Activation energies for dissociation of double strand oligonucleotide anions: evidence for watson-crick base pairing in vacuo

Gas-Phase Fragmentation as a Probe of G-Quadruplex Formation

G-quadruplex (G4) DNA is found in oncogene promoters and human telomeres and is an attractive anticancer target. Stable G4 structures form in guanine-rich sequences in the presence of metal cations and can stabilize further with specific ligand adduction. To explore the preservation and stability of this secondary structure with mass spectrometry, gas-phase collision-induced dissociation kinetics of G4-like and non-G4-like ion structures were determined in a linear quadrupole ion trap. This study focused on a sequence from the promoter of the MYC oncogene, MycG4, and a mutant non-G4-forming sequence, MycNonG4. At relatively high ion activation energies, the backbone fragmentation patterns of the MycG4 and MycNonG4 are similar, while potassium ion-stabilized G4-folded [MycG4 + 2K-7H]5- and counterpart [MycG4-5H]5- ions are essentially indistinguishable, indicating that high-energy fragmentation is not sensitive to the G4 structure. At low energies, the backbone fragmentation patterns of MycG4 and MycNonG4 are significantly different. For MycG4, fragmentation over time differed significantly between the potassium-bound and free structures, reflecting the preservation of the G4 structure in the gas phase. Kinetic measurements revealed the [MycG4 + 2K-7H]5- ions to fragment two to three times more slowly than the [MycG4-5H]5-. Results for the control MycNonG4 indicated that the phenomena noted for [MycG4 + 2K-7H]5- ions are specific to G4-folding. Therefore, our data show that gentle activation conditions can lead to fragmentation behavior that is sensitive to G-quadruplex structure, revealing differences in kinetic stabilities of isomeric structures as well as the regions of the sequence that are directly involved in forming these structures.

Do d(GCGAAGC) Cations Retain the Hairpin Structure in the Gas Phase? A Cyclic Ion Mobility Mass Spectrometry and Density Functional Theory Computational Study

d(GCGAAGC) is the smallest oligonucleotide with a well-defined hairpin structure in solution. We report a study of multiply protonated d(GCGAAGC) and its sequence-scrambled isomers, d(CGAAGCG), d(GCGAACG), and d(CGGAAGC), that were produced by electrospray ionization with the goal of investigating their gas-phase structures and dissociations. Cyclic ion mobility measurements revealed that dications of d(GCGAAGC) as well as the scrambled-sequence ions were mixtures of protomers and/or conformers that had collision cross sections (CCS) within a 439-481 Å2 range. Multiple ion conformers were obtained by electrospray under native conditions as well as from aqueous methanol. Arrival time distribution profiles were characteristic of individual isomeric heptanucleotides. Extensive Born-Oppenheimer molecular dynamics (BOMD) and density functional theory (DFT) calculations of d(GCGAAGC)2+ isomers indicated that hairpin structures were high-energy isomers of more compact distorted conformers. Protonation caused a break up of the C2···G6 pair that was associated with the formation of strong hydrogen bonds in zwitterionic phosphate anion-nucleobase cation motifs that predominated in low energy ions. Multiple components were also obtained for d(GCGAAGC)3+ trications under native and denaturing electrospray conditions. The calculated trication structures showed disruption of the G···C pairs in low energy zwitterions. A hairpin trication was calculated to be a high energy isomer. d(GCGAAGC)4+ tetracations were produced and separated by c-IMS as two major isomers. All low energy d(GCGAAGC)4+ ions obtained by DFT geometry optimizations were zwitterions in which all five purine bases were protonated, and the ion charge was balanced by a phosphate anion. Tetracations of the scrambled sequences were each formed as one dominant isomer. The CCS calculated with the MobCal-MPI method were found to closely match experimental values. Collision-induced dissociation (CID) spectra of multiply charged heptanucleotides showed nucleobase loss and backbone cleavages occurring chiefly at the terminal nucleosides. Electron-transfer-CID tandem mass spectra were used to investigate dissociations of different charge and spin states of charge-reduced heptanucleotide cation radicals.

The time complexity of self-assembly

Time efficiency of self-assembly is crucial for many biological processes. Moreover, with the advances of nanotechnology, time efficiency in artificial self-assembly becomes ever more important. While structural determinants and the final assembly yield are increasingly well understood, kinetic aspects concerning the time efficiency, however, remain much more elusive. In computer science, the concept of time complexity is used to characterize the efficiency of an algorithm and describes how the algorithm's runtime depends on the size of the input data. Here we characterize the time complexity of nonequilibrium self-assembly processes by exploring how the time required to realize a certain, substantial yield of a given target structure scales with its size. We identify distinct classes of assembly scenarios, i.e., "algorithms" to accomplish this task, and show that they exhibit drastically different degrees of complexity. Our analysis enables us to identify optimal control strategies for nonequilibrium self-assembly processes. Furthermore, we suggest an efficient irreversible scheme for the artificial self-assembly of nanostructures, which complements the state-of-the-art approach using reversible binding reactions and requires no fine-tuning of binding energies.

Mass Spectrometry of Nucleic Acid Noncovalent Complexes

Nucleic acids have been among the first targets for antitumor drugs and antibiotics. With the unveiling of new biological roles in regulation of gene expression, specific DNA and RNA structures have become very attractive targets, especially when the corresponding proteins are undruggable. Biophysical assays to assess target structure as well as ligand binding stoichiometry, affinity, specificity, and binding modes are part of the drug development process. Mass spectrometry offers unique advantages as a biophysical method owing to its ability to distinguish each stoichiometry present in a mixture. In addition, advanced mass spectrometry approaches (reactive probing, fragmentation techniques, ion mobility spectrometry, ion spectroscopy) provide more detailed information on the complexes. Here, we review the fundamentals of mass spectrometry and all its particularities when studying noncovalent nucleic acid structures, and then review what has been learned thanks to mass spectrometry on nucleic acid structures, self-assemblies (e.g., duplexes or G-quadruplexes), and their complexes with ligands.

