Electron spin-lattice relaxation of nitroxyl radicals in temperature ranges that span glassy solutions to low-viscosity liquids

Oligomeric and Fibrillar α-Synuclein Display Persistent Dynamics and Compressibility under Controlled Confinement

The roles of α-synuclein in neurotransmitter release in brain neurons and in the Parkinson's disease condition have challenged comprehensive description. To gain insight into molecular mechanistic properties that actuate α-synuclein function and dysfunction, the coupled protein and solvent dynamics of oligomer and fibril forms of human α-synuclein are examined in a low-temperature system that allows control of confinement and localization of a motionally sensitive electron paramagnetic resonance spin probe in the coupled solvent-protein regions. The rotational mobility of the spin probe resolves two distinct α-synuclein-associated solvent components for oligomers and fibrils, as for globular proteins, but with dramatically higher fluidities at each temperature, that are comparable to low-confinement, aqueous-cryosolvent mesophases. In contrast to the temperature-independent volumes of the solvent phases that surround globular and condensate-forming proteins, the higher-fluidity mesophase volume of α-synuclein oligomers and fibrils decreases with decreasing temperature, signaling a compression of this phase. This unique property and thermal hysteresis in the mobilities and component weights, together with previous high-resolution structural characterizations, suggest a model in which the dynamically disordered C-terminal domain of α-synuclein creates a compressible phase that maintains high fluidity under confinement. Robust dynamics and compressibility are fundamental molecular mechanical properties of α-synuclein oligomers and fibrils, which may contribute to dysfunction and inform about function.

A Highly Ordered Nitroxide Side Chain for Distance Mapping and Monitoring Slow Structural Fluctuations in Proteins

Site-directed spin labeling electron paramagnetic resonance (SDSL-EPR) is an established tool for exploring protein structure and dynamics. Although nitroxide side chains attached to a single cysteine via a disulfide linkage are commonly employed in SDSL-EPR, their internal flexibility complicates applications to monitor slow internal motions in proteins and to structure determination by distance mapping. Moreover, the labile disulfide linkage prohibits the use of reducing agents often needed for protein stability. To enable the application of SDSL-EPR to the measurement of slow internal dynamics, new spin labels with hindered internal motion are desired. Here, we introduce a highly ordered nitroxide side chain, designated R9, attached at a single cysteine residue via a non-reducible thioether linkage. The reaction to introduce R9 is highly selective for solvent-exposed cysteine residues. Structures of R9 at two helical sites in T4 Lysozyme were determined by X-ray crystallography and the mobility in helical sequences was characterized by EPR spectral lineshape analysis, Saturation Transfer EPR, and Saturation Recovery EPR. In addition, interspin distance measurements between pairs of R9 residues are reported. Collectively, all data indicate that R9 will be useful for monitoring slow internal structural fluctuations, and applications to distance mapping via dipolar spectroscopy and relaxation enhancement methods are anticipated.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00723-023-01618-8.

Native and nonnative reactions in ethanolamine ammonia-lyase are actuated by different dynamics

We address the contribution of select classes of solvent-coupled configurational fluctuations to the complex choreography involved in configurational and chemical steps in an enzyme by comparing native and nonnative reactions conducted at different protein internal sites. The low temperature, first-order kinetics of covalent bond rearrangement of the cryotrapped substrate radical in coenzyme B12-dependent ethanolamine ammonia-lyase (EAL) from Salmonella enterica display a kink, or increase in slope, of the Arrhenius plot with decreasing temperature. The event is associated with quenching of a select class of reaction-actuating collective fluctuations in the protein hydration layer. For comparison, a nonnative, radical reaction of the protein interior cysteine sulfhydryl group with hydrogen peroxide (H2O2) is introduced by cryotrapping EAL in an aqueous H2O2 eutectic system. The low-temperature aqueous H2O2 protein hydration and mesodomain solvent phases surrounding cryotrapped EAL are characterized by using TEMPOL spin probe electron paramagnetic resonance spectroscopy, including a freezing transition of the eutectic phase that orders the protein hydration layer. Kinetics of the cysteine-H2O2 reaction in the EAL protein interior are monitored by DEPMPO spin trapping of hydroxyl radical product. In contrast to the native reaction, the linear Arrhenius relation for the nonnative cysteine-H2O2 reaction is maintained through the solvent-protein ordering transition. The nonnative reaction is coupled to the generic local, incremental fluctuations that are intrinsic to globular proteins. The comparative approach supports the proposal that select coupled solvent-protein configurational fluctuations actuate the native reaction, and suggests that select dynamical coupling contributes to the degree of catalysis in enzymes.

Steady state effects introduced by local relaxation modes on J-driven DNP-enhanced NMR

One of solution-state Nuclear Magnetic Resonance (NMR)'s main weaknesses, is its relative insensitivity. J-driven Dynamic Nuclear Polarization (JDNP) was recently proposed for enhancing solution-state NMR's sensitivity, by bypassing the limitations faced by conventional Overhauser DNP (ODNP), at the high magnetic fields where most analytical research is performed. By relying on biradicals with inter-electron exchange couplings Jex on the order of the electron Larmor frequency ωE, JDNP was predicted to introduce a transient enhancement in NMR's nuclear polarization at high magnetic fields, and for a wide range of rotational correlation times of medium-sized molecules in conventional solvents. This communication revisits the JDNP proposal, including additional effects and conditions that were not considered in the original treatment. These include relaxation mechanisms arising from local vibrational modes that often dominate electron relaxation in organic radicals, as well as the possibility of using biradicals with Jex of the order of the nuclear Larmor frequency ωN as potential polarizing agents. The presence of these new relaxation effects lead to variations in the JDNP polarization mechanism originally proposed, and indicate that triplet-to-singlet cross-relaxation processes may lead to a nuclear polarization enhancement that persists even at steady states. The physics and potential limitations of the ensuing theoretical derivations, are briefly discussed.

Rate constants for saturation-recovery EPR and ELDOR of 14N-Spin labels

Saturation-recovery (SR)-EPR can determine electron spin-lattice relaxation rates in liquids over a wide range of effective viscosity, making it especially useful for biophysical and biomedical applications. Here, I develop exact solutions for the SR-EPR and SR-ELDOR rate constants of ¹⁴N-nitroxyl spin labels as a function of rotational correlation time and spectrometer operating frequency. Explicit mechanisms for electron spin-lattice relaxation are: rotational modulation of the N-hyperfine and electron-Zeeman anisotropies (specifically including cross terms), spin-rotation interaction, and residual frequency-independent vibrational contributions from Raman processes and local modes. Cross relaxation from mutual electron and nuclear spin flips, and direct nitrogen nuclear spin-lattice relaxation, also must be included. Both the latter are further contributions from rotational modulation of the electron-nuclear dipolar interaction (END). All the conventional liquid-state mechanisms are defined fully by the spin-Hamiltonian parameters; only the vibrational contributions contain fitting parameters. This analysis gives a firm basis for interpreting SR (and inversion recovery) results in terms of additional, less standard mechanisms.

