Variable bandwidth, high efficiency microwave resonator for control of spin-qubits in nitrogen-vacancy centers

Nitrogen-Vacancy (NV) centers in diamond are attractive tools for sensing and quantum information. Realization of this potential requires effective tools for controlling the spin degree of freedom by microwave (mw) magnetic fields. In this work, we present a planar microwave resonator optimized for microwave-optical double resonance experiments on single NV centers in diamond. It consists of a piece of wide microstrip line, which is symmetrically connected to two 50 Ω microstrip feed lines. In the center of the resonator, an Ω-shaped loop focuses the current and the mw magnetic field. It generates a relatively homogeneous magnetic field over a volume of 0.07 × 0.1 mm³. It can be operated at 2.9 GHz in both transmission and reflection modes with bandwidths of 1000 and 400 MHz, respectively. The high power-to-magnetic field conversion efficiency allows us to produce π-pulses with a duration of 50 ns with only about 200 and 50 mW microwave power in transmission and reflection, respectively. The transmission mode also offers capability for efficient radio frequency excitation. The resonance frequency can be tuned between 1.3 and 6 GHz by adjusting the length of the resonator. This will be useful for experiments on NV-centers at higher external magnetic fields and on different types of optically active spin centers.

Quintet Dinitrenes with Negative Zero-Field Splitting: Effect of Spin-Orbit Coupling on the Sign of Magnetic Anisotropy

We report on W-band EPR and quantum chemical investigation of novel organic tetraradicals with negative axial zero-field splitting (ZFS) parameter D. These belong to the class of quintet 1,3,5-tribromophenylene-2,4-dinitrenes bearing different substituents in position 6 of the benzene ring (1b, N₃; 1c, F; 1d, CN; 1e; Cl; 1f, Br). Analysis of the W-band EPR spectrum of dinitrene 1c reveals its large negative ZFS parameter D = -0.27 cm^-1. Quantum chemical calculations show that negative D gradually grows in the row of 1c(F) < 1b(N₃) < 1d(CN) < 1e(Cl) < 1f(Br) dinitrenes due to decreasing of the through-space distance between the nitrene units and neighboring bromine atoms. Shorter steric N···Br distance results in the stronger contribution of the spin-orbit coupling (SOC) to the total ZFS. The sign of D depends on the interplay of three factors: (i) the angle θ between the "easy" z-axes of the dipolar spin-spin (D_SS) and spin-orbit (D_SOC) interaction tensors, (ii) the ratio of D_SOC/D_SS values, and (iii) the rhombicity parameters E_SS/D_SS and E_SOC/D_SOC. The study demonstrates in which cases organic quintet tetraradicals may have negative ZFS owing to the presence of heavy atoms at appropriate sites nearby the nitrene units and, thus, possess the bistability property as single-molecule magnets.

Short-range ENDOR distance measurements between Gd(III) and trifluoromethyl labels in proteins

The measurement of distances in proteins can be challenging in the 5–20 Å range, which is outside the range accessible through conventional NMR and EPR methods. Recently it was demonstrated...

Characterization of a Triplet Vinylidene

Singlet vinylidenes (R₂C=C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters (D = 0.377 cm^–1 and |E|/D = 0.028) were determined, and the ¹³C hyperfine coupling tensor was obtained by ¹³C-ENDOR measurements. Most strikingly, the isotropic ¹³C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

Sequential Hydrogen Tunneling in o-Tolylmethylene

o‐Tolylmethylene 1 is a metastable triplet carbene that rearranges to o‐xylylene 2 even at temperatures as low as 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopical techniques were used to identify two conformers of 1 (anti and syn) in noble gas matrices and in frozen organic solutions. Conformer‐specific kinetic measurements revealed that the rate constants for the rearrangements of the anti and syn conformers of 1 are very similar. However, the orbital alignment in the syn conformer is less favorable for the hydrogen transfer reaction than the orbital configuration in the anti conformer. Our spectroscopic and quantum chemical investigations indicate that anti1 and syn1 rapidly interconvert via efficient quantum tunneling forming a rotational pre‐equilibrium. The subsequent second tunneling reaction, the [1,4] H migration from anti1 to 2, is rate‐limiting for the formation of 2. We here present an efficient strategy for the study of such tunneling equilibria.

