexible nucleic acid molecules by multi-frequency PELDOR. In the first step, a library with a broad collection of conformers, which are in agreement with topological constraints, NMR restraints and distances derived from PELDOR, was created. In the second step, a weighted structural ensemble of these conformers was chosen, such that it fits the multi-frequency PELDOR time traces of all doubly Ç-labelled samples simultaneously. This ensemble reflects the global structure and the conformational flexibility of the two-way DNA junction. We demonstrate this approach on a flexible bent DNA molecule, consisting of two short helical parts with a five adenine bulge at the center. The kink and twist motions between both helical parts were quantitatively determined and showed high flexibility, in agreement with a Förster Resonance Energy Transfer (FRET) study on a similar bent DNA motif. The approach presented here should be useful to describe the relative orientation of helical motifs and the conformational flexibility of nucleic acid structures, both alone and in complexes with proteins and other molecules.

A Versatile Approach to Attachment of Triarylmethyl Labels to DNA for Nanoscale Structural EPR Studies at Physiological Temperatures

Triarylmethyl (trityl, TAM) radicals are a promising class of spin labels for nanometer-scale distance measurements in biomolecules at physiological temperatures. However, to date, existing approaches to site-directed TAM labeling of DNA have been limited to label attachment at the termini of oligonucleotides, thus hindering a majority of demanded applications. Herein, we report a new versatile strategy for TAM attachment at arbitrary sites of nucleic acids. It utilizes an achiral non-nucleoside phosphoramidite monomer for automated solid-phase synthesis of oligonucleotides, which are then postsynthetically functionalized with TAM. We demonstrate a synthesis of a set of oligonucleotide complexes that are TAM-labeled at internal or terminal sites, as well as the possibility of measuring interspin distances up to ∼5-6 nm at 298 K using double quantum coherence electron paramagnetic resonance (EPR). Implementation of the developed approach strongly broadens the scope of nucleic acids and nucleoprotein complexes available for nanoscale structural EPR studies at room temperatures.

The structural flexibility of the human copper chaperone Atox1: Insights from combined pulsed EPR studies and computations

Metallochaperones are responsible for shuttling metal ions to target proteins. Thus, a metallochaperone's structure must be sufficiently flexible both to hold onto its ion while traversing the cytoplasm and to transfer the ion to or from a partner protein. Here, we sought to shed light on the structure of Atox1, a metallochaperone involved in the human copper regulation system. Atox1 shuttles copper ions from the main copper transporter, Ctr1, to the ATP7b transporter in the Golgi apparatus. Conventional biophysical tools such as X-ray or NMR cannot always target the various conformational states of metallochaperones, owing to a requirement for crystallography or low sensitivity and resolution. Electron paramagnetic resonance (EPR) spectroscopy has recently emerged as a powerful tool for resolving biological reactions and mechanisms in solution. When coupled with computational methods, EPR with site-directed spin labeling and nanoscale distance measurements can provide structural information on a protein or protein complex in solution. We use these methods to show that Atox1 can accommodate at least four different conformations in the apo state (unbound to copper), and two different conformations in the holo state (bound to copper). We also demonstrate that the structure of Atox1 in the holo form is more compact than in the apo form. Our data provide insight regarding the structural mechanisms through which Atox1 can fulfill its dual role of copper binding and transfer.

Analysis of Nitroxide-Based Distance Measurements in Cell Extracts and in Cells by Pulsed ESR Spectroscopy

Measurements of distances in cells by pulsed ESR spectroscopy afford tremendous opportunities to study proteins in native environments that are irreproducible in vitro. However, the in-cell environment is harsh towards the typical nitroxide radicals used in double electron-electron resonance (DEER) experiments. A systematic examination is performed on the loss of the DEER signal, including contributions from nitroxide decay and nitroxide side-chain cleavage. In addition, the possibility of extending the lifetime of the nitroxide radical by use of an oxidizing agent is investigated. Using this oxidizing agent, DEER distance measurements are performed on doubly nitroxide-labeled GB1, the immunoglobulin-binding domain of protein G, at varying incubation times in the cellular environment. It is found that, by comparison of the loss of DEER signal to the loss of the CW spectrum, cleavage of the nitroxide side chain contributes to the loss of DEER signal, which is significantly greater in cells than in cell extracts. Finally, local spin concentrations are monitored at varying incubation times to show the time required for molecular diffusion of a small globular protein within the cellular milieu.

