High-frequency EPR approach to the electron spin-polarization effects observed in the photosynthetic reaction centers

Time-resolved high-frequency electron paramagnetic resonance (EPR) spectroscopy was applied to study the structure and dynamics of the electron transfer pathways in the photosynthetic RC proteins. When the spin-polarized EPR spectra are recorded at the high field, the singlet-triplet mixing in the radical pairs becomes faster due to the increase of Zeeman interaction, and a sequential electron transfer polarization model, which includes both the primary and secondary radical pairs, should be considered. Application of the sequential electron transfer polarization model for the interpretation of the bacterial RC proteins with a “slow” electron transfer rate reveals the importance of the protein dynamics. It was shown that the reorganization energy for the electron transfer process between P ₈₆₅ ⁺ H^?Q_A and P ₈₆₅ ⁺ HQ _A ^? , but not the change in the structure of the donor-acceptor complex, is a dominant factor that alters the electron transfer rate. The relaxation data, obtained in the delay after laser flash experiment, have been used to estimate the magnetic interaction in the weakly coupled radical pair. High-frequency spin-polarized EPR spectra allow the quantitative characterization of isotopically labeled quinone exchange in the PS I reaction center proteins.     

Nuclear coherences in photosynthetic reaction centers following light excitation

Transient electron paramagnetic resonance is used to study the secondary radical pair in plant photosystem I. Nuclear coherences are observed in the transverse magnetization at lower temperatures following light excitation. Comparative studies of deuterated and deuterated¹⁵N-substituted cyanobacteriaS. lividus indicate assignment of these coherences to nitrogen nuclei in the primary donor and deuterons in the secondary acceptor. The modulation amplitude of a deuteron matrix line, as a function of the microwave power, reveals a distinct resonance behavior. The maximum amplitude is obtained when the Rabi frequency equals the nuclear Zeeman frequency.     

Spin-labeled photosynthetic reaction centers fromRhodobacter sphaeroides studied by electron paramagnetic resonance spectroscopy and molecular dynamics simulations

A new strategy has been applied that combines molecular dynamics (MD) simulations and electron paramagnetic resonance (EPR) spectroscopy to study the structure and conformational dynamics of the spin-labeled photosynthetic reaction center (RC) ofRhodobacter sphaeroides. This protein serves here as a model system to demonstrate the applicability of this new methodology. The RC contains five native cysteines and EPR experiments show that only one cysteine, located on the H subunit, is accessible for spin labeling. The EPR spectra calculated from MD simulation trajectories of spin labels bound to the native cysteines C156 and C234 in subunit H reveal that only the spin label side chain at position 156 provides a spectrum which agrees with the experimental EPR spectrum.     

High-field EPR, ENDOR and ELDOR on bacterial photosynthetic reaction centers

We report on recent 95 and 360 GHz high-field electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and pulsed electron-electron double resonance (PELDOR) studies of wild-type and mutant reaction centers (RCs) from the photosynthetic bacteriumRhodobacter sphaeroides. Taking advantage of the excellent spectral and temporal resolution of EPR at 95 and 360 GHz, the electron-transfer (ET) cofactors radical ions and spin-correlated radical pairs were characterized by theirg- and hyperfine-tensor components, their anisotropicT ₂ relaxation as well as by the dipolar interaction between P ₈₆₅ ^?+ Q _A ^?? radical pairs. The goal of these studies is to better understand the dominant factors determining the specificity and directionality of transmembrane ET processes in photosynthetic RC proteins. In particular, our multifrequency experiments elucidate the subtle cofactor-protein interactions, which are essential for fine-tuning the ET characteristics, e.g., the unidirectionality of the light-induced ET pathways along the A branch of the RC protein. By our high-field techniques, frozen-solution RCs of novel site-specific single and double mutants ofR. sphaeroides were studied to modulate the ET characteristics, e.g., even to the extent that dominant B branch ET prevails. The presented multifrequency EPR work culminates in first 360 GHz ENDOR results from organic nitroxide radicals as well as in first 95 GHz high-field PELDOR results from orientationally selected spin-polarized radical pairs P ₈₆₅ ^?+ Q _A ^?? , which allow to determine the full geometrical structure of the pairs even in frozen-solution RCs.     

Influence of the concentration,temperature and electric field intensity on the electron mobility in n-doped zinc sulphide

We have calculated the steady state electron mobility in n-doped zinc sulphide (in WZ and ZB phases), driven far away from equilibrium by an electric field. The dependence of the electron mobility (which depends on the nonequilibrium thermodynamic state of the sample) on the concentration, temperature and electric field strength was obtained and analyzed.     

Kinetics of reduction of bacteriochlorophyll dimer in reaction centers of photosynthetic bacteria

Kinetic curves of reduction of the oxidized bacteriochlorophyll dimer by the proximal heme of cytochrome in seven mutated reaction centers from Rps. viridis was measured in the temperature range of 295-10 K under an ambient potential when two high-potential hemes were reduced. The data are analyzed in frames of a model which accounts for slow medium dynamics (the diffusion-reaction equation) and the quantum effect in the modes responsible for reorganization of the cofactors and the medium. The model reproduces the observed nonexponential kinetics down to the lowest temperature where the fast kinetic component still survives. The modeling results in determination of an average characteristic time for re-orientation of dipoles of the protein matrix as a function of temperature. At temperatures above the glass-transition temperature, this function shows Arrhenius behavior with the activation energy varying between the values of 0.5 eV for wild type Rps. viridis and 0 eV for Rps. sulfoviridis found previously. Possible reasons for disappearance of the fast kinetic component in some reaction centers under study at low temperatures are discussed.     