Cryo-EM photosystem I structure reveals adaptation mechanisms to extreme high light in Chlorella ohadii

Photosynthesis in deserts is challenging since it requires fast adaptation to rapid night-to-day changes, that is, from dawn's low light (LL) to extreme high light (HL) intensities during the daytime. To understand these adaptation mechanisms, we purified photosystem I (PSI) from Chlorella ohadii, a green alga that was isolated from a desert soil crust, and identified the essential functional and structural changes that enable the photosystem to perform photosynthesis under extreme high light conditions. The cryo-electron microscopy structures of PSI from cells grown under low light (PSI_LL) and high light (PSI_HL), obtained at 2.70 and 2.71 Å, respectively, show that part of light-harvesting antenna complex I (LHCI) and the core complex subunit (PsaO) are eliminated from PSI_HL to minimize the photodamage. An additional change is in the pigment composition and their number in LHCI_HL; about 50% of chlorophyll b is replaced by chlorophyll a. This leads to higher electron transfer rates in PSI_HL and might enable C. ohadii PSI to act as a natural photosynthesiser in photobiocatalytic systems. PSI_HL or PSI_LL were attached to an electrode and their induced photocurrent was determined. To obtain photocurrents comparable with PSI_HL, 25 times the amount of PSI_LL was required, demonstrating the high efficiency of PSI_HL. Hence, we suggest that C. ohadii PSI_HL is an ideal candidate for the design of desert artificial photobiocatalytic systems.

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y _x Sc_3-x N@C₈₀ (x = 0-3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁-T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin-lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Improving B1 field homogeneity in dielectric tube resonators for EPR spectroscopy via controlled shaping of the dielectric insert

We propose an approach for improving the homogeneity of microwave magnetic field amplitude in a dielectric tube resonator for electron paramagnetic resonance spectroscopy at X-band. The improvement is achieved by "shaping" (controllable variation of the outer diameter of a dielectric insert along its axial direction). Various shaping scenarios based on the principle of discrete solenoids and electromagnetic calculations have been considered. The dielectric insert having the most promising shape was manufactured from a bismuth germanate single crystal. The shaped insert increases the area at B₁ > 0.9 B_1max from 5.06 to 7.36 mm. Higher sensitivity and lower likelihood of quantitative errors have been achieved in pulse EPR experiments for "long" samples (whose length was comparable to that of the dielectric insert) in a shaped dielectric insert.

Intermolecular background decay in RIDME experiments

Theoretical and experimental studies of the RIDME background reveal electron and nuclear spectral diffusion contributions.

Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet Tribromophenyl Nitrenes

Previously unknown the steric heavy atom effect on magnetic anisotropy parameters of triplet phenyl nitrenes is reported. The heavy bromine atom effect is revealed by W-band EPR and theoretical investigations of triplet 2,4,6-tribromophenyl nitrenes bearing different substituents in positions 3 and 5 of the phenyl ring (1a, H/H; 1b, CN/CN; 1c, N₃/F; 1d, N₃/N₃; 1e, Cl/Cl; 1f, Br/Br). The zero-field splitting parameters of nitrenes 1a ( D = 0.9930 cm^-1, E = 0.0261 cm^-1), 1c ( D = 1.244 cm^-1, E = 0.030 cm^-1), and 1d ( D = 1.369 cm^-1, E = 0.093 cm^-1), generated by the photolysis of the corresponding azides in frozen methylcyclohexane solution at 5 K, were determined from the W-band EPR spectra. To clarify the origin of considerable differences in the experimental D values of nitrenes 1a, 1c, and 1d, extensive DFT and CASSCF calculations of these nitrenes as well as of model nitrenes 1b, 1e, and 1f were performed. The calculations show that all nitrenes have nearly the same magnitudes of the spin-spin interactions ( D_SS ∼ 1 cm^-1), but drastically differ in the spin-orbit coupling parameter (from D_SOC = 0.087 cm^-1 for 1a to D_SOC = 0.765 cm^-1 for 1f). Comprehensive analysis of various computational data showed that the magnitude of D_SOC of nitrenes 1a-f is the function of the N···Br distance between the nitrene nitrogen and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a-1f, the smaller the N--Br distance and the larger D_SOC. These features indicate that the heavy atom effect on magnetic anisotropy of triplet phenyl nitrenes originates from the through-space rather than through-bond electronic interactions between the bromine atoms and the nitrene unit.