First Direct Evidence of Interpartner Hydride/Deuteride Exchanges for Stored Sodiated Arginine/Fructose-6-phosphate Complex Anions within Salt-Solvated Structures

Mass spectrometric investigations of noncovalent binding between low molecular weight compounds revealed the existence of gas-phase (GP) noncovalent complex (NCC) ions involving zwitterionic structures. ESI MS is used to prove the formation of stable sodiated NCC anions between fructose (F6P) and arginine (R) moieties. Theoretical calculations indicate a folded solvated salt (i.e., sodiated carboxylate interacting with phosphate) rather than a charge-solvated form. Under standard CID conditions, [(F6P+R-H+Na)-H]^- competitively forms two major product ions (PIs) through partner splitting [(R-H+Na) loss] and charge-induced cross-ring cleavage while preserving the noncovalent interactions (noncovalent product ions (NCPIs)). MS/MS experiments combined with in-solution proton/deuteron exchanges (HDXs) demonstrated an unexpected labeling of PIs, i.e., a correlated D-enrichment/D-depletion. An increase in activation time up to 3000 ms favors such processes when limited to two H/D exchanges. These results are rationalized by interpartner hydride/deuteride exchanges (⟨HDX⟩) through stepwise isomerization/dissociation of sodiated NCC-d11 anions. In addition, the D-enrichment/D-depletion discrepancy is further explained by back HDX with residual water in LTQ (selective for the isotopologue NCPIs as shown by PI relaxation experiments). Each isotopologue leads to only one back HDX unlike multiple HDXs generally observed in GP. This behavior shows that NCPIs are zwitterions with charges solvated by a single water molecule, thus generating a back HDX through a relay mechanism, which quenches the charges and prevents further back HDX. By estimating back HDX impact on D-depletion, the interpartner ⟨HDX⟩ complementarity was thus illustrated. This is the first description of interpartner ⟨HDX⟩ and selective back HDX validating salt-solvated structures.

THERAPEUTIC OLIGONUCLEOTIDES, IMPURITIES, DEGRADANTS, AND THEIR CHARACTERIZATION BY MASS SPECTROMETRY

Oligonucleotides are an emerging class of drugs that are manufactured by solid-phase synthesis. As a chemical class, they have unique product-related impurities and degradants, characterization of which is an essential step in drug development. The synthesis cycle, impurities produced during the synthesis and degradation products are presented and discussed. The use of liquid chromatography combined with mass spectrometry for characterization and quantification of product-related impurities and degradants is reviewed. In addition, sequence determination of oligonucleotides by gas-phase fragmentation and indirect mass spectrometric methods is discussed. © 2019 John Wiley & Sons Ltd. Mass Spec Rev.

Submicrometer Nanospray Emitters Provide New Insights into the Mechanism of Cation Adduction to Anionic Oligonucleotides

The electrospray-MS analysis of oligonucleotides is hampered by nonvolatile metal cations, which may produce adducts responsible for signal suppression and loss of resolution. Alternative to replacing metal cations with MS-friendly ammonium, we explored the utilization of nanospray emitters with submicrometer-diameter tips, which was shown to benefit the analysis of protein samples containing elevated salt concentrations. We demonstrated that such benefits are not limited to proteins, but extend also to oligonucleotide samples analyzed in the negative ion mode. At elevated Na⁺/Mg²⁺ concentrations, submicrometer tips produced significantly greater signal-to-noise ratios, as well as greatly reduced adducts and salt clusters, than observed when utilizing micrometer tips. These effects were marginally affected by emitter composition (i.e., borosilicate versus quartz), but varied according to salt concentration and number of oligonucleotide phosphates. The results confirmed that adduct formation is driven by the concentrating effects of the desolvation process, which leads to greatly increased solute concentrations as the volume of the droplet decreases. The process promotes cation-phosphate interactions that may not have necessarily existed in the initial sample, but nevertheless shape the observed adduct series. Therefore, such series may not accurately reflect the distribution of counterions surrounding the analyte in solution. No adverse effects were noted on specific metal interactions, such as those present in a model drug-DNA assembly. These observations indicate that the utilization of submicrometer tips represents an excellent alternative to traditional ammonium-replacement approaches, which enables the analysis of oligonucleotides in the presence of Na⁺/Mg²⁺ concentrations capable of preserving their structure and functional properties.

Modular calibrant sets for the structural analysis of nucleic acids by ion mobility spectrometry mass spectrometry

This study explored the use of modular nucleic acid (NA) standards to generate calibration curves capable of translating primary ion mobility readouts into corresponding collision cross section (CCS) data. Putative calibrants consisted of single- (ss) and double-stranded (ds) oligo-deoxynucleotides reaching up to ∼40 kDa in size (i.e., 64 bp) and ∼5700 Å(2) in CCS. To ensure self-consistency among reference CCS values, computational data obtained in house were preferred to any experimental or computational data from disparate sources. Such values were obtained by molecular dynamics (MD) simulations and either the exact hard sphere scattering (EHSS) or the projection superposition approximation (PSA) methods, and then plotted against the corresponding experimental values to generate separate calibration curves. Their performance was evaluated on the basis of their correlation coefficients and ability to provide values that matched the CCS of selected test samples mimicking typical unknowns. The results indicated that the predictive power benefited from the exclusion of higher charged species that were more susceptible to the destabilizing effects of Coulombic repulsion. The results revealed discrepancies between EHSS and PSA data that were ascribable to the different approximations used to describe the ion mobility process. Within the boundaries defined by these approximations and the challenges of modeling NA structure in a solvent-free environment, the calibrant sets enabled the experimental determination of CCS with excellent reproducibility (precision) and error (accuracy), which will support the analysis of progressively larger NA samples of biological significance.

Infrared laser dissociation of single megadalton polymer ions in a gated electrostatic ion trap: the added value of statistical analysis of individual events

In this study, we report the unimolecular dissociation mechanism of megadalton SO₃-containing poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) polymer cations and anions with the aid of infrared multiphoton dissociation coupled to charge detection ion trap mass spectrometry. A gated electrostatic ion trap ("Benner trap") is used to store and detect single gaseous polymer ions generated by positive and negative polarity in an electrospray ionization source. The trapped ions are then fragmented due to the sequential absorption of multiple infrared photons produced from a continuous-wave CO₂ laser. Several fragmentation pathways having distinct signatures are observed. Highly charged parent ions characteristically adopt a distinctive "stair-case" pattern (assigned to the "fission" process) whereas low charge species take on a "funnel like" shape (assigned to the "evaporation" process). Also, the log-log plot of the dissociation rate constants as a function of laser intensity between PAMPS positive and negative ions is significantly different.