Ctr1 Intracellular Loop Is Involved in the Copper Transfer Mechanism to the Atox1 Metallochaperone

Understanding the human copper cycle is essential to understand the role of metals in promoting neurological diseases and disorders. One of the cycles controlling the cellular concentration and distribution of copper involves the copper transporter, Ctr1; the metallochaperone, Atox1; and the ATP7B transporter. It has been shown that the C-terminus of Ctr1, specifically the last three amino acids, HCH, is involved in both copper coordination and the transfer mechanism to Atox1. In contrast, the role of the intracellular loop of Ctr1, which is an additional intracellular segment of Ctr1, in facilitating the copper transfer mechanism has not been investigated yet. Here, we combine various biophysical methods to explore the interaction between this Ctr1 segment and metallochaperone Atox1 and clearly demonstrate that the Ctr1 intracellular loop (1) can coordinate Cu(I) via interactions with the side chains of one histidine and two methionine residues and (2) closely interacts with the Atox1 metallochaperone. Our findings are another important step in elucidating the mechanistic details of the eukaryotic copper cycle.

Flexibility and conformation of the cocaine aptamer studied by PELDOR

The cocaine aptamer is a DNA three-way junction that binds cocaine at its helical junction. We studied the global conformation and overall flexibility of the aptamer in the absence and presence of cocaine by pulsed electron-electron double resonance (PELDOR) spectroscopy, also called double electron-electron resonance (DEER). The rigid nitroxide spin label Ç was incorporated pairwise into two helices of the aptamer. Multi-frequency 2D PELDOR experiments allow the determination of the mutual orientation and the distances between two Çs. Since Ç is rigidly attached to double-stranded DNA, it directly reports on the aptamer dynamics. The cocaine-bound and the non-bound states could be differentiated by their conformational flexibility, which decreases upon binding to cocaine. We observed a small change in the width and mean value of the distance distribution between the two spin labels upon cocaine binding. Further structural insights were obtained by investigating the relative orientation between the two spin-labeled stems of the aptamer. We determined the bend angle between this two stems. By combining the orientation information with a priori knowledge about the secondary structure of the aptamer, we obtained a molecular model describing the global folding and flexibility of the cocaine aptamer.

2'-Alkynylnucleotides: A Sequence- and Spin Label-Flexible Strategy for EPR Spectroscopy in DNA

Electron paramagnetic resonance (EPR) spectroscopy is a powerful method to elucidate molecular structure through the measurement of distances between conformationally well-defined spin labels. Here we report a sequence-flexible approach to the synthesis of double spin-labeled DNA duplexes, where 2'-alkynylnucleosides are incorporated at terminal and internal positions on complementary strands. Post-DNA synthesis copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions with a variety of spin labels enable the use of double electron-electron resonance experiments to measure a number of distances on the duplex, affording a high level of detailed structural information.

Complementary-addressed site-directed spin labeling of long natural RNAs

Nanoscale distance measurements by pulse dipolar Electron paramagnetic resonance (EPR) spectroscopy allow new insights into the structure and dynamics of complex biopolymers. EPR detection requires site directed spin labeling (SDSL) of biomolecule(s), which remained challenging for long RNAs up-to-date. Here, we demonstrate that novel complementary-addressed SDSL approach allows efficient spin labeling and following structural EPR studies of long RNAs. We succeeded to spin-label Hepatitis C Virus RNA internal ribosome entry site consisting of ≈330 nucleotides and having a complicated spatial structure. Application of pulsed double electron–electron resonance provided spin–spin distance distribution, which agrees well with the results of molecular dynamics (MD) calculations. Thus, novel SDSL approach in conjunction with EPR and MD allows structural studies of long natural RNAs with nanometer resolution and can be applied to systems of biological and biomedical significance.