Confinement dependence of protein-associated solvent dynamics around different classes of proteins, from the EPR spin probe perspective

Protein function is modulated by coupled solvent fluctuations, subject to the degree of confinement from the surroundings. To identify universal features of the external confinement effect, the temperature dependence of the dynamics of protein-associated solvent over 200-265 K for proteins representative of different classes and sizes is characterized by using the rotational correlation time (detection bandwidth, 10-10-10-7 s) of the electron paramagnetic resonance (EPR, X-band) spin probe, TEMPOL, which is restricted to regions vicinal to protein in frozen aqueous solution. Weak (protein surrounded by aqueous-dimethylsulfoxide cryosolvent mesodomain) and strong (no added crysolvent) conditions of ice boundary confinement are imposed. The panel of soluble proteins represents large and small oligomeric (ethanolamine ammonia-lyase, 488 kDa; streptavidin, 52.8 kDa) and monomeric (myoglobin, 16.7 kDa) globular proteins, an intrinsically disordered protein (IDP, β-casein, 24.0 kDa), an unstructured peptide (protamine, 4.38 kDa) and a small peptide with partial backbone order (amyloid-β residues 1-16, 1.96 kDa). Expanded and condensate structures of β-casein and protamine are resolved by the spin probe under weak and strong confinement, respectively. At each confinement condition, the soluble globular proteins display common T-dependences of rotational correlation times and normalized weights, for two mobility components, protein-associated domain, PAD, and surrounding mesodomain. Strong confinement induces a detectable PAD component and emulation of globular protein T-dependence by the amyloid-β peptide. Confinement uniformly impacts soluble globular protein PAD dynamics, and is therefore a generic control parameter for modulation of soluble globular protein function.

Spin-Lattice Relaxation Rates of Lipid Spin Labels as a Measure of Their Rotational Diffusion Rates in Lipid Bilayer Membranes

The spin-lattice relaxation rate (T₁^-1) of lipid spin labels obtained from saturation recovery EPR measurements in deoxygenated membranes depends primarily on the rate of the rotational diffusion of the nitroxide moiety within the lipid bilayer. It has been shown that T₁^-1 also can be used as a qualitative convenient measure of membrane fluidity that reflects local membrane dynamics; however, the relation between T₁^-1 and rotational diffusion coefficients was not provided. In this study, using data previously presented for continuous wave and saturation recovery EPR measurements of phospholipid analog spin labels, one-palmitoyl-2-(n-doxylstearoyl)phosphatidylcholine in 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine/cholesterol membranes, we show that measured T₁^-1 values are linear functions of rotational diffusion of spin labels. Thus, these linear relationships can be used to transfer T₁^-1 values into spin label rotational rates as a precise description of membrane fluidity. This linearity is independent through the wide range of conditions including lipid environment, depth in membrane, local hydrophobicity, and the anisotropy of rotational motion. Transferring the spin-lattice relaxation rates into the rotational diffusion coefficients makes the results obtained from saturation recovery EPR spin labeling easy to understand and readily comparable with other membrane fluidity data.

Resolution and characterization of confinement- and temperature-dependent dynamics in solvent phases that surround proteins in frozen aqueous solution by using spin-probe EPR spectroscopy

Spin probe electron paramagnetic resonance spectroscopy is applied to characterize the dynamics of concentric hydration and mesophase solvent domains that surround proteins within the ice boundary in frozen aqueous solutions. The solvent dynamics are tuned by variation of temperature (190-265K) and by the degree of ice boundary confinement, which is modulated by the volume of added cryosolvent (0-~50Å separation distance from protein surface). Goals are to: (1) characterize the protein-coupled solvent dynamics on correlation time scales of ~10^-10<τ<10^-7s, and spatial scales from protein surface to periphery of the surrounding solution, from the perspective of a free, small-molecule (~7Å diameter) probe, and (2) reveal properties of the solvent-protein coupling that can be correlated with protein functions, that are measureable under the same conditions. Rotational mobility of the nitroxide spin probe, TEMPOL, resolves and tracks two solvent components, the protein-associated domain (PAD; akin to hydration layer) and surrounding mesodomain, through their distinct temperature- and confinement-dependent values of τ and normalized weight. Detailed protocols are described for simulation of two-component nitroxide EPR spectra, which are categorized by line shape regime and guided by a library of template spectra and simulation parameters derived from two model soluble globular proteins. The order-disorder transition in the PAD, which is a universal feature of protein-coupled solvent dynamics, provides a well-defined, tunable property for elucidating mechanism in solvent-protein-function dynamical coupling. The low-temperature mesodomain system and EPR spin probe method are generally applicable to reveal solvent contributions to a broad range of macromolecule-mediated biological processes.

Probing Small-Angle Molecular Motions with EPR Spectroscopy: Dynamical Transition and Molecular Packing in Disordered Solids

Disordered molecular solids present a rather broad class of substances of different origin—amorphous polymers, materials for photonics and optoelectronics, amorphous pharmaceutics, simple molecular glass formers, and others. Frozen biological media in many respects also may be referred to this class. Theoretical description of dynamics and structure of disordered solids still does not exist, and only some phenomenological models can be developed to explain results of particular experiments. Among different experimental approaches, electron paramagnetic resonance (EPR) applied to spin probes and labels also can deliver useful information. EPR allows probing small-angle orientational molecular motions (molecular librations), which intrinsically are inherent to all molecular solids. EPR is employed in its conventional continuous wave (CW) and pulsed—electron spin echo (ESE)—versions. CW EPR spectra are sensitive to dynamical librations of molecules while ESE probes stochastic molecular librations. In this review, different manifestations of small-angle motions in EPR of spin probes and labels are discussed. It is shown that CW-EPR-detected dynamical librations provide information on dynamical transition in these media, similar to that explored with neutron scattering, and ESE-detected stochastic librations allow elucidating some features of nanoscale molecular packing. The possible EPR applications are analyzed for gel-phase lipid bilayers, for biological membranes interacting with proteins, peptides and cryoprotectants, for supercooled ionic liquids (ILs) and supercooled deep eutectic solvents (DESs), for globular proteins and intrinsically disordered proteins (IDPs), and for some other molecular solids.

