Pulsed EPR Spectra Research Articles

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor-Acceptor Systems.

Charge transfer (CT) in molecular electron donor-acceptor systems is pivotal for artificial photosynthesis, photocatalysis, photovoltaics and fundamental photochemistry. We summarized the recent development in study of CT and discussed its application in thermally activated delayed fluorescence (TADF) emitters. The direct experimental proof of the spin multiplicity of the charge separated (CS) state with pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy was discussed. Experimental determination of the electron exchange energy (J) of the CS state, with magnetic field effect on its yield or lifetime was introduced. The electron spin transfer accompanying the CT, studied with pulsed EPR spectra was briefly discussed. Tuning of the CT yield and kinetics with selective vibration excitation of the linker (the bridge) with IR pulse was presented. Above all, these studies show that there are more fun than simply monitoring the formation of the cations and anions and the kinetics or CS yields in this area.

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor‐Acceptor Systems

AbstractCharge transfer (CT) in molecular electron donor‐acceptor systems is pivotal for artificial photosynthesis, photocatalysis, photovoltaics and fundamental photochemistry. We summarized the recent development in study of CT and discussed its application in thermally activated delayed fluorescence (TADF) emitters. The direct experimental proof of the spin multiplicity of the charge separated (CS) state with pulsed laser excited time‐resolved electron paramagnetic resonance (TREPR) spectroscopy was discussed. Experimental determination of the electron exchange energy (J) of the CS state, with magnetic field effect on its yield or lifetime was introduced. The electron spin transfer accompanying the CT, studied with pulsed EPR spectra was briefly discussed. Tuning of the CT yield and kinetics with selective vibration excitation of the linker (the bridge) with IR pulse was presented. Above all, these studies show that there are more fun than simply monitoring the formation of the cations and anions and the kinetics or CS yields in this area.

Addressing Ligand-Based Redox in Molybdenum-Dependent Methionine Sulfoxide Reductase.

A combination of pulsed EPR, CW EPR, and X-ray absorption spectroscopies has been employed to probe the geometric and electronic structure of the E. coli periplasmic molybdenum-dependent methionine sulfoxide reductase (MsrP). 17O and 1H pulsed EPR spectra show that the as-isolated Mo(V) enzyme form does not possess an exchangeable H2O/OH- ligand bound to Mo as found in the sulfite oxidizing enzymes of the same family. The nature of the unusual CW EPR spectrum has been re-evaluated in light of new data on the MsrP-N45R variant and related small-molecule analogues of the active site. These data point to a novel "thiol-blocked" [(PDT)MoVO(SCys)(thiolate)]- structure, which is supported by new EXAFS data. We discuss these new results in the context of ligand-based and metal-based redox chemistry in the enzymatic oxygen atom transfer reaction.

Site-Directed Spin-Label EPR Spectroscopy of M2 Protein in Lipid Bilayers of Different Curvature Generation Propensity

M2 is a homotetrameric membrane protein critical to generation of membrane curvature in the budding stage of the influenza A virus life cycle. We have designed and interpreted site-directed spin-label electron paramagnetic resonance spectroscopy (SDSL-EPR) experiments on the conformation and dynamics of M2 in lipid bilayers of different propensities for membrane curvature generation. We have obtained data for full length M2 protein spin labelled at multiple sites in the C-terminal juxtamembrane region, which forms a membrane associated amphipathic helix. Continuous wave (CW) and pulsed EPR spectra provide evidence that M2 adopts two conformational states in bilayers, and that the propensity of lipids to form membranes of differing curvature dictates the relative population of these states. Multicomponent simulation of CW-EPR spectra further show lipid bilayers of the highly curved cubic phases stabilize an immobilized conformation of M2, hypothesized to be relevant for curvature generation. We have also characterized the size distribution of large unilamellar vesicles of lipids of different curvature generation propensities alone and in the presence of full-length M2 using dynamic light scattering (DLS).