Study of a DNA Duplex by Nuclear Magnetic Resonance and Molecular Dynamics Simulations. Validation of Pulsed Dipolar Electron Paramagnetic Resonance Distance Measurements Using Triarylmethyl-Based Spin Labels

Pulse dipole-dipole electron paramagnetic resonance (EPR) spectroscopy (double electron-electron resonance [DEER] or pulse electron-electron double resonance [PELDOR] and double quantum coherence [DQC]) allows for measurement of distances in biomolecules and can be used at low temperatures in a frozen solution. Recently, the possibility of distance measurement in a nucleic acid at a physiological temperature using pulse EPR was demonstrated. In these experiments, triarylmethyl (TAM) radicals with long memory time of the electron spin served as a spin label. In addition, the duplex was immobilized on modified silica gel particles (Nucleosil DMA); this approach enables measurement of interspin distances close to 4.5 nm. Nevertheless, the possible influence of TAM on the structure of a biopolymer under study and validity of the data obtained by DQC are debated. In this paper, a combination of molecular dynamics (MD) and nuclear magnetic resonance (NMR) methods was used for verification of interspin distances measured by the X-band DQC method. NMR is widely used for structural analysis of biomolecules under natural conditions (room temperature and an aqueous solution). The ultraviolet (UV) melting method and thermal series (1)H NMR in the range 5-95 °C revealed the presence of only the DNA duplex in solution at oligonucleotide concentrations 1 μM to 1.1 mM at temperatures below 40 °C. The duplex structures and conformation flexibility of native and TAM-labeled DNA complexes obtained by MD simulation were the same as the structure obtained by NMR refinement. Thus, we showed that distance measurements at physiological temperatures by the X-band DQC method allow researchers to obtain valid structural information on an unperturbed DNA duplex using terminal TAM spin labels.

Conformation and EPR characterization of rigid, 310 -helical peptides with TOAC spin labels: Models for short distances

For 3D-structure determination in biophysical systems EPR is rapidly gaining ground. Proteins labeled specifically with two nitroxide spin labels can be prepared, and several EPR methods are available for distance determination, which makes it possible to determine distance constraints. However, such methods require frozen solutions, potentially causing non-physiological states of the sample. Here, we target spin- spin interaction in liquid solution at room temperature using rigid model compounds. A series of 310 -helical peptides, based on α-aminoisobutyric acid (Aib), is synthesized with pairs of spin labels separated by three, four, and five amino acids. To avoid flexibility, the noncoded nitroxyl-containing α-amino acid TOAC that is rigidly connected with the peptide backbone, is used. The EPR spectra of the peptides show a decreasing amount of coupling between the two spin labels within this series. We suggest through-bond interaction as the dominating mechanism for exchange interaction (J) and find a stronger J-coupling than in the corresponding Ala-based TOAC-peptides investigated previously (Hanson, et al., J Am Chem Soc 1996, 118, 7618-7625). We speculate that stronger coupling in Aib- vs Ala- peptides is due to intrinsically stronger through-bond interaction in the Aib-based peptides.

High-resolution measurement of long-range distances in RNA: pulse EPR spectroscopy with TEMPO-labeled nucleotides

Distance measurements in RNAs by pulse EPR with TEMPO-labeled nucleotides allow for model free conversion of distances into base-pair separation.

Structural information at atomic resolution of biomolecular assemblies, such as RNA and RNA protein complexes, is fundamental to comprehend biological function. Modern spectroscopic methods offer exceptional opportunities in this direction. Here we present the capability of pulse EPR to report high-resolution long-range distances in RNAs by means of a recently developed spin labeled nucleotide, which carries the TEMPO group directly attached to the nucleobase and preserves Watson–Crick base-pairing. In a representative RNA duplex with spin-label separations up to 28 base pairs (≈8 nm) we demonstrate that the label allows for a model-free conversion of inter-spin distances into base-pair separation (Δbp) if broad-band pulse excitation at Q band frequencies (34 GHz) is applied. The observed distance distribution increases from ±0.2 nm for Δbp = 10 to only ±0.5 nm for Δbp = 28, consistent with only small deviations from the “ideal” A-form RNA structure. Molecular dynamics (MD) simulations conducted at 20 °C show restricted conformational freedom of the label. MD-generated structural deviations from an “ideal” A-RNA geometry help disentangle the contributions of local flexibility of the label and its neighboring nucleobases and global deformations of the RNA double helix to the experimental distance distributions. The study demonstrates that our simple but strategic spin labeling procedure can access detailed structural information on RNAs at atomic resolution over distances that match the size of macromolecular RNA complexes.