Structural properties of supercooled deep eutectic solvents: choline chloride–thiourea compared to reline

Deep eutectic solvents (DESs) are eutectic mixtures of hydrogen bond acceptors and hydrogen bond donors which melt at much lower temperatures than the individual components. DESs attract growing interest because...

Evidence for an Ordering Transition near 120 K in an Intrinsically Disordered Protein, Casein

Intrinsically disordered proteins (IDPs) are proteins that possess large unstructured regions. Their importance is increasingly recognized in biology but their characterization remains a challenging task. We employed field swept Electron Spin Echoes in pulsed EPR to investigate low-temperature stochastic molecular librations in a spin-labeled IDP, casein (the main protein of milk). For comparison, a spin-labeled globular protein, hen egg white lysozyme, is also investigated. For casein these motions were found to start at 100 K while for lysozyme only above 130 K, which was ascribed to a denser and more ordered molecular packing in lysozyme. However, above 120 K, the motions in casein were found to depend on temperature much slower than those in lysozyme. This abrupt change in casein was assigned to an ordering transition in which peptide residues rearrange making the molecular packing more rigid and/or more cohesive. The found features of molecular motions in these two proteins turned out to be very similar to those known for gel-phase lipid bilayers composed of conformationally ordered and conformationally disordered lipids. This analogy with a simpler molecular system may appear helpful for elucidation properties of molecular packing in IDPs.

Solute Diffusion into Polymer Swollen by Supercritical CO2 by High-Pressure Electron Paramagnetic Resonance Spectroscopy and Chromatography

High-pressure electron paramagnetic resonance (EPR) was used to measure translational diffusion coefficients (D_tr) of a TEMPONE spin probe in poly(D,L-lactide) (PDLLA) and swollen in supercritical CO₂. D_tr was measured on two scales: macroscopic scale (>1 μm), by measuring spin probe uptake by the sample; and microscopic scale (<10 nm), by using concentration-dependent spectrum broadening. Both methods yield similar translational diffusion coefficients (in the range 5-10 × 10^-12 m²/s at 40-60 °C and 8-10 MPa). Swollen PDLLA was found to be homogeneous on the nanometer scale, although the TEMPONE spin probe in the polymer exhibited higher rotational mobility (τ_corr = 6 × 10^-11 s) than expected, based on its D_tr. To measure distribution coefficients of the solute between the swollen polymer and the supercritical medium, supercritical chromatography with sampling directly from the high-pressure vessel was used. A distinct difference between powder and bulk polymer samples was only observed at the start of the impregnation process.

13C isotope enrichment of the central trityl carbon decreases fluid solution electron spin relaxation times

Electron spin relaxation times for perdeuterated Finland trityl 99% enriched in ¹³C at the central carbon (¹³C₁-dFT) were measured in phosphate buffered saline (pH = 7.2) (PBS) solution at X-band. The anisotropic ¹³C₁ hyperfine (A_x = A_y = 18 ± 2, A_z = 162 ± 1 MHz) and g values (2.0033, 2.0032, 2.00275) in a 9:1 trehalose:sucrose glass at 293 K and in 1:1 PBS:glycerol at 160 K were determined by simulation of spectra at X-band and Q-band. In PBS at room temperature the tumbling correlation time, τ_R, is 0.29 ± 0.02 ns. The linewidths are broadened by incomplete motional averaging of the hyperfine anisotropy and T₂ is 0.13 ± 0.02 µs, which is shorter than the T₂ ~ 3.8 µs for natural abundance dFT at low concentration in PBS. T₁ for ¹³C₁-dFT in deoxygenated PBS is 5.9 ± 0.5 µs, which is shorter than for natural abundance dFT in PBS (16 µs) but much longer than in air-saturated solution (0.48 ± 0.04 µs). The tumbling dependence of T₁ in PBS, 3:1 PBS:glycerol (τ_R = 0.80 ± 0.05 ns, T₁ = 9.7 ± 0.7 µs) and 1:1 PBS:glycerol (τ_R = 3.4 ± 0.3 ns, T₁ = 12.0 ± 1.0 µs) was modeled with contributions to the relaxation predominantly from modulation of hyperfine anisotropy and a local mode. The 1/T₁ rate for the 1% ¹²C₁-dFT in the predominantly ¹³C labeled sample is about a factor of 6 more strongly concentration dependent than for natural abundance ¹²C₁-trityl, which reflects the importance of Heisenberg exchange with molecules with different resonance frequencies and faster relaxation rates. In glassy matrices at 160 K, T₁ and T_m for ¹³C₁-dFT are in good agreement with previously reported values for ¹²C₁-dFT consistent with the expectation that modulation of nuclear hyperfine does not contribute to electron spin relaxation in a rigid lattice.

Control of Solvent Dynamics around the B12-Dependent Ethanolamine Ammonia-Lyase Enzyme in Frozen Aqueous Solution by Using Dimethyl Sulfoxide Modulation of Mesodomain Volume

The temperature-dependent structure and dynamics of two concentric solvent phases, the protein-associated domain (PAD) and the mesodomain, that surround the ethanolamine ammonia-lyase (EAL) protein from Salmonella typhimurium in frozen polycrystalline aqueous solution are addressed by using electron paramagnetic resonance spectroscopy of the paramagnetic nitroxide spin probe, TEMPOL, over the temperature ( T) range 190-265 K. Dimethyl sulfoxide (DMSO), added at 0.5, 2.0, and 4.0% v/v and present at the maximum freeze concentration at T ≤ 245 K, varies the volume of the interstitial aqueous DMSO mesodomain ( V_meso) relative to a fixed PAD volume ( V_PAD). The increase in V_meso/ V_PAD from 0.8 to 6.0 is quantified by the partitioning of TEMPOL between the two phases. As V_meso/ V_PAD is increased, the Arrhenius parameters for activated TEMPOL rotational motion in the mesodomain remain uniform, whereas the parameters for TEMPOL in the PAD show a progressive transformation toward the mesodomain values (higher mobility). An order-disorder transition (ODT) in the PAD is detected by the exclusion of TEMPOL from the PAD into the mesodomain. The ODT T value is systematically lowered by increased V_meso/ V_PAD (from 215 to 200 K), and PAD ordering kinks the mesodomain Arrhenius dependence. Thus there is reciprocity in PAD-mesodomain solvent coupling. The results are interpreted as a dominant influence of ice-boundary confinement on the PAD solvent structure and dynamics, which is transmitted through the mesodomain and which decreases with mesodomain volume at increased added DMSO. The systematic tuning of PAD and mesodomain solvent dynamics by the variation of added DMSO is an incisive approach for the resolution of contributions of protein-solvent dynamical coupling to EAL catalysis.