Reactions of Cyclopentadienylidenes with CF3I: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CF₃I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF₃I at 3 K. Bimolecular reactions between the carbenes and CF₃I were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF₃ group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25-30 K.

Thermally Activated Delayed Fluorescence in a Y3 N@C80 Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies

The endohedral fullerene Y3 N@C80 exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable-temperature steady-state and time-resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y3 N@C80 exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X- and W-band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y3 N@C80 /PFO film is demonstrated opening the possibility for red-emitting fullerene-based organic light-emitting diodes (OLEDs).

Synthesis and characterization of an Fe(i) cage complex with high stability towards strong H-acids

The new iron(i) dioximate showed an unrivaled stability towards strong acids. This calls for a reassessment of the electrocatalytic activity of similar low-valent Co and Fe cage complexes, which have shown to be effective HER electrocatalysts.

Jim Hyde and the ENDOR Connection: A Personal Account

In this minireview, we report on our year-long EPR work, such as electron-nuclear double resonance (ENDOR), pulse electron double resonance (PELDOR) and ELDOR-detected NMR (EDNMR) at X-band and W-band microwave frequencies and magnetic fields. This report is dedicated to James S. Hyde and honors his pioneering contributions to the measurement of spin interactions in large (bio)molecules. From these interactions, detailed information is revealed on structure and dynamics of macromolecules embedded in liquid-solution or solid-state environments. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultra-fast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy and its multi-frequency extensions to new horizons concerning sensitivity of detection, selectivity of molecular interactions and time resolution. Among the most important advances is the upgrading of EPR to high magnetic fields, very much in analogy to what happened in NMR. The ongoing progress in EPR spectroscopy is exemplified by reviewing various multi-frequency electron-nuclear double-resonance experiments on organic radicals, light-generated donor-acceptor radical pairs in photosynthesis, and site-specifically nitroxide spin-labeled bacteriorhodopsin, the light-driven proton pump, as well as EDNMR and ENDOR on nitroxides. Signal and resolution enhancements are particularly spectacular for ENDOR, EDNMR and PELDOR on frozen-solution samples at high Zeeman fields. They provide orientation selection for disordered samples approaching single-crystal resolution at canonical g-tensor orientations-even for molecules with small g-anisotropies. Dramatic improvements of EPR detection sensitivity could be achieved, even for short-lived paramagnetic reaction intermediates. Thus, unique structural and dynamic information is revealed that can hardly be obtained by other analytical techniques. Micromolar concentrations of sample molecules have become sufficient to characterize stable and transient reaction intermediates of complex molecular systems-offering exciting applications for physicists, chemists, biochemists and molecular biologists.

Electron transfer pathways in a light, oxygen, voltage (LOV) protein devoid of the photoactive cysteine

Blue-light absorption by the flavin chromophore in light, oxygen, voltage (LOV) photoreceptors triggers photochemical reactions that lead to the formation of a flavin-cysteine adduct. While it has long been assumed that adduct formation is essential for signaling, it was recently shown that LOV photoreceptor variants devoid of the photoactive cysteine can elicit a functional response and that flavin photoreduction to the neutral semiquinone radical is sufficient for signal transduction. Currently, the mechanistic basis of the underlying electron- (eT) and proton-transfer (pT) reactions is not well understood. We here reengineered pT into the naturally not photoreducible iLOV protein, a fluorescent reporter protein derived from the Arabidopsis thaliana phototropin-2 LOV2 domain. A single amino-acid substitution (Q489D) enabled efficient photoreduction, suggesting that an eT pathway is naturally present in the protein. By using a combination of site-directed mutagenesis, steady-state UV/Vis, transient absorption and electron paramagnetic resonance spectroscopy, we investigate the underlying eT and pT reactions. Our study provides strong evidence that several Tyr and Trp residues, highly conserved in all LOV proteins, constitute the eT pathway for flavin photoreduction, suggesting that the propensity for photoreduction is evolutionary imprinted in all LOV domains, while efficient pT is needed to stabilize the neutral semiquinone radical.