Compaction of Duplex Nucleic Acids upon Native Electrospray Mass Spectrometry

We report on the fate of nucleic acids conformation in the gas phase as sampled using native mass spectrometry coupled to ion mobility spectrometry. On the basis of several successful reports for proteins and their complexes, the technique has become popular in structural biology, and the conformation survival becomes more and more taken for granted. Surprisingly, we found that DNA and RNA duplexes, at the electrospray charge states naturally obtained from native solution conditions (≥100 mM aqueous NH₄OAc), are significantly more compact in the gas phase compared to the canonical solution structures. The compaction is observed for all duplex sizes (gas-phase structures are more compact than canonical B-helices by ∼20% for 12-bp, and by up to ∼30% for 36-bp duplexes), and for DNA and RNA alike. Molecular modeling (density functional calculations on small helices, semiempirical calculations on up to 12-bp, and molecular dynamics on up to 36-bp duplexes) demonstrates that the compaction is due to phosphate group self-solvation prevailing over Coulomb repulsion. Molecular dynamics simulations starting from solution structures do not reproduce the experimental compaction. To be experimentally relevant, molecular dynamics sampling should reflect the progressive structural rearrangements occurring during desolvation. For nucleic acid duplexes, the compaction observed for low charge states results from novel phosphate-phosphate hydrogen bonds formed across both grooves at the very late stages of electrospray.

The Contribution of the Activation Entropy to the Gas-Phase Stability of Modified Nucleic Acid Duplexes

Tricyclo-DNA (tcDNA) is a sugar-modified analogue of DNA currently tested for the treatment of Duchenne muscular dystrophy in an antisense approach. Tandem mass spectrometry plays a key role in modern medical diagnostics and has become a widespread technique for the structure elucidation and quantification of antisense oligonucleotides. Herein, mechanistic aspects of the fragmentation of tcDNA are discussed, which lay the basis for reliable sequencing and quantification of the antisense oligonucleotide. Excellent selectivity of tcDNA for complementary RNA is demonstrated in direct competition experiments. Moreover, the kinetic stability and fragmentation pattern of matched and mismatched tcDNA heteroduplexes were investigated and compared with non-modified DNA and RNA duplexes. Although the separation of the constituting strands is the entropy-favored fragmentation pathway of all nucleic acid duplexes, it was found to be only a minor pathway of tcDNA duplexes. The modified hybrid duplexes preferentially undergo neutral base loss and backbone cleavage. This difference is due to the low activation entropy for the strand dissociation of modified duplexes that arises from the conformational constraint of the tc-sugar-moiety. The low activation entropy results in a relatively high free activation enthalpy for the dissociation comparable to the free activation enthalpy of the alternative reaction pathway, the release of a nucleobase. The gas-phase behavior of tcDNA duplexes illustrates the impact of the activation entropy on the fragmentation kinetics and suggests that tandem mass spectrometric experiments are not suited to determine the relative stability of different types of nucleic acid duplexes. Graphical Abstract ᅟ.

A simple heated-capillary modification improves the analysis of non-covalent complexes by Z-spray electrospray ionization

RATIONALE

The observation of intact non-covalent complexes by electrospray ionization mass spectrometry (ESI-MS) hinges on the ability to minimize in-source activation processes that take place during analyte desolvation. We explored the merits of replacing the sampling cone of a standard Z-spray source with a heated capillary that makes the desolvation process slower and more gradual. We employed well-characterized protein-RNA, RNA-RNA, and DNA-DNA assemblies to compare the alternative configurations.

METHODS

Mass analysis evaluated the integrity of the complexes, whereas traveling wave ion mobility experiments assessed the stability of biomolecular structure. Analyses were performed back-to-back on the same samples on a Synapt G2 HDMS equipped with either the standard sampling cone or the heated-capillary apparatus. In each configuration, the source/capillary temperature was varied in controlled fashion, while keeping all other desolvation parameters constant to monitor the in-source dissociation of selected DNA duplexes. Ion mobility data were obtained from the same precursor by using the alternative configurations under the same settings.

RESULTS

Monitoring the percentage of associated complex demonstrated that the heated capillary provided softer desolvation that was more conducive to the detection of intact non-covalent interactions. This configuration failed to produce complete dissociation of 14 bp and 24 bp duplexes, even when the source/capillary temperature was increased well above their solution melting points. Analyzed by IMS-MS, a selected construct displayed just one conformation with the heated capillary, but two with the standard sampling cone.

CONCLUSIONS

The heated capillary minimizes in-source activation processes that can lead to unintended dissociation of complexes and perturbation of biomolecular structure, which rely on the integrity of non-covalent interactions. This effect can be attributed to the attenuation of the supersonic expansion typical of the Z-spray geometry and the greater ability to control the energy imparted to the system. This hardware modification will be expected to benefit the analysis of biomolecular structure performed on this particular instrumental platform.

Multiphoton dissociation of electrosprayed megadalton-sized DNA ions in a charge-detection mass spectrometer

Charge detection mass spectrometry in combination with a linear electrostatic ion trap coupled to a continuous wavelength infrared CO2 laser has been used to study the multiphoton dissociation of DNA macromolecular ions. Samples, with masses ranging from 2.23 to 31.5 MDa, include single strand circular M13mp18, double strand circular M13mp18, and double strand linear LambdaPhage DNA fragments. Their activation energies for unimolecular dissociation were determined. Activation energy values slightly increase as a function of the molecular weight. The most important result is the difference between the fragmentations observed for hybridized double-strands and dimers of single strands.

Structure and dynamics of oligonucleotides in the gas phase

By combining ion-mobility mass spectrometry experiments with sub-millisecond classical and ab initio molecular dynamics we fully characterized, for the first time, the dynamic ensemble of a model nucleic acid in the gas phase under electrospray ionization conditions. The studied oligonucleotide unfolds upon vaporization, loses memory of the solution structure, and explores true gas-phase conformational space. Contrary to our original expectations, the oligonucleotide shows very rich dynamics in three different timescales (multi-picosecond, nanosecond, and sub-millisecond). The shorter timescale dynamics has a quantum mechanical nature and leads to changes in the covalent structure, whereas the other two are of classical origin. Overall, this study suggests that a re-evaluation on our view of the physics of nucleic acids upon vaporization is needed.