Molecular order and T1-relaxation, cross-relaxation in nitroxide spin labels

Interpretation of saturation-recovery EPR experiments on nitroxide spin labels whose angular rotation is restricted by the orienting potential of the environment (e.g., membranes) currently concentrates on the influence of rotational rates and not of molecular order. Here, I consider the dependence on molecular ordering of contributions to the rates of electron spin-lattice relaxation and cross relaxation from modulation of N-hyperfine and Zeeman anisotropies. These are determined by the averages 〈cos²θ〉 and 〈cos⁴θ〉, where θ is the angle between the nitroxide z-axis and the static magnetic field, which in turn depends on the angles that these two directions make with the director of uniaxial ordering. For saturation-recovery EPR at 9 GHz, the recovery rate constant is predicted to decrease with increasing order for the magnetic field oriented parallel to the director, and to increase slightly for the perpendicular field orientation. The latter situation corresponds to the usual experimental protocol and is consistent with the dependence on chain-labelling position in lipid bilayer membranes. An altered dependence on order parameter is predicted for saturation-recovery EPR at high field (94 GHz) that is not entirely consistent with observation. Comparisons with experiment are complicated by contributions from slow-motional components, and an unexplained background recovery rate that most probably is independent of order parameter. In general, this analysis supports the interpretation that recovery rates are determined principally by rotational diffusion rates, but experiments at other spectral positions/field orientations could increase the sensitivity to order parameter.

NMR Signal Quenching from Bound Biradical Affinity Reagents in DNP Samples

We characterize the effect of specifically bound biradicals on the NMR spectra of dihydrofolate reductase from E. coli. Dynamic nuclear polarization methods enhance the signal-to-noise of solid state NMR experiments by transferring polarization from unpaired electrons of biradicals to nuclei. There has been recent interest in colocalizing the paramagnetic polarizing agents with the analyte of interest through covalent or noncovalent specific interactions. This experimental approach broadens the scope of dynamic nuclear polarization methods by offering the possibility of selective signal enhancements and the potential to work in a broad range of environments. Paramagnetic compounds can have other effects on the NMR spectroscopy of nearby nuclei, including broadening of nuclear resonances due to the proximity of the paramagnetic agent. Understanding the distance dependence of these interactions is important for the success of the technique. Here we explore paramagnetic signal quenching due to a bound biradical, specifically a biradical-derivatized trimethoprim ligand of E. coli dihydrofolate reductase. Biradical-derivatized trimethoprim has nanomolar affinity for its target, and affords strong and selective signal enhancements in dynamic nuclear polarization experiments. In this work, we show that, although the trimethoprim fragment is well ordered, the biradical (TOTAPOL) moiety is disordered when bound to the protein. The distance dependence in bleaching of NMR signal intensity allows us to detect numerous NMR signals in the protein. We present the possibility that static disorder and electron spin diffusion play roles in this observation, among other contributions. The fact that the majority of signals are observed strengthens the case for the use of high affinity or covalent radicals in dynamic nuclear polarization solid state NMR enhancement.

Dynamic Nuclear Polarization of β-Cyclodextrin Macromolecules

¹H dynamic nuclear polarization and nuclear spin-lattice relaxation rates have been studied in amorphous complexes of β-cyclodextrins doped with different concentrations of the TEMPO radical. Nuclear polarization increased up to 10% in the optimal case, with a behavior of the buildup rate (1/T_POL) and of the nuclear spin-lattice relaxation rate (1/T_1n) consistent with a thermal mixing regime. The temperature dependence of 1/T_1n and its increase with the radical concentration indicate a relaxation process arising from the modulation of the electron-nucleus coupling by the glassy dynamics. The high-temperature relaxation is driven by molecular motions, and 1/T_1n was studied at room temperature in liquid solutions for dilution levels close to the ones typically used for in vivo studies.

A new approach to distance measurements between two spin labels in the >10 nm range

ESR spectroscopy can be efficiently used to acquire the distance between two spin labels placed on a macromolecule by measuring their mutual dipolar interaction frequency, as long as the distance is not greater than ∼10 nm. Any hope to significantly increase this figure is hampered by the fact that all available spin labels have a phase memory time (T_m), restricted to the microseconds range, which provides a limited window during which the dipolar interaction frequency can be measured. Thus, due to the inverse cubic dependence of the dipolar frequency over the labels' separation distance, evaluating much larger distances, e.g. 20 nm, would require to have a T_m that is ∼200 microsecond, clearly beyond any hope. Here we propose a new approach to greatly enhancing the maximum measured distance available by relying on another type of dipole interaction-mediated mechanism called spin diffusion. This mechanism operates and can be evaluated during the spin lattice relaxation time, T₁ (commonly in the milliseconds range), rather than only during T_m. Up until recently, the observation of spin diffusion in solid electron spin systems was considered experimentally impractical. However, recent developments have enabled its direct measurement by means of high sensitivity pulsed ESR that employs intense short magnetic field gradients, thus opening the door to the subsequent utilization of these capabilities. The manuscript presents the subject of spin diffusion, the ways it can be directly measured, and a theoretical discussion on how intramolecular spin-pair distance, even in the range of 20-30 nm, could be accurately extracted from spin diffusion measurements.

Versatile Trityl Spin Labels for Nanometer Distance Measurements on Biomolecules In Vitro and within Cells

Structure determination of biomacromolecules under in-cell conditions is a relevant yet challenging task. Electron paramagnetic resonance (EPR) distance measurements in combination with site-directed spin labeling (SDSL) are a valuable tool in this endeavor but the usually used nitroxide spin labels are not well-suited for in-cell measurements. In contrast, triarylmethyl (trityl) radicals are highly persistent, exhibit a long relaxation time and a narrow spectral width. Here, the synthesis of a versatile collection of trityl spin labels and their application in in vitro and in-cell trityl-iron distance measurements on a cytochrome P450 protein are described. The trityl labels show similar labeling efficiencies and better signal-to-noise ratios (SNR) as compared to the popular methanethiosulfonate spin label (MTSSL) and enabled a successful in-cell measurement.

Long Distance Measurements up to 160 Å in the GroEL Tetradecamer Using Q-Band DEER EPR Spectroscopy

Current distance measurements between spin-labels on multimeric protonated proteins using double electron-electron resonance (DEER) EPR spectroscopy are generally limited to the 15-60 Å range. Here we show how DEER experiments can be extended to dipolar evolution times of ca. 80 μs, permitting distances up to 170 Å to be accessed in multimeric proteins. The method relies on sparse spin-labeling, supplemented by deuteration of protein and solvent, to minimize the deleterious impact of multispin effects and substantially increase the apparent spin-label phase memory relaxation time, complemented by high sensitivity afforded by measurements at Q-band. We demonstrate the approach using the tetradecameric molecular machine GroEL as an example. Two engineered surface-exposed mutants, R268C and E315C, are used to measure pairwise distance distributions with mean values ranging from 20 to 100 Å and from 30 to 160 Å, respectively, both within and between the two heptameric rings of GroEL. The measured distance distributions are consistent with the known crystal structure of apo GroEL. The methodology presented here should significantly expand the use of DEER for the structural characterization of conformational changes in higher order oligomers.

Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety

In the search for long-lived quantum coherence in spin systems, vanadium(IV) complexes have shown record phase memory times among molecular systems. When nuclear spin-free ligands are employed, vanadium(IV) complexes can show at low temperature sufficiently long quantum coherence times, Tm, to perform quantum operations, but their use in real devices operating at room temperature is still hampered by the rapid decrease of T1 caused by the efficient spin-phonon coupling. In this work we have investigated the effect of different coordination environments on the magnetization dynamics and the quantum coherence of two vanadium(IV)-based potential molecular spin qubits in the solid state by introducing a unique structural difference, i.e., an oxovanadium(IV) in a square pyramidal versus a vanadium(IV) in an octahedral environment featuring the same coordinating ligand, namely, the 1,3-dithiole-2-thione-4,5-dithiolate. This investigation, performed by a combined approach of alternate current (ac) susceptibility measurements and continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopies revealed that the effectiveness of the vanadyl moiety in enhancing quantum coherence up to room temperature is related to a less effective mechanism of spin-lattice relaxation that can be quantitatively evaluated by the exponent n (ca. 3) of the temperature dependence of the relaxation rate. A more rapid collapse is observed for the non-oxo counterpart (n = 4) hampering the observation of quantum coherence at room temperature. Record coherence time at room temperature (1.04 μs) and Rabi oscillations are also observed for the vanadyl derivative in a very high concentrated material (5 ± 1%) as a result of the additional benefit provided by the use of a nuclear spin-free ligand.

Dynamics of 4-oxo-TEMPO-d16-(15)N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295K

ESR spectra and electron spin relaxation of nitroxide radical in 4-oxo-TEMPO-d16-(15)N in propylene glycol were studied at X-band in the temperature range 10-295K. The spin-lattice relaxation in the liquid viscous state determined from the resonance line shape is governed by three mechanisms occurring during isotropic molecular reorientations. In the glassy state below 200K the spin-lattice relaxation, phase relaxation and electron spin echo envelope modulations (ESEEM) were studied by pulse spin echo technique using 2-pulse and 3-pulse induced signals. Electron spin-lattice relaxation is governed by a single non-phonon relaxation process produced by localized oscillators of energy 76cm(-1). Electron spin dephasing is dominated by a molecular motion producing a resonance-type peak in the temperature dependence of the dephasing rate around 120K. The origin of the peak is discussed and a simple method for the peak shape analysis is proposed, which gives the activation energy of a thermally activated motion Ea=7.8kJ/mol and correlation time τ0=10(-8)s. The spin echo amplitude is strongly modulated and FT spectrum contains a doublet of lines centered around the (2)D nuclei Zeeman frequency. The splitting into the doublet is discussed as due to a weak hyperfine coupling of nitroxide unpaired electron with deuterium of reorienting CD3 groups.

Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning

We report a strategy to push the limits of solid-state NMR sensitivity far beyond its current state-of-the-art. The approach relies on the use of dynamic nuclear polarization and demonstrates unprecedented DNP enhancement factors for experiments performed at sample temperatures much lower than 100 K, and can translate into 6 orders of magnitude of experimental time-savings. This leap-forward was made possible thanks to the employment of cryogenic helium as the gas to power magic angle sample spinning (MAS) for dynamic nuclear polarization (DNP) enhanced NMR experiments. These experimental conditions far exceed what is currently possible and allows currently reaching sample temperatures down to 30 K while conducting experiments with improved resolution (thanks to faster spinning frequencies, up to 25 kHz) and highly polarized nuclear spins. The impressive associated gains were used to hyperpolarize the surface of an industrial catalyst as well as to hyperpolarize organic nano-assemblies (self-assembling peptides in our case), for whom structures cannot be solved using diffraction techniques. Sustainable cryogenic helium sample spinning significantly enlarges the realm and possibilities of the MAS-DNP technique and is the route to transform NMR into a versatile but also sensitive atomic-level characterization tool.

Multifrequency Pulsed EPR and the Characterization of Molecular Dynamics

In fluid solution, motion-dependent processes dominate electron spin-lattice relaxation for nitroxides and semiquinones at frequencies between 250 MHz and 34 GHz. For triarylmethyl radicals, motion-dependent processes dominate spin-lattice relaxation at frequencies below about 3 GHz. The frequency dependence of relaxation provides invaluable information about dynamic processes occurring with characteristic times on the order of the electron Zeeman frequency. Relaxation mechanisms, methods of measuring spin-lattice relaxation, and motional processes for nitroxide, semiquinone, and triarylmethyl radicals are discussed.

Frequency dependence of electron spin-lattice relaxation for semiquinones in alcohol solutions

The spin-lattice relaxation rates at 293 K for three anionic semiquinones (2,5-di-t-butyl-1,4-benzosemiquinone, 2,6-di-t-butyl-1,4-benzosemiquinone, and 2,3,5,6-tetramethoxy-1,4-benzosemiquinone) were studied at up to 8 frequencies between 250 MHz and 34 GHz in ethanol or methanol solution containing high concentrations of OH(-). The relaxation rates are about a factor of 2 faster at lower frequencies than at 9 or 34 GHz. However, in perdeuterated alcohols the relaxation rates exhibit little frequency dependence, which demonstrates that the dominant frequency-dependent contribution to relaxation is modulation of dipolar interactions with solvent nuclei. The relaxation rates were modeled as the sum of two frequency-independent contributions (spin rotation and a local mode) and two frequency-dependent contributions (modulation of dipolar interaction with solvent nuclei and a much smaller contribution from modulation of g anisotropy). The correlation time for modulation of the interaction with solvent nuclei is longer than the tumbling correlation time of the semiquinone and is consistent with hydrogen bonding of the alcohol to the oxygen atoms of the semiquinones.