ELDOR-detected NMR: A general and robust method for electron-nuclear hyperfine spectroscopy?

ELDOR-detected NMR (EDNMR) performed at higher magnetic fields is becoming an increasingly popular alternative to conventional ENDOR for the characterization of electron-nuclear hyperfine interactions owing to its enhanced sensitivity. However there are two key problems that limit its widespread adoption, with factors controlling: (i) lineshape distortions and; (ii) overall spectral resolution, still largely understood only at a qualitative level. Indeed highly anisotropic (dipolar) coupled species are particularly problematic in the EDNMR experiment. Nor is it clear as to whether line intensities measured in EDNMR can provide quantitative information. Here we describe how all these problems can be overcome for a nitroxide radical as model system. We introduce a simulation procedure/protocol for the simulation of EDNMR line-shapes collected over a range of high turning angle (HTA) pulse lengths. It is shown that spectral line-shapes can be robustly reproduced and that the intensities of spectral lines and the spin nutation behavior can be quantitatively assessed. This broadens the scope of the EDNMR experiment as a generally applicable, quantitative double resonance method.

Local water sensing: water exchange in bacterial photosynthetic reaction centers embedded in a trehalose glass studied using multiresonance EPR

Pulsed EPR spectroscopies and isotope labeled water are applied to detect and quantify the local water in a bacterial reaction center embedded into a trehalose glass.

Computing distance distributions from dipolar evolution data with overtones: RIDME spectroscopy with Gd(iii)-based spin labels

Extraction of distance distributions between high-spin paramagnetic centers from relaxation induced dipolar modulation enhancement (RIDME) data is affected by the presence of overtones of dipolar frequencies.

Protein Immobilization Capabilities of Sucrose and Trehalose Glasses: The Effect of Protein/Sugar Concentration Unraveled by High-Field EPR

Disaccharide glasses are increasingly used to immobilize proteins at room temperature for structural/functional studies and long-term preservation. To unravel the molecular basis of protein immobilization, we studied the effect of sugar/protein concentration ratios in trehalose or sucrose matrixes, in which the bacterial photosynthetic reaction center (RC) was embedded as a model protein. The structural, dynamical, and H-bonding characteristics of the sugar-protein systems were probed by high-field W-band EPR of a matrix-dissolved nitroxide radical. We discovered that RC immobilization and thermal stabilization, being independent of the protein concentration in trehalose, occur in sucrose only at sufficiently low sugar/protein ratios. EPR reveals that only under such conditions does sucrose form a microscopically homogeneous matrix that immobilizes, via H-bonds, the nitroxide probe. We conclude that the protein immobilization capability depends critically on the propensity of the glass-forming sugar to create intermolecular H-bond networks, thus establishing long-range, homogeneous connectivity within the matrix.

Multifrequency Multiresonance EPR Investigation of Halogen-bonded Complexes Involving Neutral Nitroxide Radicals

Halogen-bonded complexes with neutral nitroxide radicals as the Lewis base have been investigated in liquid and frozen solutions by multifrequency CW and pulse EPR spectroscopies, including ENDOR and ELDOR-detected NMR (EDNMR) techniques. The non-covalent interaction with iodopentafluorobenzene as halogen-bond donor is shown to affect a variety of EPR parameters of the stable nitroxide radicals. In liquid solution, only bulk effects on the EPR signal, i.e. isotropic g value, isotropic 14N hyperfine coupling and linewidth, are observed. Experiments on frozen solutions allow for a more in-depth dissection of complexing effects. W-band EPR spectra at cryogenic temperatures exhibit multiple signal components of different 14N hyperfine interactions and spectral widths. This demonstrates the coexistence of several halogen-bonded complexes that differ in donor-acceptor binding geometries. These complexes have different relaxation properties, which allow for their spectral discrimination. 19F ENDOR experiments prove the origin of these effects to be different specific intermolecular interactions rather than a consequence of changes in the solvation environment. The EPR spectra yet reveal a strong influence of solvent composition on the amount of the various complexes formed. The introduced methodology for the characterization of such adducts improves our understanding of halogen bonding and could be helpful in the development of tailor-made donors and complexes for specific applications.