Supramolecular reactivity in the gas phase: investigating the intrinsic properties of non-covalent complexes

The high vacuum inside a mass spectrometer offers unique conditions to broaden our view on the reactivity of supramolecules. Because dynamic exchange processes between complexes are efficiently suppressed, the intrinsic and intramolecular reactivity of the complexes of interest is observed. Besides this, the significantly higher strength of non-covalent interactions in the absence of competing solvent allows processes to occur that are unable to compete in solution. The present review highlights a series of examples illustrating different aspects of supramolecular gas-phase reactivity ranging from the dissociation and formation of covalent bonds in non-covalent complexes through the reactivity in the restricted inner phase of container molecules and step-by-step mechanistic studies of organocatalytic reaction cycles to cage contraction reactions, processes induced by electron capture, and finally dynamic molecular motion within non-covalent complexes as unravelled by hydrogen-deuterium exchange processes performed in the gas phase.

Nucleic acid ion structures in the gas phase

Nucleic acids are diverse polymeric macromolecules that are essential for all life forms. These biomolecules possess a functional three-dimensional structure under aqueous physiological conditions. Mass spectrometry-based approaches have on the other hand opened the possibility to gain structural information on nucleic acids from gas-phase measurements. To correlate gas-phase structural probing results with solution structures, it is therefore important to grasp the extent to which nucleic acid structures are preserved, or altered, when transferred from the solution to a fully anhydrous environment. We will review here experimental and theoretical approaches available to characterize the structure of nucleic acids in the gas phase (with a focus on oligonucleotides and higher-order structures), and will summarize the structural features of nucleic acids that can be preserved in the gas phase on the experiment time scale.

B-DNA characteristics are preserved in double stranded d(A)3·d(T)3 and d(G)3·d(C)3 mini-helixes: conclusions from DFT/M06-2X study

We report the results of the first comprehensive DFT study on the d(A)3·d(T)3 and d(G)3·d(C)3 nucleic acid duplexes. The ability of mini-helixes to preserve the conformation of B-DNA in the gas phase and under the influence of such factors as: solvent, uncompensated charge, and counter-ions was evaluated using M06-2X functional with 6-31G(d,p) basis set. The accuracy of the models was ascertained based on their ability to reproduce key structural features of natural B-DNA. Analysis of the helicity suggests that the helical conformations adopt geometrical parameters which are close to those of the B-DNA form. The torsion angles fall somewhere between the values observed for BI/BII conformational classes. The comparative analysis of parameters of isolated Watson-Crick base pairs versus B-DNA-like conformations indicates the same tendency of base-pair polarization and hydration. Specifically, effects of polarization of nucleobases in continuum type dielectric medium mimicking water are stronger than those caused by the presence of backbone. Polar environment as well as the presence of counterions stabilizes duplexes, facilitating helix formation. Substantial conformational changes of nucleotides upon duplex formation decrease the binding energy. In spite of structural and energetic changes, the placement of a mini-helix into the gas phase does not lead to significant disruption of the structure. On the contrary, the duplex preserves its helicity and the strands remain bound.

Energetics from slow infrared multiphoton dissociation of biomolecules

Photodissociation kinetics of the protonated pentapeptide leucine enkephalin measured using a cw CO(2) laser and a Fourier-transform mass spectrometer are reported. A short induction period, corresponding to the time required to raise the internal energy of the ion population to a (dissociating) steady state, is observed. After this induction period, the dissociation data are accurately fit by first-order kinetics. A plot of the log of the unimolecular dissociation rate constant, k(uni), as a function of the log of laser power is linear at low laser powers (<9 W, k(uni) <0.05 s(-1)), but tapers off at high laser power (9-33 W, k(uni) = 0.05-7 s(-1)). The entire measured dissociation curve can be accurately fit by an exponential function plus a constant. The experiment is simulated using a master equation formalism. In the model, the laser radiation is described as an energetically flat-topped distribution which is spatially uniform. This description is consistent with experimental results which indicate that ion motion within the cell averages out spatial inhomogeneities in the laser light. The model has several adjustable parameters. The effect of varying these parameters on the calculated kinetics and power dependence curves is discussed. A procedure for determining a limited range of threshold dissociation energy, E(o), which fits both the measured induction period and power dependence curves, is presented. Using this procedure, E(o) of leucine enkephalin is determined to be 1.12-1.46 eV. This result is consistent with, although less precise than, values measured previously using blackbody infrared radiative dissociation. Although the blackbody dissociation results were used as a starting point to search for fits of the master equation model to experiment, these results demonstrate that it is, in principle, possible to determine a limited range of E(o) from slow infrared multiphoton dissociation data alone.

Pushing the Limit of Infrared Multiphoton Dissociation to Megadalton-Size DNA Ions

We report the use of infrared multiphoton dissociation (IRMPD) for the determination of relative activation energies for unimolecular dissociation of megadalton DNA ions. Single ions with masses in the megadalton range were stored in an electrostatic ion trap for a few tens of milliseconds and the image current generated by the roundtrips of ions in the trap was recorded. While being trapped, single ions were irradiated by a CO2 laser and fragmented, owing to multiphoton IR activation. The analysis of the single-ion image current during the heating period allows us to measure changes in the charge of the trapped ion. We estimated the activation energy associated with the dissociation of megadalton-size DNA ions in the frame of an Arrhenius-like model by analyzing a large set of individual ions in order to construct a frequency histogram of the dissociation rates for a collection of ions.

Isomer-selected photoelectron spectroscopy of isolated DNA oligonucleotides: phosphate and nucleobase deprotonation at high negative charge states

Fractionation according to ion mobility and mass-to-charge ratio has been used to select individual isomers of deprotonated DNA oligonucleotide multianions for subsequent isomer-resolved photoelectron spectroscopy (PES) in the gas phase. Isomer-resolved PE spectra have been recorded for tetranucleotides, pentanucleotides, and hexanucleotides. These were studied primarily in their highest accessible negative charge states (3-, 4-, and 5-, respectively), as provided by electrospraying from room temperature solutions. In particular, the PE spectra obtained for pentanucleotide tetraanions show evidence for two coexisting classes of gas-phase isomeric structures. We suggest that these two classes comprise: (i) species with excess electrons localized exclusively at deprotonated phosphate backbone sites and (ii) species with at least one deprotonated base (in addition to several deprotonated phosphates). By permuting the sequence of bases in various [A(5-x)T(x)](4-) and [GT(4)](4-) pentanucleotides, we have established that the second type of isomer is most likely to occur if the deprotonated base is located at the first or last position in the sequence. We have used a combination of molecular mechanics and semiempirical calculations together with a simple electrostatic model to explore the photodetachment mechanism underlying our photoelectron spectra. Comparison of predicted to measured photoelectron spectra suggests that a significant fraction of the detected electrons originates from the DNA bases (both deprotonated and neutral).