Conformations of Human Telomeric G-Quadruplex Studied Using a Nucleotide-Independent Nitroxide Label

Guanine-rich oligonucleotides can form a unique G-quadruplex (GQ) structure with stacking units of four guanine bases organized in a plane through Hoogsteen bonding. GQ structures have been detected in vivo and shown to exert their roles in maintaining genome integrity and regulating gene expression. Understanding GQ conformation is important for understanding its inherent biological role and for devising strategies to control and manipulate functions based on targeting GQ. Although a number of biophysical methods have been used to investigate structure and dynamics of GQs, our understanding is far from complete. As such, this work explores the use of the site-directed spin labeling technique, complemented by molecular dynamics simulations, for investigating GQ conformations. A nucleotide-independent nitroxide label (R5), which has been previously applied for probing conformations of noncoding RNA and DNA duplexes, is attached to multiple sites in a 22-nucleotide DNA strand derived from the human telomeric sequence (hTel-22) that is known to form GQ. The R5 labels are shown to minimally impact GQ folding, and inter-R5 distances measured using double electron-electron resonance spectroscopy are shown to adequately distinguish the different topological conformations of hTel-22 and report variations in their occupancies in response to changes of the environment variables such as salt, crowding agent, and small molecule ligand. The work demonstrates that the R5 label is able to probe GQ conformation and establishes the base for using R5 to study more complex sequences, such as those that may potentially form multimeric GQs in long telomeric repeats.

Doubly Spin-Labeled RNA as an EPR Reporter for Studying Multicomponent Supramolecular Assemblies

mRNAs are involved in complicated supramolecular complexes with human 40S and 80S ribosomes responsible for the protein synthesis. In this work, a derivative of nonaribonucleotide pUUCGUAAAA with nitroxide spin labels attached to the 5'-phosphate and to the C8 atom of the adenosine in sixth position (mRNA analog) was used for studying such complexes using double electron-electron resonance/pulsed electron-electron double resonance spectroscopy. The complexes were assembled with participation of tRNA(Phe), which targeted triplet UUC of the derivative to the ribosomal peptidyl site and predetermined location of the adjacent GUA triplet coding for Val at the aminoacyl (A) site. The interspin distances were measured between the two labels of mRNA analog attached to the first nucleotide of the peptidyl site bound codon and to the third nucleotide of the A site bound codon, in the absence/presence of second tRNA bound at the A site. The values of the obtained interspin distances agree with those calculated for available near-atomic structures of similar complexes of 40S and 80S ribosomes, showing that neither 60S subunit nor tRNA at the A site have a noticeable effect on arrangement of mRNA at the codon-anticodon interaction area. In addition, the shapes of distance distributions in four studied ribosomal complexes allowed conclusions on conformational flexibility of mRNA in these complexes. Overall, the results of this study are the first, to our knowledge, demonstration of double electron-electron resonance/pulsed electron-electron double resonance application for measurements of intramolecular distances in multicomponent supramolecular complexes involving intricate cellular machineries and for evaluating dynamic properties of ligands bound to these machineries.