Effect of Dehydrated Trehalose Matrix on the Kinetics of Forward Electron Transfer Reactions in Photosystem I

The effect of dehydration on the kinetics of forward electron transfer (ET) has been studied in cyanobacterial photosystem I (PS I) complexes in a trehalose glassy matrix by time-resolved optical and EPR spectroscopies in the 100 fs to 1 ms time domain. The kinetics of the flash-induced absorption changes in the subnanosecond time domain due to primary and secondary charge separation steps were monitored by pump–probe laser spectroscopy with 20-fs low-energy pump pulses centered at 720 nm. The back-reaction kinetics of P700 were measured by high-field time-resolved EPR spectroscopy and the forward kinetics of A 1A • − / A 1 B • − → F X ${\rm{A}}_{{\rm{1A}}}^{ \bullet - }/{\rm{A}}_{1{\rm{B}}}^{ \bullet - } \to {{\rm{F}}_{\rm{X}}}$ by time-resolved optical spectroscopy at 480 nm. The kinetics of the primary ET reactions to form the primary P 700 • + A 0 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_0^{ \bullet - }$ and the secondary P 700 • + A 1 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_1^{ \bullet - }$ ion radical pairs were not affected by dehydration in the trehalose matrix, while the yield of the P 700 • + A 1 • − ${\rm{P}}_{700}^{ \bullet + }{\rm{A}}_1^{ \bullet - }$ was decreased by ~20%. Forward ET from the phylloquinone molecules in the A 1 A • − ${\rm{A}}_{1{\rm{A}}}^{ \bullet - }$ and A 1 B • − ${\rm{A}}_{1{\rm{B}}}^{ \bullet - }$ sites to the iron–sulfur cluster FX slowed from ~220 ns and ~20 ns in solution to ~13 μs and ~80 ns, respectively. However, as shown by EPR spectroscopy, the ~15 μs kinetic phase also contains a small contribution from the recombination between A 1 B • − ${\rm{A}}_{1{\rm{B}}}^{ \bullet - }$ and P 700 • + . ${\rm{P}}_{700}^{ \bullet + }.$ These data reveal that the initial ET reactions from P700 to secondary phylloquinone acceptors in the A- and B-branches of cofactors (A1A and A1B) remain unaffected whereas ET beyond A1A and A1B is slowed or prevented by constrained protein dynamics due to the dry trehalose glass matrix.

Erratum to: 36th International Symposium on Intensive Care and Emergency Medicine: Brussels, Belgium. 15-18 March 2016

[This corrects the article DOI: 10.1186/s13054-016-1208-6.].

Paramagnetic Molecular Grippers: The Elements of Six-State Redox Switches

The development of semiquinone-based resorcin[4]arene cavitands expands the toolbox of switchable molecular grippers by introducing the first paramagnetic representatives. The semiquinone (SQ) states were generated electrochemically, chemically, and photochemically. We analyzed their electronic, conformational, and binding properties by cyclic voltammetry, ultraviolet/visible (UV/vis) spectroelectrochemistry, electron paramagnetic resonance (EPR) and transient absorption spectroscopy, in conjunction with density functional theory (DFT) calculations. The utility of UV/vis spectroelectrochemistry and EPR spectroscopy in evaluating the conformational features of resorcin[4]arene cavitands is demonstrated. Guest binding properties were found to be enhanced in the SQ state as compared to the quinone (Q) or the hydroquinone (HQ) states of the cavitands. Thus, these paramagnetic SQ intermediates open the way to six-state redox switches provided by two conformations (open and closed) in three redox states (Q, SQ, and HQ) possessing distinct binding ability. The switchable magnetic properties of these molecular grippers and their responsiveness to electrical stimuli has the potential for development of efficient molecular devices.