Enantioselective supramolecular devices in the gas phase. Resorcin[4]arene as a model system

This review describes the state-of-art in the field of the gas-phase reactivity of diastereomeric complexes formed between a chiral artificial receptor and a biologically active molecule. The presented experimental approach is a ligand-displacement reaction carried out in a nano ESI-FT-ICR instrument, supported by a thermodynamic MS-study and molecular-mechanics and molecular-dynamics (MM/MD) computational techniques. The noncovalent ion–molecule complexes are ideal for the study of chiral recognition in the absence of complicating solvent and counterion effects.

Higher-order structure of nucleic acids in the gas phase: top-down analysis of base-pairing interactions

Non-ergodic as well as ergodic activation methods are capable of maintaining the integrity of base pairs during the top-down analysis of nucleic acids. Here, we investigate the significance of this characteristic in the investigation of higher-order structures of increasing complexity. We show that cognate fragments produced by typical backbone cleavages may not be always detected as separate sequence ions, but rather as individual products that remain associated through mutual pairing contacts. This effect translates into unintended masking of cleavage events that take place in double-stranded regions, thus leading to the preferential detection of fragments originating from unpaired regions. Such effect is determined by the stability of the weak non-covalent association between complementary stretches, which is affected by base composition, length of the double-stranded structure, and charge of the precursor ion selected for analysis. Although such effect may prevent the achievement of full sequence coverage for primary structure determination, it may provide the key to correctly differentiate double- versus single-stranded regions, in what could be defined as gas-phase footprinting experiments. In light of the critical role played by base pairs in defining the higher-order structure of nucleic acids, these approaches will be expected to support an increased utilization of mass spectrometry for the investigation of nucleic acid structure and dynamics.

Applications of mass spectrometry to the study of siRNA

RNA interference (RNAi) has quickly become a well-established laboratory tool for regulating gene expression and is currently being explored for its therapeutic potential. The design and use of double-stranded RNA oligonucleotides as therapeutics to trigger the RNAi mechanism and a greater effort to understand the RNAi pathway itself is driving the development of analytical techniques that can characterize these oligonucleotides. Electrospray (ESI) and MALDI have been used routinely to analyze oligonucleotides and their ability to provide mass and sequence information has made them ideal for this application. Reviewed here is the work done to date on the use of ESI and MALDI for the study of RNAi oligonucleotides as well as the strategies and issues associated with siRNA analysis by mass spectrometry. While there is not a large body of literature on the specific application of mass spectrometry to RNAi, the work done in this area is a good demonstration of the range of experiments that can be conducted and the value that ESI and MALDI can provide to the RNAi field.

Feasibility study of hydrogen-bonded nucleic acid base pairs in gas and water phases — A theoretical study

To investigate the base pair binding probabilities for nucleic acid bases, numerous models were studied for contacts between adenine, thymine, guanine, cytosine, and uracil using density functional theory (DFT) in combination with the 6–311G* basis set. We obtained an assessment for the energy given by our calculations in gas and aqueous phases, which showed that it should be incorporated into hydrogen bonding and propeller rotational energies. The 42 complexes of base pairs (5 regular and 37 irregular base pairs) were proposed and their hydrogen-bonding (H-bonding) properties were verified. The hydrogen bonds in some irregular base pairs, including CC, UU, and TT (series 1), were stronger than in regular GC and AT base pairs. Also, the strength of the hydrogen bonds in the proposed base pairs, including CU, GG, GU, and TU (series 2), were similar to regular base pairs from an energetic point of view. The propeller rotations revealed a higher rotational barrier energy (6–7.5 kcal/mol; 1 cal = 4.184 J) for irregular base pairs (series 1 and 2) than regular GC and AT ones (1–3 kcal/mol). Nevertheless, the trend in these affinities of the complex contact probabilities and their biological properties were confirmed by our calculations.

Bioanalytical LC-MS of therapeutic oligonucleotides

Therapeutic oligonucleotides (OGNTs) are important biopharmaceutical drugs for the future, due to their ability to selectively reduce or knockout the expression of target genes. For the development of OGNTs, reliable and relatively high-throughput bioanalytical methods are required to perform the quantitative bioanalysis of OGNTs and their metabolites in biological fluids (e.g., plasma, urine and tissue). Although immunoaffinity methods, especially ELISA, are currently widely applied for this purpose, the potential of LC-MS in OGNT analysis is under investigation. Owing to its inherent ability to monitor the individual target OGNTs as well as their metabolites, LC-MS is now evolving into the method-of-choice for the bioanalysis of OGNTs. In this paper, the state-of-the-art of bioanalytical LC-MS of OGNTs and their metabolites in biological fluids is critically reviewed and its advantages and limitations highlighted. Finally, the future perspective of bioanalytical LC-MS, that is, lower detection levels and potential generic LC-MS methodology, is discussed.

Asymmetric charge partitioning upon dissociation of DNA duplexes

Upon collisional activation, a series of DNA duplexes exhibited a significant degree of asymmetric dissociation with respect to charge partitioning among the single strands. That is, the charge states of the single strand product ions did not equal q/2 for even precursor charge states or (q + 1)/2 and (q-1)/2 for odd precursor charge states (where q is the charge of the precursor). The factors that affect this asymmetric charge partitioning were assessed. The smaller, lower charged duplexes resulted in more symmetric dissociation compared with larger duplexes in higher charge states, which displayed a high degree of asymmetry upon dissociation. The composition of the duplexes influenced charge partitioning, with those containing a greater number of A/T base pairs showing more symmetric dissociation relative to the more G/C rich duplexes. The use of higher collisional energies resulted in significantly more asymmetric dissociation. Comparisons were made with the dissociation behavior previously studied for protein noncovalent complexes and past studies of the gas-phase conformations and dissociation of DNA complexes.