Radiation of an electron in an electric field

By the methods of classical theory, the motion of an electron in the field of a longitudinal electric wave is analyzed. This field may be taken as a model of the accelerating field at the axis of a linear accelerator. It was shown that the radiation of electrons in this field is analogous to the radiation in a constant homogeneous electric field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 96–99, May, 1976.The authors are sincerely grateful to Prof. V. A. Bagrov for formulating the problem and for his constant interest in the work.     

Influence of electric field on weak localization

The influence of an electric field on weak localization is investigated. The conductance of thin Au and Ag films is measured for different applied electric fields. The electronic temperature of the conduction electrons is directly measured by the magneto-conductance of the films. The electric field does not alter the conductance and discriminates between the theories by Altshuler et al. and Tsuzuki.     

A D-band (130 GHz) EPR study of the primary electron donor triplet state in photosynthetic reaction centers ofRhodobacter sphaeroides R26

A high-field (D-band, 130 GHz) electron spin echo-detected spectrum of the primary electron donor triplet state,³P, in quinone-depleted photosynthetic reaction centers from the bacteriumRhodobacter sphaeroides R26 is obtained. It shows a significantg-anisotropy, which is larger than that of the primary donor oxidized state, P^+?. Simulation gives the tripletg-tensor principal values of 2.0037, 2.0028, and 2.0022 (precision ±0.0001), assuming that theg-tensor is coaxial to a zerofield splitting tensor. The³P spectral lineshape reveals an orientational anisotropy of the triplet quantum yield. We explain this anisotropy as arising from the difference in the main values and relative orientations between theg-tensors of P^+? and I _A ^?? in the primary radical pair (the triplet state’s precursor).     

The magnetic field dependence of the electron spin polarization in consecutive spin correlated radical pairs in type I photosynthetic reaction centres

The magnetic field/microwave frequency dependence of the spin polarized EPR spectra of the sequential spin correlated radical pairs P⁺A^? ₁ and P⁺F^? _x in type I photosynthetic reaction centres is investigated. Experimental data are presented for photosystem (PS) I and reaction centres of heliobacteria at × band (9.7 GHz) and K band (24 GHz). In photosystem I at ambient temperatures the lifetime of A ^? ₁ is ~290 ns and both states are observable by transient EPR. In heliobacteria, electron transfer to F_x occurs within ~600 ps and only the state P⁺F^? _x is observed. The experimental data show a net polarization of P⁺ in the state P⁺F^? _x, which displays a clear dependence on the strength of the external field. The net polarization generated in sequential radical pairs is expected to pass through a maximum as a function of the Zeeman energy when the characteristic time of singlet-triplet mixing is comparable with the lifetime of the precursor. In PS I, the precursor lifetime (290ns) is much longer than the characteristic time of singlet-triplet mixing at × band (9 GHz, 3 kG) and K band (24 GHz, 8 kG). As a result, the observable net polarization decreases with the field strength in this region. In contrast, in heliobacteria, the precursor lifetime (600 ps) is much shorter than the characteristic time of singlet-triplet mixing, and the net polarization increases in the same range of Zeeman energies. The polarization patterns in these two systems can be described using the specific limiting cases of a short lived and long lived precursor radical pair and written as a sum of several contributions. The spectra are simulated on this basis using parameters derived entirely from independent experimental data, and good agreement between the experimental polarization patterns is obtained. The calculated polarization patterns are sensitive to spin dynamics on a timescale much shorter than the spectrometer response time, and the expected influence of a 10 ns component in the electron transfer, as observed optically in some PS I, preparations is discussed. No significant influence from such a component is found in the spin polarization patterns of PS I from the cyanobacterium Synechocystis 6803.     

Observation of velocity-independent electron transport in the reversed field pinch

Confinement of runaway electrons has been observed for the first time in a reversed field pinch during improved-confinement plasmas in the Madison Symmetric Torus. Energy-resolved hard-x-ray flux measurements have been used to determine the velocity dependence of the electron diffusion coefficient, utilizing computational solutions of the Fokker-Planck transport equation. With improved-confinement, the fast electron diffusivity drops by 2 orders of magnitude and is independent of velocity. This suggests a change in the transport mechanism away from stochastic magnetic field diffusion.     

Effect of the electric field on electron chemiemission from a semiconductor surface

It is found that, if the energy being released due to the formation of product molecules in the course of a heterogeneous reaction is lower than the electron work function, the current density of electron chemiemission from the surface of any semiconductor exponentially depends on electric field strength E,j = Cexp(ΒE), where C and Β are coefficients. Theoretical results qualitatively agree with experimental data obtained for the heterogeneous recombination of hydrogen atoms on the silicon surface in weak electric fields (0<E<5 x 10⁶V/m).     

Holtzmark electric field distribution in a two-dimensional electron fluid

The Holtzmark distribution for the thermal electric field at a neutral point in a two-dimensional and uncorrelated electron fluid is given by $\text{β(1 + β}{}^2\text{)}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$, with $\text{β =}\text{E}\text{E}{}_0$, and E₀ = πnZ_pe in terms of the density n and perturber Z_pe.     

Influence of an electric field on the magnetoabsorption in a field of resonant laser radiation

The frequency dependence of the coefficient of interband magnetoabsorption of a weak electromagnetic wave propagating in a constant electric field and in a field of resonant laser radiation at a frequency equal to the cyclotron frequency (infrared magnetic resonance) is calculated. The specific features observed in interband absorption of the electromagnetic wave in a uniform electric field are considered for the case in which the frequency of laser radiation is equal to the confinement frequency in a parabolic quantum well (infrared quantum-well resonance).