Rapid-scan EPR of immobilized nitroxides

X-band electron paramagnetic resonance spectra of immobilized nitroxides were obtained by rapid scan at 293 K. Scan widths were 155 G with 13.4 kHz scan frequency for (14)N-perdeuterated tempone and for T4 lysozyme doubly spin labeled with an iodoacetamide spirocyclohexyl nitroxide and 100 G with 20.9 kHz scan frequency for (15)N-perdeuterated tempone. These wide scans were made possible by modifications to our rapid-scan driver, scan coils made of Litz wire, and the placement of highly conducting aluminum plates on the poles of a Bruker 10″ magnet to reduce resistive losses in the magnet pole faces. For the same data acquisition time, the signal-to-noise for the rapid-scan absorption spectra was about an order of magnitude higher than for continuous wave first-derivative spectra recorded with modulation amplitudes that do not broaden the lineshapes.

Molecular organization of cytochrome c2 near the binding domain of cytochrome bc1 studied by electron spin-lattice relaxation enhancement

Measurements of specific interactions between proteins are challenging. In redox systems, interactions involve surfaces near the attachment sites of cofactors engaged in interprotein electron transfer (ET). Here we analyzed binding of cytochrome c₂ to cytochrome bc₁ by measuring paramagnetic relaxation enhancement (PRE) of spin label (SL) attached to cytochrome c₂. PRE was exclusively induced by the iron atom of heme c₁ of cytochrome bc₁, which guaranteed that only the configurations with SL to heme c₁ distances up to ∼30 Å were detected. Changes in PRE were used to qualitatively and quantitatively characterize the binding. Our data suggest that at low ionic strength and under an excess of cytochrome c₂ over cytochrome bc₁, several cytochrome c₂ molecules gather near the binding domain forming a “cloud” of molecules. When the cytochrome bc₁ concentration increases, the cloud disperses to populate additional available binding domains. An increase in ionic strength weakens the attractive forces and the average distance between cytochrome c₂ and cytochrome bc₁ increases. The spatial arrangement of the protein complex at various ionic strengths is different. Above 150 mM NaCl the lifetime of the complexes becomes so short that they are undetectable. All together the results indicate that cytochrome c₂ molecules, over the range of salt concentration encompassing physiological ionic strength, do not form stable, long-lived complexes but rather constantly collide with the surface of cytochrome bc₁ and ET takes place coincidentally with one of these collisions.

Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments

Thanks to instrumental and theoretical development, notably the access to high-power and high-frequency microwave sources, high-field dynamic nuclear polarization (DNP) on solid-state NMR currently appears as a promising solution to enhance nuclear magnetization in many different types of systems. In magic-angle-spinning DNP experiments, systems of interest are usually dissolved or suspended in glass-forming matrices doped with polarizing agents and measured at low temperature (down to ∼100K). In this work, we discuss the influence of sample conditions (radical concentration, sample temperature, etc.) on DNP enhancements and various nuclear relaxation times which affect the absolute sensitivity of DNP spectra, especially in multidimensional experiments. Furthermore, DNP-enhanced solid-state NMR experiments performed at 9.4 T are complemented by high-field CW EPR measurements performed at the same magnetic field. Microwave absorption by the DNP glassy matrix is observed even below the glass transition temperature caused by softening of the glass. Shortening of electron relaxation times due to glass softening and its impact in terms of DNP sensitivity is discussed.

A diamond-based scanning probe spin sensor operating at low temperature in ultra-high vacuum

We present the design and performance of an ultra-high vacuum (UHV) low temperature scanning probe microscope employing the nitrogen-vacancy color center in diamond as an ultrasensitive magnetic field sensor. Using this center as an atomic-size scanning probe has enabled imaging of nanoscale magnetic fields and single spins under ambient conditions. In this article we describe an experimental setup to operate this sensor in a cryogenic UHV environment. This will extend the applicability to a variety of molecular systems due to the enhanced target spin lifetimes at low temperature and the controlled sample preparation under UHV conditions. The instrument combines a tuning-fork based atomic force microscope (AFM) with a high numeric aperture confocal microscope and the facilities for application of radio-frequency (RF) fields for spin manipulation. We verify a sample temperature of <50 K even for strong laser and RF excitation and demonstrate magnetic resonance imaging with a magnetic AFM tip.

ESR lineshape and 1H spin-lattice relaxation dispersion in propylene glycol solutions of nitroxide radicals--joint analysis

Electron Spin Resonance (ESR) spectroscopy and Nuclear Magnetic Relaxation Dispersion (NMRD) experiments are reported for propylene glycol solutions of the nitroxide radical: 4-oxo-TEMPO-d16 containing (15)N and (14)N isotopes. The NMRD experiments refer to (1)H spin-lattice relaxation measurements in a broad frequency range (10 kHz-20 MHz). A joint analysis of the ESR and NMRD data is performed. The ESR lineshapes give access to the nitrogen hyperfine tensor components and the rotational correlation time of the paramagnetic molecule. The NMRD data are interpreted in terms of the theory of paramagnetic relaxation enhancement in solutions of nitroxide radicals, recently presented by Kruk et al. [J. Chem. Phys. 138, 124506 (2013)]. The theory includes the effect of the electron spin relaxation on the (1)H relaxation of the solvent. The (1)H relaxation is caused by dipole-dipole interactions between the electron spin of the radical and the proton spins of the solvent molecules. These interactions are modulated by three dynamic processes: relative translational dynamics of the involved molecules, molecular rotation, and electron spin relaxation. The sensitivity to rotation originates from the non-central positions of the interacting spin in the molecules. The electronic relaxation is assumed to stem from the electron spin-nitrogen spin hyperfine coupling, modulated by rotation of the radical molecule. For the interpretation of the NMRD data, we use the nitrogen hyperfine coupling tensor obtained from ESR and fit the other relevant parameters. The consistency of the unified analysis of ESR and NMRD, evaluated by the agreement between the rotational correlation times obtained from ESR and NMRD, respectively, and the agreement of the translation diffusion coefficients with literature values obtained for pure propylene glycol, is demonstrated to be satisfactory.

Electron spin-lattice relaxation mechanisms of rapidly-tumbling nitroxide radicals

Electron spin relaxation times at 295 K were measured at frequencies between 250 MHz and 34 GHz for perdeuterated 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (PDT) in five solvents with viscosities that result in tumbling correlation times, τR, between 4 and 50 ps and for three (14)N/(15)N pairs of nitroxides in water with τR between 9 and 19 ps. To test the impact of structure on relaxation three additional nitroxides with τR between 10 and 26 ps were studied. In this fast tumbling regime T2(-1)~T1(-1) at frequencies up to about 9 GHz. At 34 GHz T2(-1)>T1(-1) due to increased contributions to T2(-1) from incomplete motional averaging of g-anisotropy, and T2(-1)-T1(-1) is proportional to τR. The contribution to T1(-1) from spin rotation is independent of frequency and decreases as τR increases. Spin rotation dominates T1(-1) at 34 GHz for all τR studied, and at all frequencies studied for τR=4 ps. The contribution to T1(-1) from modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as τR increases; it dominates at low frequencies for τR>~15 ps. The contribution from modulation of g anisotropy is significant only at 34 GHz. Inclusion of a thermally-activated process was required to account for the observation that for most of the radicals, T1(-1) was smaller at 250 MHz than at 1-2 GHz. The significant (15)N/(14)N isotope effect, the small H/D isotope effect, and the viscosity dependence of the magnitude of the contribution from the thermally-activated process suggest that it arises from intramolecular motions of the nitroxide ring that modulate the isotropic A values.