Electron capture dissociation mass spectrometry of metallo-supramolecular complexes

The electron capture dissociation (ECD) of metallo-supramolecular dinuclear triple-stranded helicate Fe(2)L(3)(4+) ions was determined by Fourier transform ion cyclotron resonance mass spectrometry. Initial electron capture by the di-iron(II) triple helicate ions produces dinuclear double-stranded complexes analogous to those seen in solution with the monocationic metal centers Cu(I) or Ag(I). The gas-phase fragmentation behavior [ECD, collision-induced dissociation (CID), and infrared multiphoton dissociation (IRMPD)] of the di-iron double-stranded complexes, (i.e., MS(3) of the ECD product) was compared with the ECD, CID, and IRMPD of the Cu(I) and Ag(I) complexes generated from solution. The results suggest that iron-bound dimers may be of the form Fe(I)(2)L(2)(2+) and that ECD by metallo-complexes allows access, in the gas phase, to oxidation states and coordination chemistry that cannot be accessed in solution.

A role for the MS analysis of nucleic acids in the post-genomics age

The advances of mass spectrometry in the analysis of nucleic acids have tracked very closely the exciting developments of instrumentation and ancillary technologies, which have taken place over the years. However, their diffusion in the broader life sciences community has been and will be linked to the ever evolving focus of biomedical research and its changing demands. Before the completion of the Human Genome Project, great emphasis was placed on sequencing technologies that could help accomplish this project of exceptional scale. After the publication of the human genome, the emphasis switched toward techniques dedicated to the exploration of sequences not coding for actual protein products, which amount to the vast majority of transcribed elements. The broad range of capabilities offered by mass spectrometry is rapidly advancing this platform to the forefront of the technologies employed for the structure-function investigation of these noncoding elements. Increasing focus on the characterization of functional assemblies and their specific interactions has prompted a re-evaluation of what has been traditionally construed as nucleic acid analysis by mass spectrometry. Inspired by the accelerating expansion of the broader field of nucleic acid research, new applications to fundamental biological studies and drug discovery will help redefine the evolving role of MS-analysis of nucleic acids in the post-genomics age.

Chain and conformation stability of solid-state DNA: implications for room temperature storage

There is currently wide interest in room temperature storage of dehydrated DNA. However, there is insufficient knowledge about its chemical and structural stability. Here, we show that solid-state DNA degradation is greatly affected by atmospheric water and oxygen at room temperature. In these conditions DNA can even be lost by aggregation. These are major concerns since laboratory plastic ware is not airtight. Chain-breaking rates measured between 70 degrees C and 140 degrees C seemed to follow Arrhenius' law. Extrapolation to 25 degrees C gave a degradation rate of about 1-40 cuts/10(5) nucleotides/century. However, these figures are to be taken as very tentative since they depend on the validity of the extrapolation and the positive or negative effect of contaminants, buffers or additives. Regarding the secondary structure, denaturation experiments showed that DNA secondary structure could be preserved or fully restored upon rehydration, except possibly for small fragments. Indeed, below about 500 bp, DNA fragments underwent a very slow evolution (almost suppressed in the presence of trehalose) which could end in an irreversible denaturation. Thus, this work validates using room temperature for storage of DNA if completely protected from water and oxygen.

A new method for the determination of the relative affinity of a ligand against various DNA sequences by electrospray ionization mass spectrometry. Application to a polyamide minor groove binder

A new method for the determination of the relative affinity of a ligand against various dsDNA sequences is presented by using electrospray ionization time-of-flight mass spectrometry (ESI-QTOF) mass spectrometry. The principle is described here through the complexation of double-stranded DNA by a polyamide ligand including twelve N-methylpyrrole rings. However this method could be applied to other ligands especially when dissociation constants (Kd) are in nanomolar range. This method does not require knowing the ligand concentration accurately. It allows determination of the relative affinity of a ligand against various dsDNA sequences for 1 : 1 complex stoichiometries in a quick manner without labeling.

Influence of the charge state on the structures and interactions of vancomycin antibiotics with cell-wall analogue peptides: experimental and theoretical studies

Charge matters! The charge state significantly influences the conformation and the binding energy between vancomycin antibiotic and bacterial cell-wall analogue peptides (see figure). Surface-induced dissociation (SID) studies provide a quantitative comparison between the stabilities of different charge states of the complex.In this study we examined the effect of the charge state on the energetics and dynamics of dissociation of the noncovalent complex between the vancomycin and the cell-wall peptide analogue N(alpha),N(epsilon)-diacetyl-L-Lys-D-Ala-D-Ala (V-Ac(2)LKdAdA). The binding energies between the vancomycin and the peptide were obtained from the RRKM (Rice, Ramsperger, Kassel, Marcus) modeling of the time- and energy-resolved surface-induced dissociation (SID) experiments. Our results demonstrate that the stability of the complex towards fragmentation increases in the order: doubly protonated<singly protonated<deprotonated. Dissociation of the singly protonated and singly deprotonated complex is characterized by very large entropy effects, which indicate a substantial increase in the conformational flexibility of the resulting products. The experimental threshold energies of (1.75+/-0.08) eV ((40.3+/-1.8) kcal mol(-1)) and (1.34+/-0.08) eV ((30.9+/-1.8) kcal mol(-1)) obtained for the deprotonated and singly protonated complexes, respectively, are in excellent agreement with the results of density functional theory calculations. The increased stability of the deprotonated complex observed experimentally is attributed to the presence of three charged sites in the deprotonated complex, as compared with only one charged site in the singly protonated complex. The low binding energy of (0.93+/-0.04) eV ((21.4+/-0.9) kcal mol(-1)) obtained for the doubly protonated complex suggests that this ion is destabilized by Coulomb repulsion between the singly protonated vancomycin and the singly protonated peptide comprising the complex.