Site Directed Spin Label EPR Spectroscopy of Influenza a M2 Protein

M2 is a membrane protein critical to the life cycle of influenza A. We have capitalized on the expanding body of biochemical and high-resolution structural data available for the M2 protein to design and interpret site-directed spin labeling electron paramagnetic resonance spectroscopy (SDSL-EPR) experiments on the conformation and dynamics of the homotetrameric M2 protein embedded in lipid bilayers. We have obtained data for three different M2 constructs (M2TM 22-46, M2TMC 23-60 and full length M2 protein) spin-labeled at multiple sites within the transmembrane and C-terminal domains. CW and pulsed EPR spectra show evidence that M2 adopts multiple conformational states in bilayers, and that cholesterol content dictates the relative populations of the states. Furthermore comparison of full-length M2 protein and a M2TMC 23-60 peptide demonstrates that the C-terminal juxtamembrane region in both constructs forms an amphipathic membrane surface helix.

Probing the Conformation and Dynamics of Influenza a M2 Protein using Site-Directed Spin-Label EPR Spectroscopy

M2 is a membrane protein critical to the life cycle of influenza A. We have capitalized on the expanding body of high-resolution structural data available for the 97 amino acid M2 protein to design and interpret site-directed spin labeling electron paramagnetic resonance spectroscopy (SDSL-EPR) experiments on the conformation and dynamics of the homotetrameric M2 protein embedded in lipid bilayers. We have obtained data for three different M2 constructs (M2TM 22-46, M2TMC 23-60 and full length M2 protein) spin-labeled at multiple sites within the transmembrane and C-terminal domains. CW and pulsed EPR spectra show evidence that M2 adopts multiple conformational states in bilayers, and that cholesterol content dictates the relative populations of the states.

Spin dynamics of paramagnetic centers with anisotropic g tensor and spin of 1/2

The influence of g tensor anisotropy on spin dynamics of paramagnetic centers having real or effective spin of 1/2 is studied. The g anisotropy affects both the excitation and the detection of EPR signals, producing noticeable differences between conventional continuous-wave (cw) EPR and pulsed EPR spectra. The magnitudes and directions of the spin and magnetic moment vectors are generally not proportional to each other, but are related to each other through the g tensor. The equilibrium magnetic moment direction is generally parallel to neither the magnetic field nor the spin quantization axis due to the g anisotropy. After excitation with short microwave pulses, the spin vector precesses around its quantization axis, in a plane that is generally not perpendicular to the applied magnetic field. Paradoxically, the magnetic moment vector precesses around its equilibrium direction in a plane exactly perpendicular to the external magnetic field. In the general case, the oscillating part of the magnetic moment is elliptically polarized and the direction of precession is determined by the sign of the g tensor determinant (g tensor signature). Conventional pulsed and cw EPR spectrometers do not allow determination of the g tensor signature or the ellipticity of the magnetic moment trajectory. It is generally impossible to set a uniform spin turning angle for simple pulses in an unoriented or ‘powder’ sample when g tensor anisotropy is significant.

Radical formation in cytochrome c oxidase

The formation of radicals in bovine cytochrome c oxidase (bC cO), during the O 2 redox chemistry and proton translocation, is an unresolved controversial issue. To determine if radicals are formed in the catalytic reaction of bC cO under single turnover conditions, the reaction of O 2 with the enzyme, reduced by either ascorbate or dithionite, was initiated in a custom-built rapid freeze quenching (RFQ) device and the products were trapped at 77 K at reaction times ranging from 50 μs to 6 ms. Additional samples were hand mixed to attain multiple turnover conditions and quenched with a reaction time of minutes. X-band (9 GHz) continuous wave electron paramagnetic resonance (CW-EPR) spectra of the reaction products revealed the formation of a narrow radical with both reductants. D-band (130 GHz) pulsed EPR spectra allowed for the determination of the g-tensor principal values and revealed that when ascorbate was used as the reductant the dominant radical species was localized on the ascorbyl moiety, and when dithionite was used as the reductant the radical was the SO 2 − ion. When the contributions from the reductants are subtracted from the spectra, no evidence for a protein-based radical could be found in the reaction of O 2 with reduced bC cO. As a surrogate for radicals formed on reaction intermediates, the reaction of hydrogen peroxide (H 2O 2) with oxidized bC cO was studied at pH 6 and pH 8 by trapping the products at 50 μs with the RFQ device to determine the initial reaction events. For comparison, radicals formed after several minutes of incubation were also examined, and X-band and D-band analysis led to the identification of radicals on Tyr-244 and Tyr-129. In the RFQ measurements, a peroxyl (R O O ) species was formed, presumably by the reaction between O 2 and an amino acid-based radical. It is postulated that Tyr-129 may play a central role as a proton loading site during proton translocation by ejecting a proton upon formation of the radical species and then becoming reprotonated during its reduction via a chain of three water molecules originating from the region of the propionate groups of heme a 3. This article is part of a Special Issue entitled: “Allosteric cooperativity in respiratory proteins”.