EPR characterization of Mn(ii) complexes for distance determination with pulsed dipolar spectroscopy

EPR properties of four Mn(ii) complexes and Tikhonov regularization-based analysis of RIDME data containing dipolar overtones are presented.

Möbius–Hückel topology switching in an expanded porphyrin cation radical as studied by EPR and ENDOR spectroscopy

The symmetry of the arrangement of objects has fascinated philosophers, artists and scientists for a long time, and still does.

RIDME Spectroscopy with Gd(III) Centers

The relaxation induced dipolar modulation enhancement (RIDME) technique is applied at W-band microwave frequencies around 94 GHz to a pair of Gd(III) complexes that are connected by a rodlike spacer, and the extraction of the interspin distance distribution is discussed. A dipolar pattern derived from RIDME experimental data is a superposition of Pake-like dipolar patterns corresponding to the fundamental dipolar interaction and higher harmonics thereof. Intriguingly, the relative weights of the stretched patterns do not depend significantly on mixing time. As much larger modulation depths can be achieved than in double electron-electron resonance distance measurements at the same frequency, Gd(III)-Gd(III) RIDME may become attractive for structural characterization of biomacromolecules and biomolecular complexes.

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates

Transition-metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt(II) cage complexes with the largest (among the transition-metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10(-32) m(3) at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way toward the applications of cobalt(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance.

High-field ELDOR-detected NMR study of a nitroxide radical in disordered solids: towards characterization of heterogeneity of microenvironments in spin-labeled systems

The combination of high-field EPR with site-directed spin-labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in probing the polarity and proticity characteristics of protein/matrix systems. This information is concluded from the principal components of the nitroxide Zeeman (g), nitrogen hyperfine (A) and quadrupole (P) tensors of the spin labels attached to specific sites. Recent multi-frequency high-field EPR studies underlined the complexity of the problem to treat the nitroxide microenvironment in proteins adequately due to inherent heterogeneities which result in several principal x-components of the nitroxide g-tensor. Concomitant, but distinctly different nitrogen hyperfine components could, however, not be determined from high-field cw EPR experiments owing to the large intrinsic EPR linewidth in fully protonated guest/host systems. It is shown in this work that, using the W-band (95GHz) ELDOR- (electron-electron double resonance) detected NMR (EDNMR) method, different principal nitrogen hyperfine, Azz, and quadrupole, Pzz, tensor values of a nitroxide radical in glassy 2-propanol matrix can be measured with high accuracy. They belong to nitroxides with different hydrogen-bond situations. The satisfactory resolution and superior sensitivity of EDNMR as compared to the standard ENDOR (electron-nuclear double resonance) method are demonstrated.

Hydrogen bonding of nitroxide spin labels in membrane proteins

On the basis of experiments at 275 GHz, we reconsider the dependence of the continuous-wave EPR spectra of nitroxide spin-labeled protein sites in sensory- and bacteriorhodopsin on the micro-environment.

Structural and dynamical characteristics of trehalose and sucrose matrices at different hydration levels as probed by FTIR and high-field EPR

Some organisms can survive complete dehydration and high temperatures by adopting an anhydrobiotic state in which the intracellular medium contains large amounts of disaccharides, particularly trehalose and sucrose. Trehalose is most effective also in protecting isolated in vitro biostructures. In an attempt to clarify the molecular mechanisms of disaccharide bioprotection, we compared the structure and dynamics of sucrose and trehalose matrices at different hydration levels by means of high-field W-band EPR and FTIR spectroscopy. The hydration state of the samples was characterized by FTIR spectroscopy and the structural organization was probed by EPR using a nitroxide radical dissolved in the respective matrices. Analysis of the EPR spectra showed that the structure and dynamics of the dehydrated matrices as well as their evolution upon re-hydration differ substantially between trehalose and sucrose. The dehydrated trehalose matrix is homogeneous in terms of distribution of the residual water and spin-probe molecules. In contrast, dehydrated sucrose forms a heterogeneous matrix. It is comprised of sucrose polycrystalline clusters and several bulk water domains. The amorphous form was found only in 30% (volume) of the sucrose matrix. Re-hydration leads to a structural homogenization of the sucrose matrix, whilst in the trehalose matrix several domains develop differing in the local water/radical content and radical mobility. The molecular model of the matrices provides an explanation for the different protein-matrix dynamical coupling observed in dried ternary sucrose and trehalose matrices, and accounts for the superior efficacy of trehalose as a bioprotectant. Furthermore, for bacterial photosynthetic reaction centers it is shown that at low water content the protein-matrix coupling is modulated by the sugar/protein molar ratio in sucrose matrices only. This effect is suggested to be related to the preference for sucrose, rather than trehalose, as a bioprotective disaccharide in some anhydrobiotic organisms.