Early structural evolution of native cytochrome c after solvent removal

Electrospray ionization transfers thermally labile biomolecules, such as proteins, from solution into the gas phase, where they can be studied by mass spectrometry. Covalent bonds are generally preserved during and after the phase transition, but it is less clear to what extent noncovalent interactions are affected by the new gaseous environment. Here, we present atomic-level computational data on the structural rearrangement of native cytochrome c immediately after solvent removal. The first structural changes after desolvation occur surprisingly early, on a timescale of picoseconds. For the time segment of up to 4.2 ns investigated here, we observed no significant breaking of native noncovalent bonds; instead, we found formation of new noncovalent bonds. This generally involves charged residues on the protein surface, resulting in transiently stabilized intermediate structures with a global fold that is essentially the same as that in solution. Comparison with data from native electron capture dissociation experiments corroborates both its mechanistic postulations and our computational predictions, and suggests that global structural changes take place on a millisecond timescale not covered by our simulations.

Which electrospray-based ionization method best reflects protein-ligand interactions found in solution? a comparison of ESI, nanoESI, and ESSI for the determination of dissociation constants with mass spectrometry

We present a comparison of three different electrospray-based ionization techniques for the investigation of noncovalent complexes with mass spectrometry. The features and characteristics of standard electrospray ionization (ESI), chip-based nanoESI, and electrosonic spray ionization (ESSI) mounted onto a hybrid quadrupole time-of-flight mass spectrometer were compared in their performance to determine the dissociation constant (KD) of the model system hen egg white lysozyme (HEWL) binding to N,N',N''-triacetylchitotriose (NAG3). The best KD value compared with solution data were found for ESSI, 19.4 +/- 3.6 microM. Then, we determined the KDs of the two nucleotide binding sites of adenylate kinase (AK), where we obtained KDs of 2.2 +/- 0.8 microM for the first and 19.5 +/- 8.0 microM for the second binding site using ESSI. We found a weak charge state dependence of the KD for both protein-ligand systems, where for all ionization techniques the KD value decreases with increasing charge state. We demonstrate that ESSI is very gentle and insensitive to instrumental parameters, and the KD obtained is in good agreement with solution phase results from the literature. In addition, we tried to determine the KD for the lymphocyte-specific kinase LCK binding to a kinase inhibitor using nanoESI due to the very low amount of sample available. In this case, we found KD values with a strong charge state dependence, which were in no case close to literature values for solution phase.

Studies of the intermolecular DNA triplexes of C+.GC and T.AT triplets by electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry

Formation and stabilities of four 14-mer intermolecular DNA triplexes, consisting of third strands with repeating sequence CTCT, CCTT, CTT, or TTT, were studied by electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) in the gas phase. The gas-phase stabilities of the triplexes were compared with their CD spectra and melting behaviors in solution, and parallel correlation between two phases were obtained. In the presence of 20 mM NH(4) (+) (pH 5.5), the formation of the TTT triplex was not detected in both solution and the gas phase. Other triplexes showed the same order, CTCT > CCTT > CTT, of ion abundances in mass spectra and T(m) values in solution. The more stable triplexes are those that contained higher percentage of C(+).GC triplets and an alternating CT sequence. However, the CCTT with the same C(+).GC triplets as the CTCT showed a higher stability than the latter during the gas-phase dissociation. Furthermore, a biphasic triplex-to-duplex-to-single transition was detected in the gas phase, while a monophasic triplex-to-single dissociation was observed in solution. The present results reveal that hydrogen bonds and electrostatic interactions dominate in the gas phase, while base stacking and hydrophobic interactions dominate in solution to stabilize the triplexes. Moreover, weak acidic conditions (pH 5-6) promote the formation of the parallel triplexes.

Elucidating the intermolecular interactions within a desolvated protein-ligand complex. An experimental and computational study

The first detailed study of the intermolecular hydrogens bonds (H-bonds) within a desolvated, noncovalent protein-ligand complex is reported. Using both experimental and computational methods, the intermolecular H-bonds stabilizing protonated and deprotonated ions of a complex composed of a single chain fragment (scFv) of a monoclonal antibody and its native trisaccharide ligand, alphaGal[alphaAbe] alphaMan (1), are characterized. Using the blackbody infrared radiative dissociation-functional group replacement (BIRD/FGR) technique, three H-bond donor-acceptor pairs within the gaseous (scFv + 1)n+ ions are identified and quantified. Additional sites of interaction on the protein and ligand, for which the binding partner could not be elucidated, are also identified. Comparison of the gas-phase interaction maps with the crystal structure suggests that at least two of the specific H-bonds are conserved upon transfer of the complex from solution to the gas phase by electrospray ionization. However, new (nonspecific) interactions can also form in the gas phase. Notably, the nature and strength of the intermolecular interactions can vary significantly with charge state, and striking differences in the structures of the (scFv + 1)n+ and (scFv + 1)n- ions are evident. Intermolecular H-bonds are also identified from molecular dynamics (MD) simulations performed at the +8 and -8 charge states. Agreement is found for a majority of intermolecular interactions predicted for the (scFv + 1)8+ ion by the MD simulation and BIRD/FGR method; the agreement is less favorable in the case of the (scFv + 1)8- ion. However, both the computational and experimental results point to structural differences between the +8 and -8 ions. The computational results also provide insights into the structural changes that accompany the loss of interfacial waters from the complex.

A theoretical study of hydration effects on the prototropic tautomerism of selenouracils

The prototropic tautomerism of 2-, 4-selenouracil and 2,4-diselenouracil has been studied using density functional theory (DFT) methods, at the B3LYP/6-311 + G(3df,2p)//B3LYP/6-31G(d,p) level. The relative stability order of selenouracil tautomers does not resemble that of uracil tautomers, but it is similar to that of thiouracils, even though the energy gaps between the different tautomers of selenouracils are smaller than for thiouracils. The tautomerism activation barriers are high enough as to conclude that only the oxo-selenone or the diselenone structures should be found in the gas phase. The specific interaction with one water molecule reduces these barriers by a half, but still the oxo-selenone form is always the most stable tautomer. The addition of a second water molecule has a relatively small effect, as well as bulk effects, evaluated by means of a continuum-polarized model. For isolated 2- and 4-selenouracils, the more favorable tautomerization process corresponds to a hydrogen transfer towards the selenium atom, the activation barriers for transfer towards the oxygen atom being much higher. This situation changes when specific and bulk effects are included, and the latter process becomes the more favorable one. For 2,4-diselenouracil the more favorable tautomerization, in the gas phase, corresponds to the H shift from N1 to the Se atom at C2, while solvation effects favor the transfer from N3 to the Se atom at C4.