Electron spin resonance studies of trityl OX063 at a concentration optimal for DNP

We have performed temperature-dependent electron spin resonance (ESR) measurements of the stable free radical trityl OX063, an efficient polarizing agent for dissolution dynamic nuclear polarization (DNP), at the optimum DNP concentration (15 mM). We have found that (i) when compared to the W-band electron spin-lattice relaxation rate T1e(-1) of other free radicals used in DNP at the same concentration, trityl OX063 has slower T1e(-1) than BDPA and 4-oxo-TEMPO. At T > 20 K, the T1e(-1)vs. T data of trityl OX063 appears to follow a power law dependence close to the Raman process prediction whereas at T < 10 K, electronic relaxation slows and approaches the direct process behaviour. (ii) Gd(3+) doping, a factor known to enhance DNP, of trityl OX063 samples measured at W-band resulted in monotonic increases of T1e(-1) especially at temperatures below 20-40 K while the ESR lineshapes remained essentially unchanged. (iii) The high frequency ESR spectrum can be fitted with an axial g-tensor with a slight g-anisotropy: g(x) = g(y) = 2.00319(3) and g(z) = 2.00258(3). Although the ESR linewidth D monotonically increases with field, the temperature-dependent T1e(-1) is almost unchanged as the ESR frequency is increased from 9.5 GHz to 95 GHz, but becomes faster at 240 GHz and 336 GHz. The ESR properties of trityl OX063 reported here may provide insights into the efficiency of DNP of low-γ nuclei performed at various magnetic fields, from 0.35 T to 12 T.

Adiabatic and fast passage ultra-wideband inversion in pulsed EPR

We demonstrate that adiabatic and fast passage ultra-wideband (UWB) pulses can achieve inversion over several hundreds of MHz and thus enhance the measurement sensitivity, as shown by two selected experiments. Technically, frequency-swept pulses are generated by a 12 GS/s arbitrary waveform generator and upconverted to X-band frequencies. This pulsed UWB source is utilized as an incoherent channel in an ordinary pulsed EPR spectrometer. We discuss experimental methodologies and modeling techniques to account for the response of the resonator, which can strongly limit the excitation bandwidth of the entire non-linear excitation chain. Aided by these procedures, pulses compensated for bandwidth or variations in group delay reveal enhanced inversion efficiency. The degree of bandwidth compensation is shown to depend critically on the time available for excitation. As a result, we demonstrate optimized inversion recovery and double electron electron resonance (DEER) experiments. First, virtually complete inversion of the nitroxide spectrum with an adiabatic pulse of 128ns length is achieved. Consequently, spectral diffusion between inverted and non-inverted spins is largely suppressed and the observation bandwidth can be increased to increase measurement sensitivity. Second, DEER is performed on a terpyridine-based copper (II) complex with a nitroxide-copper distance of 2.5nm. As previously demonstrated on this complex, when pumping copper spins and observing nitroxide spins, the modulation depth is severely limited by the excitation bandwidth of the pump pulse. By using fast passage UWB pulses with a maximum length of 64ns, we achieve up to threefold enhancement of the modulation depth. Associated artifacts in distance distributions when increasing the bandwidth of the pump pulse are shown to be small.

Temperature Dependence of Electron Spin Relaxation of 2,2-diphenyl-1-picrylhydrazyl in Polystyrene

The electron spin relaxation rates for the stable radical DPPH (2,2-diphenyl-1-picrylhydrazyl) doped into polystyrene were studied by inversion recovery and electron spin echo at X-band and Q-band between 20 and 295 K. At low concentration (340 μM, 0.01%) spin-lattice relaxation was dominated by the Raman process and a local mode. At high concentration (140 mM, 5%) relaxation is orders of magnitude faster than at the lower concentration, and 1/T₁ is approximately linearly dependent on temperature. Spin lattice relaxation rates are similar at X-band and Q-band. The temperature dependence of spin echo dephasing was faster at about 140 K than at higher or lower temperatures, which is attributed to a wagging motion of the phenyl groups.

Translational diffusion in paramagnetic liquids by 1H NMR relaxometry: nitroxide radicals in solution

For nitroxide radicals in solution one can identify three frequency regimes in which (1)H spin-lattice relaxation rate of solvent molecules depend linearly on square root of the (1)H resonance frequency. Combining a recently developed theory of nuclear (proton) spin-lattice relaxation in solutions of nitroxide radicals [D. Kruk et al., J. Chem. Phys. 137, 044512 (2012)] with properties of the spectral density function associated with translational dynamics, relationships between the corresponding linear changes of the relaxation rate (for (14)N spin probes) and relative translational diffusion coefficient of the solvent and solute molecules have been derived (in analogy to (15)N spin probes [E. Belorizky et al., J. Phys. Chem. A 102, 3674 (1998)]). This method allows a simple and straightforward determination of diffusion coefficients in spin-labeled systems, by means of (1)H nuclear magnetic resonance (NMR) relaxometry. The approach has thoroughly been tested by applying to a large set of experimental data-(1)H spin-lattice relaxation dispersion results for solutions of different viscosity (decalin, glycerol, propylene glycol) of (14)N and (15)N spin probes. The experiments have been performed versus temperature (to cover a broad range of translational diffusion coefficients) using field cycling spectrometer which covers three decades in (1)H resonance frequency, 10 kHz-20 MHz. The limitations of NMR relaxometry caused by the time scale of the translational dynamics as well as electron spin relaxation have been discussed. It has been shown that for spin-labeled systems NMR relaxometry gives access to considerably faster diffusion processes than for diamagnetic systems.