Pulse dipolar ESR of doubly labeled mini TAR DNA and its annealing to mini TAR RNA

Pulse dipolar electron-spin resonance in the form of double electron electron resonance was applied to strategically placed, site-specifically attached pairs of nitroxide spin labels to monitor changes in the mini TAR DNA stem-loop structure brought on by the HIV-1 nucleocapsid protein NCp7. The biophysical structural evidence was at Ångstrom-level resolution under solution conditions not amenable to crystallography or NMR. In the absence of complementary TAR RNA, double labels located in both the upper and the lower stem of mini TAR DNA showed in the presence of NCp7 a broadened distance distribution between the points of attachment, and there was evidence for several conformers. Next, when equimolar amounts of mini TAR DNA and complementary mini TAR RNA were present, NCp7 enhanced the annealing of their stem-loop structures to form duplex DNA-RNA. When duplex TAR DNA-TAR RNA formed, double labels initially located 27.5 Å apart at the 3'- and 5'-termini of the 27-base mini TAR DNA relocated to opposite ends of a 27 bp RNA-DNA duplex with 76.5 Å between labels, a distance which was consistent with the distance between the two labels in a thermally annealed 27-bp TAR DNA-TAR RNA duplex. Different sets of double labels initially located 26-27 Å apart in the mini TAR DNA upper stem, appropriately altered their interlabel distance to ~35 Å when a 27 bp TAR DNA-TAR RNA duplex formed, where the formation was caused either through NCp7-induced annealing or by thermal annealing. In summary, clear structural evidence was obtained for the fraying and destabilization brought on by NCp7 in its biochemical function as an annealing agent and for the detailed structural change from stem-loop to duplex RNA-DNA when complementary RNA was present.

Anomalously Rapid Hydration Water Diffusion Dynamics Near DNA Surfaces

The emerging Overhauser effect dynamic nuclear polarization (ODNP) technique measures the translational mobility of water within the vicinity (5-15 Å) of preselected sites. The work presented here expands the capabilities of the ODNP technique and illuminates an important, previously unseen, property of the translational diffusion dynamics of water at the surface of DNA duplexes. We attach nitroxide radicals (i.e., spin labels) to multiple phosphate backbone positions of DNA duplexes, allowing ODNP to measure the hydration dynamics at select positions along the DNA surface. With a novel approach to ODNP analysis, we isolate the contributions of water molecules at these sites that undergo free translational diffusion from water molecules that either loosely bind to or exchange protons with the DNA. The results reveal that a significant population of water in a localized volume adjacent to the DNA surface exhibits fast, bulk-like characteristics and moves unusually rapidly compared to water found in similar probe volumes near protein and membrane surfaces. Control studies show that the observation of these characteristics are upheld even when the DNA duplex is tethered to streptavidin or the mobility of the nitroxides is altered. This implies that, as compared to protein or lipid surfaces, it is an intrinsic feature of the DNA duplex surface that it interacts only weakly with a significant fraction of the surface hydration water network. The displacement of this translationally mobile water is energetically less costly than that of more strongly bound water by up to several kBT and thus can lower the activation barrier for interactions involving the DNA surface.

EPR spectroscopy identifies Met and Lys residues that are essential for the interaction between the CusB N-terminal domain and metallochaperone CusF

Copper plays a key role in all living organisms by serving as a cofactor for a large variety of proteins and enzymes involved in electron transfer, oxidase and oxygenase activities, and the detoxification of oxygen radicals. Due to its toxicity, a conserved homeostasis mechanism is required. In E. coli, the CusCFBA efflux system is a copper-regulating system and is responsible for transferring Cu(I) and Ag(I) out of the periplasm domain into the extracellular domain. Two of the components of this efflux system, the CusF metallochaperone and the N-terminal domain of CusB, have been thought to play significant roles in the function of this efflux system. Resolving the metal ion transport mechanism through this efflux system is vital for understanding metal- and multidrug-resistant microorganisms. This work explores one aspect of the E. coli resistance mechanism by observing the interaction between the N-terminal domain of CusB and the CusF protein, using electron paramagnetic resonance (EPR) spectroscopy, circular dichroism (CD), and chemical cross-linking. The data summarized here show that M36 and M38 of CusB are important residues for both the Cu(I) coordination to the CusB N-terminal domain and the interaction with CusF, and K32 is essential for the interaction with CusF. In contrast, the K29 residue is less consequential for the interaction with CusF, whereas M21 is mostly important for the proper interaction with CusF.