PNA/dsDNA complexes: site specific binding and dsDNA biosensor applications

The ability of peptide nucleic acids (PNA) to form specific higher-order (i.e., three- and four-stranded) complexes with DNA makes it an ideal structural probe for designing strand-specific dsDNA biosensors. Higher-order complexes are formed between a dye-labeled charge-neutral PNA probe and complementary dsDNA. Addition of a light-harvesting cationic conjugated polymer (CCP) yields supramolecular structures held together by electrostatic forces that incorporate the CCP and the dye-labeled PNA/DNA complexes. Optimization of optical properties allows for excitation of the CCP and subsequent fluorescence resonance energy transfer (FRET) to the PNA-bound dye. In the case of noncomplementary dsDNA, complexation between the probe and target does not occur, and dye emission is weak. The binding between PNA and noncomplementary and complementary dsDNA was examined by several methods. Gel electrophoresis confirms specificity of binding and the formation of higher-order complexes. Nano-electrospray mass spectrometry gives insight into the stoichiometric composition, including PNA/DNA, PNA(2)/DNA, PNA/DNA(2), and PNA(2)/DNA(2) complexes. Finally, structural characteristics and binding-site specificity were examined using ion mobility mass spectrometry in conjunction with molecular dynamics. These results give possible conformations for each of the higher-order complexes formed and show exclusive binding of PNA to the complementary stretch of DNA for all PNA/DNA complexes. Overall, the capability and specificity of binding indicates that the CCP/PNA assay is a feasible detection method for dsDNA and eliminates the need for thermal denaturing steps typically required for DNA hybridization probe assays.

Application of FT-ICR-MS for the study of proton-transfer reactions involving biomolecules

Fourier transform ion cyclotron resonance mass spectrometry, combined with modern ionization (fast atom bombardment , electrospray ionization, matrix-assisted laser desorption-ionization), fragmentation (collision-induced dissociation, surface-induced dissociation, one-photon ultraviolet photodissociation, infrared multiphoton dissociation, blackbody infrared radiative dissociation, electron-capture dissociation), and separation (high-performance liquid chromatography, liquid chromatography, capillary electrophoresis) techniques is now becoming one of the most attractive and frequently used instrumental platforms for gas-phase studies of biomolecules such as amino acids, bioamines, peptides, polypeptides, proteins, nucleobases, nucleosides, nucleotides, polynucleotides, nucleic acids, saccharides, polysaccharides, etc. Since it gives the possibilities to trap the ions from a few seconds up to thousands of seconds, it is often applied to study ion/molecule reactions in the gas phase, particularly proton-transfer reactions which provide important information on acid-base properties. These properties determine in part the three-dimensional structure of biomolecules, most of their intramolecular and intermolecular interactions, and consequently their biological activity. They also indicate the form (unionized, zwitterionic, protonated, or deprotonated) which the biomolecule may take in a nonpolar environment.

B-DNA helix stability in a solvent-free environment

B-DNA is the most common DNA helix conformation under physiological conditions. However, when the amount of water in a DNA solution is decreased, B-to-A helix transitions have been observed. To understand what type of helix conformations exist in a solvent-free environment, a series of poly d(CG)(n) and mixed sequence DNA duplexes from 18 to 30 bp were examined with circular dichroism (CD), ESI-MS, ion mobility, and molecular dynamics. From the CD spectra, it was observed that all sequences had B-form helices in solution. However, the solvent-free results were more complex. For the poly d(CG)(n) series, the 18 bp duplex had an A-form helix conformation, both A- and B-helices were present for the 22 bp duplex, and only B-helices were observed for the 26 and 30 bp duplexes. Since these sequences were all present as B-DNA in solution, the observed solvent-free structures illustrate that smaller helices with fewer base pairs convert to A-DNA more easily than larger helices in the absence of solvent. A similar trend was observed for the mixed sequence duplexes where both an A- and B-helix were present for the 18 bp duplex, while only B-helices occur for the larger 22, 26, and 30 bp duplexes. Since the solvent-free B-helices appear at smaller sizes for the mixed sequences than for the pure d(CG)(n) duplexes, the pure d(CG)(n) duplexes have a greater A-philicity.

Intermolecular Interactions in Biomolecular Systems Examined by Mass Spectrometry

With the development of electrospray and matrix-assisted laser desorption ionization, mass spectrometry (MS) evolved into a powerful tool in the field of biochemistry. Whereas MS is primarily analytical in nature, an increasing number of MS research groups employ the method to address fundamental biochemical questions. Probing the interaction of noncovalently bound molecules in the mass spectrometer is one of the most interesting MS-based experiments possible today, with the potential of making a significant contribution to the basic understanding of the structure and function of biochemical complexes. Here we review a number of current research efforts employing primarily MS techniques to investigate intermolecular interactions in biochemical systems. Examples chosen include the interaction of biomolecules with solvent molecules; interactions between nucleic-acid molecules, in particular, interactions in duplex and quadruplex structures; and interactions between proteins involved in neurodegenerative diseases. Finally we conclude by presenting a few examples of very large biomolecular assemblies in the mega-Dalton range analyzed by MS.

Improving the performance of a quadrupole time-of-flight instrument for macromolecular mass spectrometry

We modified and optimized a first generation quadrupole time-of-flight (Q-TOF) 1 to perform tandem mass spectrometry on macromolecular protein complexes. The modified instrument allows isolation and subsequent dissociation of high-mass protein complexes through collisions with argon molecules. The modifications of the Q-TOF 1 include the introduction of (1) a flow-restricting sleeve around the first hexapole ion bridge, (2) a low-frequency ion-selecting quadrupole, (3) a high-pressure hexapole collision cell, (4) high-transmission grids in the multicomponent ion lenses, and (5) a low repetition rate pusher. Using these modifications, we demonstrate the experimental isolation of ions up to 12 800 mass-to-charge units and detection of product ions up to 38 150 Da, enabling the investigation of the gas-phase stability, protein complex topology, and quaternary structure of protein complexes. Some of the data reveal a so-far unprecedented new mechanism in gas-phase dissociation of protein oligomers whereby a tetramer complex dissociates into two dimers. These data add to the current debate whether gas-phase structures of protein complexes do retain some of the structural features of the corresponding species in solution. The presented low-cost modifications on a Q-TOF 1 instrument are of interest to everyone working in the fields of macromolecular mass spectrometry and more generic structural biology.