High-field EPR on membrane proteins - crossing the gap to NMR

In this review on advanced EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR concerning the measurement of molecular interactions in large biomolecules. From these interactions, detailed information can be revealed on structure and dynamics of macromolecules embedded in solution- or solid-state environments. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed to new horizons the limits of EPR spectroscopy and its multifrequency extensions concerning the sensitivity of detection, the selectivity with respect to interactions, and the resolution in frequency and time domains. One of the most important advances has been the extension of EPR to high magnetic fields and microwave frequencies, very much in analogy to what happens in NMR. This is exemplified by referring to ongoing efforts for signal enhancement in both NMR and EPR double-resonance techniques by exploiting dynamic nuclear or electron spin polarization via unpaired electron spins and their electron-nuclear or electron-electron interactions. Signal and resolution enhancements are particularly spectacular for double-resonance techniques such as ENDOR and PELDOR at high magnetic fields. They provide greatly improved orientational selection for disordered samples that approaches single-crystal resolution at canonical g-tensor orientations - even for molecules with small g-anisotropies. Exchange of experience between the EPR and NMR communities allows for handling polarization and resolution improvement strategies in an optimal manner. Consequently, a dramatic improvement of EPR detection sensitivity could be achieved, even for short-lived paramagnetic reaction intermediates. Unique structural and dynamic information is thus revealed that can hardly be obtained by any other analytical techniques. Micromolar quantities of sample molecules have become sufficient to characterize stable and transient reaction intermediates of complex molecular systems - offering highly interesting applications for chemists, biochemists and molecular biologists. In three case studies, representative examples of advanced EPR spectroscopy are reviewed: (I) High-field PELDOR and ENDOR structure determination of cation-anion radical pairs in reaction centers from photosynthetic purple bacteria and cyanobacteria (Photosystem I); (II) High-field ENDOR and ELDOR-detected NMR spectroscopy on the oxygen-evolving complex of Photosystem II; and (III) High-field electron dipolar spectroscopy on nitroxide spin-labelled bacteriorhodopsin for structure-function studies. An extended conclusion with an outlook to further developments and applications is also presented.

An improved coupling design for high-frequency TE011 electron paramagnetic resonance cavities

In high-frequency electron paramagnetic resonance (EPR) spectroscopy the sample is usually accommodated in a single-mode cylindrical TE(011) microwave cavity. This cavity stands out in terms of flexibility for various types of EPR experiments due to convenient control of its resonance frequency and easy waveguide-to-cavity microwave coupling. In continuous wave and in pulsed EPR it is, however, essential to be able to vary the coupling efficiency over a large range. We present a new mechanical design to vary the microwave coupling to the cavity using a movable metal sphere. This coupling sphere is shifted in the plane of the iris wall inside the coupling waveguide. The design allows for a compact and robust construction of the EPR probehead that can be easily accommodated inside a limited space of helium flow cryostat. The construction details and characterization of the coupling element for 95 GHz (W-band) EPR as well as for 34 GHz (Q-band) are presented.