A versatile approach for site-directed spin labeling and structural EPR studies of RNAs

Site-directed spin labeling (SDSL) is widely applied for structural studies of biopolymers by electron paramagnetic resonance (EPR). However, SDSL of long RNA sequences still remains a challenging task. Here, we propose a novel SDSL approach potentially suitable for long natural RNAs, which is based on the attachment of a linker containing an aliphatic amino group to the target nucleotide residue followed by selective coupling of a spin label to this amino group. Such a linker can be attached to the desired RNA residue via a sequence-specific reaction with the derivatives of oligodeoxyribonucleotides. To verify this approach, we applied it to model RNA duplex with known structure and expected distance between corresponding residues. A new 2,5-bis(spirocyclohexane)-substituted spin label with advanced stability and relaxation properties has been used, and the distance distribution measured using Q-band (34 GHz) pulsed double electron-electron resonance corresponds well to the expected one. We have additionally validated the obtained results by studying a similar RNA duplex, where the linker with the aliphatic amino group was introduced via solid-phase synthesis. Although this novel SDSL approach does not provide an advantage in precision of molecular distance measurements, we believe that its applicability to long RNAs is a crucial benefit for future structural studies using pulse EPR.

EPR spectroscopy shows that the blood carrier protein, human serum albumin, closely interacts with the N-terminal domain of the copper transporter, Ctr1

Copper is an essential metal whose localization within the cells must be carefully controlled to avoid copper dependent redox cycling. Although most of the key proteins involved in cellular copper transfer have been identified, fundamental questions regarding the copper transfer mechanism have yet to be resolved. One of the blood carrier proteins believed to be involved in copper transfer to the cell is human serum albumin (HSA). However, direct evidence for close interaction between HSA and the extracellular domain of the copper transporter Ctr1 has not yet been found. By utilizing EPR spectroscopy, we show here that HSA closely interacts with the first 14 amino acids of the Ctr1, even without the presence of copper ions.

Conformational dynamics of nucleic acid molecules studied by PELDOR spectroscopy with rigid spin labels

Nucleic acid molecules can adopt a variety of structures and exhibit a large degree of conformational flexibility to fulfill their various functions in cells. Here we describe the use of Pulsed Electron-Electron Double Resonance (PELDOR or DEER) to investigate nucleic acid molecules where two cytosine analogs have been incorporated as spin probes. Because these new types of spin labels are rigid and incorporated into double stranded DNA and RNA molecules, there is no additional flexibility of the spin label itself present. Therefore the magnetic dipole-dipole interaction between both spin labels encodes for the distance as well as for the mutual orientation between the spin labels. All of this information can be extracted by multi-frequency/multi-field PELDOR experiments, which gives very precise and valuable information about the structure and conformational flexibility of the nucleic acid molecules. We describe in detail our procedure to obtain the conformational ensembles and show the accuracy and limitations with test examples and application to double-stranded DNA.

An Integrated Spin-Labeling/Computational-Modeling Approach for Mapping Global Structures of Nucleic Acids

The technique of site-directed spin labeling (SDSL) provides unique information on biomolecules by monitoring the behavior of a stable radical tag (i.e., spin label) using electron paramagnetic resonance (EPR) spectroscopy. In this chapter, we describe an approach in which SDSL is integrated with computational modeling to map conformations of nucleic acids. This approach builds upon a SDSL tool kit previously developed and validated, which includes three components: (i) a nucleotide-independent nitroxide probe, designated as R5, which can be efficiently attached at defined sites within arbitrary nucleic acid sequences; (ii) inter-R5 distances in the nanometer range, measured via pulsed EPR; and (iii) an efficient program, called NASNOX, that computes inter-R5 distances on given nucleic acid structures. Following a general framework of data mining, our approach uses multiple sets of measured inter-R5 distances to retrieve "correct" all-atom models from a large ensemble of models. The pool of models can be generated independently without relying on the inter-R5 distances, thus allowing a large degree of flexibility in integrating the SDSL-measured distances with a modeling approach best suited for the specific system under investigation. As such, the integrative experimental/computational approach described here represents a hybrid method for determining all-atom models based on experimentally-derived distance measurements.

