Yoshio Kodera

Distance between tyrosines Z+ and D+ in plant Photosystem II as determined by pulsed EPR

Abstract A ‘2 + 1’ electron spin echo method was applied to estimate the dipole interaction between tyrosines D + and Z + in Mn-depleted preparations of plant Photosystem II. The value of dipole interaction obtained corresponds to the distance of approx. 30 A between the two types of tyrosine radical.

An electron spin echo envelope modulation study of the primary acceptor quinone in Zn‐substituted plant photosystem II

A Fe2+ ion on the acceptor side of plant photosystem II has been substituted by Zn2+ and an anion radical of the primary acceptor quinone, Q−A, has been studied by electron spin echo method. The electron spin echo modulation shows the interaction of the unpaired electron of Q−A with nitrogen nuclei of the histidine and, probably, alanine residues situated nearby. The comparison of the modulation spectra of Q−A with those of the anion radical of plastoquinone‐9 stabilized in protonated and deuterated isopropanol matrices allows one to distinguish between the spectrum lines due to the quinone protons and due to the protons of other molecules that form hydrogen bonds with the oxygen atoms of the quinones.

EPR STUDY OF TRAPPED TYROSINE Z+ IN CA-DEPLETED PHOTOSYSTEM II

Abstract The dependence of the light-induced transient EPR signal intensity of Y Z + on the illumination temperature was measured in Ca-depleted PS II in the presence of DCMU. The maximum yield of Y Z + , about 70% of Y D + EPR intensity, was obtained by illumination at 245 K, and trapped by immediate freezing in liquid nitrogen. Analysis of the relative intensities of Y Z + and modified multiline EPR signals under various illumination periods and temperatures shows that Y Z + has been trapped in the S 1 state of the oxygen evolving center, and the trapped Y Z + EPR converted to the multiline signal through the advancement from the S 1 to S 2 state in the dark at 273 K. Spin-lattice relaxation times of Y D + and Y Z + in the Ca-depleted and Mn-depleted PS II were measured by pulsed EPR. From the relaxation enhancements of these radicals due to the dipole interaction with the Mn cluster, the distance from Y Z + to the Mn cluster was estimated to be 15–20 A. Using the 2 + 1 ESE method, the distance between Y Z + and Y D + was determined to be 29–30 A.

Distance of P680 from the manganese complex in Photosystem II studied by time resolved EPR

The flash-induced transient EPR signal of P680+ was observed in PS II membranes at temperatures between 77 and 220 K. Below 180 K, the half decay time, t12, of P680+ transient signal was about 2 ms and re-reduction of P680+ has been ascribed to the back reaction with QA. Above 200 K, the decay rate increased with an activation energy of 15.6 kJ/mol. These results were coincident with previous optical data of P680+. Spin-lattice relaxation rates, 1/T1+ of P680+ and Tyr.D+ EPR signals, were determined by microwave power saturation experiments and by T2 measurements using a pulsed EPR in the S1 and S2 states of oxygen-evolving complex (OEC) at 90 K. The ratio of distances of both radical species from the manganese cluster in OEC was derived from the difference of T1 values in the S1 and S2 states. The result has shown that the distance from OEC to P680 was 0.91 times as far as that to Tyr.D. On the basis of the temperature-dependence of the half saturation power P12 of Tyr.D+ in the S1QAFe2+ and S2QAFe2+ states we estimated that the distance from Tyr.D to the Mn-cluster in OEC to be 24–27 A. The distance from R680 to the manganese cluster was estimated to be 21–26 A. The distance of Tyr.D from QAFe2+ was supposed to be 26–33 A, from 1/T1 values in QAFe2+ and QAFe2+ at 90 K.

Distances from tyrosine D+ to the manganese cluster and the acceptor iron in Photosystem II as determined by selective hole burning in EPR spectra

Abstract Dipolar interactions of tyrosine D + (Y + D ) radical with the Mn-cluster in oxygen-evolving center (OEC) and with Fe 2+ ion on the acceptor side of Photosystem II was studied by a novel electron spin echo method employing selective hole burning in EPR spectra with subsequent detection of the hole broadening. The mechanism of hole broadening is analyzed by fluctuation of the local dipolar field at the site of Y + D caused by a random spin flip of these transition metal ions. Experimental data was analyzed using a simple theory (Dzuba, S.A., Kodera, Y., Hara, H. and Kawamori, A. (1993) J. Magn. Res. A 102, 257–260). Based on this theory, the distance from Y + D to Fe 2+ ion has been determined to be less than 52 A in Mn-depleted PS II. The distance from Y + D to the spin center of the Mn-cluster in OEC was estimated to be 28 A in S 2 state and 30 A in S 1 state. Furthermore, the distribution of the distance was estimated to be in the range 4–6 A.

The EPR dipolar broadening of irradiated solids as detected using FID and ESE following selective hole burning

A novel electron spin echo method employing detection of FID and transient echo shape following selective hole burning in the EPR spectrum has been applied to study γ-irradiated malonic acid. The dipolar lineshapes of stabilized free radicals are separated into bulk and pairwise contributions; both of them have proved to be Lorentzian.

Dipolar interactions of spin probes in organic glasses studied by electron spin echo

Abstract A novel electron spin echo method, employing selective hole-burning in EPR spectra with subsequent detection of free induction decay and echo temporal shape, has been applied to study dipole-dipole interactions of TEMPO nitroxides dissolved in glassy ethanol and water-glycerol mixture. The results obtained suggest clustering of guest molecules in these matrices.