Preparation of cysteine-34-nitroxide spin labeled human α₁-microglobulin

α(1)-Microglobulin (α(1)m) is a protein of yet unresolved function occurring in blood plasma and urine. It consists of a lipocaline type of fold with two cysteine residues forming a disulfide bridge and the third cysteine-34 remaining a free, somewhat reactive thiol. A number of investigations point to an interaction with heme and we have recently reported, that heme binding triggers the formation of a stable α(1)m trimer upon modification of cysteine-34 with 2-iodoacetamide, i.e., [α(1)m(heme)(2)](3) [J.F. Siebel, R.L. Kosinsky, B. Åkerström, M. Knipp, Insertion of heme b into the structure of the Cys34-carbamidomethylated human lipocalin α(1)-microglobulin-formation of a [(heme)(2)(α(1)-microglobulin)](3) complex, ChemBioChem 13 (2012) 879-887]. For further structural and functional investigations, an improved purification protocol for α(1)m was sought, in particular yielding an untagged amino acid sequence. The method reported herein improves the speed and the yield of the protein production even when an expression plasmid without tag was applied. Furthermore, for the purpose of future structural studies using electron paramagnetic resonance (EPR) techniques, in accordance to the modification with 2-iodoacetamide (α(1)m(AM)), the protein was modified with 3-(2-iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (3-(2-iodoacetamido)-PROXYL) yielding the nitroxide spin labeled α(1)m(N-O). The extinction coefficient of the protein was calibrated using magnetic circular dichroism (MCD) spectroscopy of tryptophan (ε(280nm)=40,625M(-1)cm(-1)). The parallel quantification by absorbance spectroscopy (protein) and cw-EPR spectroscopy (radical spin) determined the degree of spin labeling to 90%. Characterization of the protein by circular dichroism (CD) spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) upon tryptic digestion further demonstrated the similar fold of α(1)m(AM) and α(1)m(N-O), but also established the modification of cystein-34 as well as the formation of the cysteine-72-cysteine-169 disulfide bond.

Incorporation of a high potential quinone reveals that electron transfer in Photosystem I becomes highly asymmetric at low temperature

Photosystem I (PS I) has two nearly identical branches of electron-transfer co-factors. Based on point mutation studies, there is general agreement that both branches are active at ambient temperature but that the majority of electron-transfer events occur in the A-branch. At low temperature, reversible electron transfer between P(700) and A(1A) occurs in the A-branch. However, it has been postulated that irreversible electron transfer from P(700) through A(1B) to the terminal iron-sulfur clusters F(A) and F(B) occurs via the B-branch. Thus, to study the directionality of electron transfer at low temperature, electron transfer to the iron-sulfur clusters must be blocked. Because the geometries of the donor-acceptor radical pairs formed by electron transfer in the A- and B-branch differ, they have different spin-polarized EPR spectra and echo-modulation decay curves. Hence, time-resolved, multiple-frequency EPR spectroscopy, both in the direct-detection and pulse mode, can be used to probe the use of the two branches if electron transfer to the iron-sulfur clusters is blocked. Here, we use the PS I variant from the menB deletion mutant strain of Synechocyctis sp. PCC 6803, which is unable to synthesize phylloquinone, to incorporate 2,3-dichloro-1,4-naphthoquinone (Cl(2)NQ) into the A(1A) and A(1B) binding sites. The reduction midpoint potential of Cl(2)NQ is approximately 400 mV more positive than that of phylloquinone and is unable to transfer electrons to the iron-sulfur clusters. In contrast to previous studies, in which the iron-sulfur clusters were chemically reduced and/or point mutations were used to prevent electron transfer past the quinones, we find no evidence for radical-pair formation in the B-branch. The implications of this result for the directionality of electron transfer in PS I are discussed.

Computational strategy for tuning spectral properties of red fluorescent proteins

Computational methods of quantum chemistry are used to characterize structures and vertical excitation energies of the S(0)-S(1) optical transitions in the chromophore binding pockets of the red fluorescent proteins DsRed and of its artificial mutant mCherry. As previously shown, optimizing the equilibrium geometry configurations with B3LYP density functional theory, followed by ZINDO calculations of the electronic excitations, yields positions of the optical bands in good agreement with experimental data. These large scale quantum calculations elucidate the role of the hydrogen bonded network as well as point mutations in the absorption spectra of the DsRed and mCherry proteins. The effect of an external electric field applied to the fluorescent protein chromophores is examined and shows that such fields may result in large shifts in spectral bands. These strategies can be applied for rational design of the fluorescent proteins by site-directed mutagenesis.