CW dipolar broadening EPR spectroscopy and mechanically aligned bilayers used to measure distance and relative orientation between two TOAC spin labels on an antimicrobial peptide

An EPR membrane alignment technique was applied to measure distance and relative orientations between two spin labels on a protein oriented along the surface of the membrane. Previously we demonstrated an EPR membrane alignment technique for measuring distances and relative orientations between two spin labels using a dual TOAC-labeled integral transmembrane peptide (M2δ segment of Acetylcholine receptor) as a test system. In this study we further utilized this technique and successfully measured the distance and relative orientations between two spin labels on a membrane peripheral peptide (antimicrobial peptide magainin-2). The TOAC-labeled magainin-2 peptides were mechanically aligned using DMPC lipids on a planar quartz support, and CW-EPR spectra were recorded at specific orientations. Global analysis in combination with rigorous spectral simulation was used to simultaneously analyze data from two different sample orientations for both single- and double-labeled peptides. We measured an internitroxide distance of 15.3Å from a dual TOAC-labeled magainin-2 peptide at positions 8 and 14 that closely matches with the 13.3Å distance obtained from a model of the labeled magainin peptide. In addition, the angles determining the relative orientations of the two nitroxides have been determined, and the results compare favorably with molecular modeling. This study demonstrates the utility of the technique for proteins oriented along the surface of the membrane in addition to the previous results for proteins situated within the membrane bilayer.

New developments in spin labels for pulsed dipolar EPR

Spin labelling is a chemical technique that enables the integration of a molecule containing an unpaired electron into another framework for study. Given the need to understand the structure, dynamics, and conformational changes of biomacromolecules, spin labelling provides a relatively non-intrusive technique and has certain advantages over X-ray crystallography; which requires high quality crystals. The technique relies on the design of binding probes that target a functional group, for example, the thiol group of a cysteine residue within a protein. The unpaired electron is typically supplied through a nitroxide radical and sterically shielded to preserve stability. Pulsed electron paramagnetic resonance (EPR) techniques allow small magnetic couplings to be measured (e.g., <50 MHz) providing information on single label probes or the dipolar coupling between multiple labels. In particular, distances between spin labels pairs can be derived which has led to many protein/enzymes and nucleotides being studied. Here, we summarise recent examples of spin labels used for pulse EPR that serve to illustrate the contribution of chemistry to advancing discoveries in this field.

Experimental mapping of DNA duplex shape enabled by global lineshape analyses of a nucleotide-independent nitroxide probe

Sequence-dependent variation in structure and dynamics of a DNA duplex, collectively referred to as ‘DNA shape’, critically impacts interactions between DNA and proteins. Here, a method based on the technique of site-directed spin labeling was developed to experimentally map shapes of two DNA duplexes that contain response elements of the p53 tumor suppressor. An R5a nitroxide spin label, which was covalently attached at a specific phosphate group, was scanned consecutively through the DNA duplex. X-band continuous-wave electron paramagnetic resonance spectroscopy was used to monitor rotational motions of R5a, which report on DNA structure and dynamics at the labeling site. An approach based on Pearson's coefficient analysis was developed to collectively examine the degree of similarity among the ensemble of R5a spectra. The resulting Pearson's coefficients were used to generate maps representing variation of R5a mobility along the DNA duplex. The R5a mobility maps were found to correlate with maps of certain DNA helical parameters, and were capable of revealing similarity and deviation in the shape of the two closely related DNA duplexes. Collectively, the R5a probe and the Pearson's coefficient-based lineshape analysis scheme yielded a generalizable method for examining sequence-dependent DNA shapes.

Physiological-temperature distance measurement in nucleic acid using triarylmethyl-based spin labels and pulsed dipolar EPR spectroscopy

Resolving the nanometer-scale structure of biomolecules in natural conditions still remains a challenging task. We report the first distance measurement in nucleic acid at physiological temperature using electron paramagnetic resonance (EPR). The model 10-mer DNA duplex has been labeled with reactive forms of triarylmethyl radicals and then immobilized on a sorbent in water solution and investigated by double quantum coherence EPR. We succeeded in development of optimal triarylmethyl-based labels, approach for site-directed spin labeling and efficient immobilization procedure that, working together, allowed us to measure as long distances as ~4.6 nm with high accuracy at 310 K (37 °C).