Site-Directed EPR-Studies on the Effect of Bilayer Cholesterol Content on M2 Conformational Equilibrium

M2 is a homotetrameric membrane protein of influenza A that plays multiple roles in viral replication. Structural studies have shown that M2 conformation is dependent on the hydrophobic environment. In vivo influenza A membranes contain up to 40% cholesterol. Using side directed spin-labeling EPR, we studied the effect of bilayer cholesterol content on the conformation of a 38-residue M2 peptide spanning the transmembrane region and its C-terminal extension. CW and pulsed EPR spectra show evidence that M2 adopts multiple conformational states in bilayers, and that cholesterol content dictates the relative populations of the states.

Ammoniated Electrons Stabilized at the Surface of MgO

The reaction of excess electrons at the surface of MgO with ammonia leads to surface ammoniated electrons analogous to those formed when alkali metals are dissolved in anhydrous ammonia. Surface excess electrons are found to be solvated by up to three ammonia molecules, and well-resolved CW and pulsed EPR spectra allow for a precise description of the unpaired electron spin density distribution over the solvent molecules. The large majority of the electron spin density resides in the first-shell nitrogen fragments. HYSCORE spectra allow obtaining for the first time the full hyperfine interaction of the solvated electron with the ammonia protons, which is consistent with a small and negative spin density in the (1)H 1s orbital. Furthermore, the hyperfine and nuclear quadrupole tenors of the second-shell nitrogens could be unravelled.

XSophe: An integrated molecular approach to the analysis of continuous wave and pulsed EPR spectra from metalloprotein

XSophe: An integrated molecular approach to the analysis of continuous wave and pulsed EPR spectra from metalloprotein

Direct Detection of the Proton-Containing Group Coordinated to Mo(V) in the High pH Form of Chicken Liver Sulfite Oxidase by Refocused Primary ESEEM Spectroscopy: Structural and Mechanistic Implications

A refocused primary electron spin−echo envelope modulation (RP ESEEM) technique and an adjustable frequency S/C-band pulsed EPR spectrometer have been used to produce ESEEM spectra with the lines due to nearby protons being greatly enhanced relative to those due to distant matrix protons. Application of this technique to the high pH (hpH) form of the Mo(V) center of sulfite oxidase has enabled nearby protons to be directly detected for the first time. Simulation of the RP ESEEM spectrum of the hpH form suggests the presence of two nearby protons that have distributed hyperfine interactions (hfi); these protons are ascribed to a MoV−OH group with strong H-bonding interactions to other nearby proton donors or to the presence of a coordinated H2O ligand. The RP ESEEM technique promises to be widely applicable to the investigation of mutant forms of SO with altered Mo centers and paramagnetic centers in other metalloproteins where a nearby proton of interest is often masked by much more numerous distant protons and where high spectral resolution is not required. The distinctive differences in the CW and pulsed EPR spectra of the lpH and hpH forms are proposed to result from differences of the MoV−OH torsional angle and variations in the H-bonding interactions, which control the orientation of the MoV−OH proton(s) relative to the half-filled 4dxy orbital. The large isotropic hfi for the lpH form is suggested to result from an intramolecular MoV−OH···Scys hydrogen bonding interaction that places the proton of the MoV−OH group in the equatorial plane of the square pyramidal oxo-Mo(V) center of SO.

Triplet States of Oligothiophenes and Electron Transfer to C60: A Pulsed EPR Investigation

Abstract Triplet states of oligothiophenes and C60 were excited by a pulsed laser light using a Nd:YAG laser. The finestructure values D, E of different endcapped oligothiophenes (EC3T, EC4T, and ECST) were determined from pulsed EPR-spectra. Time resolution was applied to study the kinetics and relaxation times. A photoinduced electron transfer from EC4T to C60 in liquid solutions was investigated in detail. Fourier transform of the free induction decays results in spectra of the transient triplet state and of the transient C60 monoradical anion. Electron transfer rates were deduced from the observed triplet quenching rates at different temperatures.