Frequency dependence of electron spin relaxation times in aqueous solution for a nitronyl nitroxide radical and perdeuterated-tempone between 250 MHz and 34 GHz

Electron spin relaxation times of perdeuterated tempone (PDT) 1 and of a nitronyl nitroxide (2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl) 2 in aqueous solution at room temperature were measured by 2-pulse electron spin echo (T(2)) or 3-pulse inversion recovery (T(1)) in the frequency range of 250 MHz to 34 GHz. At 9 GHz values of T(1) measured by long-pulse saturation recovery were in good agreement with values determined by inversion recovery. Below 9 GHz for 1 and below 1.5 GHz for 2,T(1)~T(2), as expected in the fast tumbling regime. At higher frequencies T(2) was shorter than T(1) due to incomplete motional averaging of g and A anisotropy. The frequency dependence of 1/T(1) is modeled as the sum of spin rotation, modulation of g and A-anisotropy, and a thermally-activated process that has maximum contribution at about 1.5 GHz. The spin lattice relaxation times for the nitronyl nitroxide were longer than for PDT by a factor of about 2 at 34 GHz, decreasing to about a factor of 1.5 at 250 MHz. The rotational correlation times, τ(R) are calculated to be 9 ps for 1 and about 25 ps for 2. The longer spin lattice relaxation times for 2 than for 1 at 9 and 34 GHz are due predominantly to smaller contributions from spin rotation that arise from slower tumbling. The smaller nitrogen hyperfine couplings for the nitronyl 2 than for 1 decrease the contribution to relaxation due to modulation of A anisotropy. However, at lower frequencies the slower tumbling of 2 results in a larger value of ωτ(R) (ω is the resonance frequency) and larger values of the spectral density function, which enhances the contribution from modulation of anisotropic interactions for 2 to a greater extent than for 1.

1H NMR relaxation in glycerol solutions of nitroxide radicals: effects of translational and rotational dynamics

(1)H spin-lattice relaxation rates in glycerol solutions of selected nitroxide radicals at temperatures between 200 K and 400 K were measured at 15 MHz and 25 MHz. The frequency and temperature conditions were chosen in such a way that the relaxation rates go through their maximum values and are affected by neither the electron spin relaxation nor the electron-nitrogen nucleus hyperfine coupling, so that the focus could be put on the mechanisms of motion. By comparison with (1)H spin-lattice relaxation results for pure glycerol, it has been demonstrated that the inter-molecular electron spin-proton spin dipole-dipole interactions are affected not only by relative translational motion of the solvent and solute molecules, but also by their rotational dynamics as the interacting spins are displaced from the molecular centers; the eccentricity effects are usually not taken into account. The (1)H relaxation data have been decomposed into translational and rotational contributions and their relative importance as a function of frequency and temperature discussed in detail. It has been demonstrated that neglecting the rotational effects on the inter-molecular interactions leads to non-realistic conclusions regarding the translational dynamics of the paramagnetic molecules.

First determination of the spin relaxation properties of a nitronyl nitroxide in solution by electron spin echoes at X-band: a comparison with Tempone

We studied by electron spin echo pulse methods the spin relaxation properties of a phenyl nitronyl nitroxide radical (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, PTIO) at X-band in fluid toluene solution in a wide temperature range, and in a water/glycerol 1:1 mixture near room temperature. The relaxation properties of PTIO have been compared with that of Tempone, as a widely used nitroxide. By a new procedure, based on experimental results on the temperature dependences of the relaxation times T(1) and T(2), and on the approximation of an isotropic brownian rotational diffusion, we separated non-secular, spin rotational and residual terms from the transverse relaxation rate to isolate secular and pseudosecular contributions. By comparing the results for the two radicals we found the differences in the magnetic properties that give rise to slower transverse (T(2)) and longitudinal (T(1)) electron spin relaxation for PTIO in the whole temperature range explored in this work.

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T(1)∼T(2) in the range of 0.50-1.1 μs at ambient temperature. Both spin-lattice relaxation T(1) and spin-spin relaxation T(2) are longer for (15)N- than for (14)N-nitroxides. The dominant contributions to T(1) are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T(1) on nitrogen nuclear spin state m(I) was observed for both (14)N and (15)N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.

Resolving Conformational and Rotameric Exchange in Spin-Labeled Proteins Using Saturation Recovery EPR

The function of many proteins involves equilibria between conformational substates, and to elucidate mechanisms of function it is essential to have experimental tools to detect the presence of conformational substates and to determine the time scale of exchange between them. Site-directed spin labeling (SDSL) has the potential to serve this purpose. In proteins containing a nitroxide side chain (R1), multicomponent electron paramagnetic resonance (EPR) spectra can arise either from equilibria involving different conformational substates or rotamers of R1. To employ SDSL to uniquely identify conformational equilibria, it is thus essential to distinguish between these origins of multicomponent spectra. Here we show that this is possible based on the time scale for exchange of the nitroxide between distinct environments that give rise to multicomponent EPR spectra; rotamer exchange for R1 lies in the ≈0.1-1 μs range, while conformational exchange is at least an order of magnitude slower. The time scales of exchange events are determined by saturation recovery EPR, and in favorable cases, the exchange rate constants between substates with lifetimes of approximately 1-70 μs can be estimated by the approach.

Electron spin relaxation rates for semiquinones between 25 and 295K in glass-forming solvents

Electron spin lattice relaxation rates for five semiquinones (2,5-di-t-butyl-1,4-benzosemiquinone, 2,5-di-t-amyl-1,4-benzosemiquinone, 2,5-di-phenyl-1,4-benzosemiquinone, 2,6-di-t-butyl-1,4-benzosemiquinone, tetrahydroxy-1,4-benzosemiquione) were studied by long-pulse saturation recovery EPR in 1:4 glycerol:ethanol, 1:1 glycerol:ethanol, and triethanolamine between 25 and 295K. Although the dominant process changes with temperature, relaxation rates vary smoothly with temperature, even near the glass transition temperatures, and could be modeled as the sum of contributions that have the temperature dependence that is predicted for the direct, Raman, local mode and tumbling-dependent processes. At 85K, which is in a temperature range where the Raman process dominates, relaxation rates along the g(xx) (g approximately 2.006) and g(yy) (g approximately 2.005) axes are about 2.7-1.5 times faster than along the g(zz) axis (g=2.0023). In highly viscous triethanolamine, contributions from tumbling-dependent processes are negligible. At temperatures above 100K relaxation rates in triethanolamine are unchanged between X-band (9.5GHz) and Q-band (34GHz), so the process that dominates in this temperature interval was assigned as a local mode rather than a thermally activated process. Because the largest proton hyperfine couplings are only 2.2G, spin rotation makes a larger contribution than tumbling-dependent modulation of hyperfine anisotropy. Since g anisotropy is small, tumbling-dependent modulation of g anisotropy makes a smaller contribution than spin rotation at X-band. Although there was negligible impact of methyl rotation on T(1), rotation of t-butyl or t-amyl methyl groups enhances spin echo dephasing between 85 and 150K.