S.A. Dikanov

Parameters of quadrupole coupling of 14N nuclei in chlorophyll a cations determined by the electron spin echo method

Modulation effects induced by interaction of an unpaired electron with 14N nuclei of pyrrole rings in the electron spin echo from chlorophyll a radical cations have been investigated. The modulation frequencies have been determined by Fourier analysis of the echo signal decay. The modulation frequencies approximate to the quadrupole resonance frequencies of 14N nuclei.

Ratio analysis of electron spin echo modulation envelopes in disordered matrices and application to the structure of solvated electrons in 2-methyltetrahydrofuran glass

A new analysis for two pulse and three pulse electron spin echo modulation data is presented which is independent of the echo decay function. The experimental ratios of maxima to minima of the echo modulation curve are compared with simulated ones to obtain the number of interacting nuclei, their distances, and their isotropic coupling by a least squares fit. From the best simulation, the empirical echo decay function can be determined and the original experimental data can be simulated. This new method is applied to new three pulse and old two pulse [L. Kevan et al., J. Chem. Phys. 63, 409 (1975)] electron spin echo data on solvated electrons in 2‐methyltetrahydrofuran (MTHF) glass at 77 K. This analysis together with previous second moment data gives a revised model for electron solvation in this matrix based on a statistical orientation of the MTHF ring carbon planes toward the electron. In this statistical configuration model three equivalent MTHF molecules form the first solvation shell with their ri...

An ESEEM study of the oxidized electron donor of plant photosystem II:evidence that D^ is a neutral tyrosine radical

Abstract Electron spin-echo envelope modulation (ESEEM) has been performed on the tyrosyl radical D in photosystem II of plants and algae. This radical was investigated in PS II-enriched subchloroplast particles, and in perdeuterated and 15N-substituted Chlorella vulgaris algae. From the ESEEM analysis in the time and frequency domain, two proton hyperfine interactions were determined. One of these protons has hyperfine parameters aiso=27.2 MHz, T11=−3.1 MHz and T22/T11=0.5. The second proton, which is easily exchangeable by deuterium, has parameters aiso=±1.6 MHz, T11=∓8.4 MHz and T22/T11=1. The hyperfine parameters of the first proton are characteristic of a β-methylene proton, whereas those of the second indicate that it is a proton hydrogen bonded to the tyrosyl oxygen. We conclude that the oxidized tyrosyl donor is a neutral hydrogen-bonded radical.

Comparative modulation analysis of electron spin echo signals from oxidized chlorophyll a in vitro and P 700 centres in chloroplasts

Abstract A comparative analysis of electron spin echo modulation effects from 14 N nuclei of pyrrole rings for Chla + and P 700 + has been performed, yielding parameters of quadrupole and isotropic hyperfine couplings with nitrogen nuclei in these paramagnetic centres.

Spectrometer dead time: effect on electron spin echo modulation spectra in disordered systems

Abstract The effect of spectrometer dead time on line shapes in ESE modulation spectra for disordered systems with S = 1 2 , I = 1 2 has been studied by means of numerical calculations.

Electron spin echo envelope modulation spectroscopy (ESEEM) of the radical cations of 14N and 15N bacteriochlorophyll a

Abstract An electron spin echo envelope modulation frequency analysis is performed of the 14 N and 15 N bacteriochlorophyll a + cation in solid solution. Hyperfine couplings of 2.0 and 3.1 MHz are derived which correspond to the perpendicular component of the axial hyperfine coupling tensor. Seven quadrupole frequencies were measured, six of which group in two sets of three zero-field quadrupole splittings.

Elimination of the matrix contribution to electron spin echo modulation

Abstract A method is proposed permitting the determination of the hfi parameters of nuclei in the nearest surroundings of a paramagnetic center by analysis of ESE modulation spectra, and an estimate of the number of these nuclei by analysis of ESE modulation envelopes in preliminary processing by appropriate frequency filters. The method is illustrated by the study of CH 2 OD radicals in a glassy CH 3 OD matrix.

Electron spin-echo studies of weak hyperfine interactions with ligands in some VO2+ complexes in frozen glassy solutions

Abstract Methods of envelope modulation function calculations in the case of paramagnetic centers having an axially symmetric g factor are described. Parameters of the hyperfine interactions between unpaired electrons of VO2+ ions and molecular deuterons have been obtained from the analysis of envelope modulation patterns for the two- and three-pulse electron spin echo of VO2+ complexes with D2O, CH3OD, and CD3OH. On this basis structural parameters of the first coordination sphere of VO2+ complexes with water and methyl alcohol have been determined.

Dependence of electron spin-echo modulation amplitude on the microwave field intensity

Abstract An analytical expression describing the dependence of the amplitude of ESE modulation on the intensity of the microwave field has been derived and verified experimentally.

Very high frequency (135 GHz) EPR of the oxidized primary donor of the photosynthetic bacteria Rb. sphaeroides R-26 and Rps. viridis and of Y . D , (signal II) of plant photosystem II

mm EPR spectroscopy reveals significant differences between the g-tensors of the oxidized primary donor in reaction centers of the photosynthetic bacteria Rb. sphaeroides R-26 and Rps. viridis: the principal values are g, = 2.00159, gw = 2.00224 and g, = 2.00402, and g, = 2.00180, gw = 2.00235 and g, = 2.00380, respectively. Similarly, the g-tensors of the Yi tyrosyl radical in the D, subunit of the reaction center of plant photosystem I1 and of oxidized tyrosine in vitro are found to be quite different, with principal values of g, = 2.00212, gV = 2.00426 and g, = 2.00752, and g,, = 2.00181, gw = 2.00381 and g, = 2.00613, respectively. The observed differences probably reflect differences in the molecular structure of the radicals investigated. In this communication we report on the amxlication of 2 mm (135 GHz) EPR spectroscopy on a number of photosynthetic preparations with the aim to determine the principal values of the g-tensor of some co-factors: the primary donor of two purple bacteria and the tyrosyl donor Y; located on the D, subunit of the reaction center of plant photosystem 11. The much enhanced resolution of 2 mm EPR allows such a determination even for protonated, randomly-oriented preparations. It is shown that the two purple bacteria differ considerably in the g-anisotropy of their primary donors, while the g-anisotropy of the YL radical is appreciately different from that of oxidized tyrosyl in vitro. Since these differences most probably reflect differences in the detailed molecular structure of the co-factors, it is hoped that our data will be an incentive for correlating theoretically the observed g-anisotropies and the co-factor structure with the aid of the recently resolved crystal structure of the reaction centers (the purple bacteria5s6i7) or the models based on these resolved structures (plant photosystem IIh9).