EPR relaxation measurements of Photosystem II reaction centers: influence of S-state oxidation and temperature

Abstract

The spin-lattice relaxation time of the EPR signal of the tyrosine radical D+ has been measured with electron spin echo spectroscopy in the range 5–30 K in Photosystem II preparations with intact oxygen evolving complex (OEC). The charge storage state of the OEC was set by illumination with a series of flashes and monitored by measuring the multiline EPR signal of the S2-state. The OEC was synchronized to 100% S1 initial state by dark-adaptation and one preflash. Agreeing with previous work (De Groot, A., Plijter, J.J., Evelo, R., Babcock, G.T. and Hoff, A.J. (1986) Biochim. Biophys. Acta 848, 8–15), the spin-lattice relaxation curves were foundto be bi-phasic. The average relaxation time τ¯ of each S-state was calculated from the data obtained for the 0—3 flash sample and the known S-state distribution,τ¯was found to be maximal in the S1-state. It decreased about 40% for the S2-state, was essentially the same for the S2- and the S3-states and decreased again by about 55% for the S0-state. These results are similar to those obtained earlier by cw EPR (Styring, S. and Rutherford, A.W. (1988) Biochemistry 27, 4915–4923). At 5 K the two exponentials describing the relaxation curves had characteristic timesτf, and τs that differed by an order of magnitude. Their amplitude was about equal, except for S0 where the faster process predominated. At 20 K the characteristic time of both the fast and the slow process was reduced by a factor of about five;their amplitudes were again about equal. The observed relaxation times τf and τs were deconvoluted as a function of S-state by an approximate method. At 5 K it was found that τf was about twice as fast for S0 and S3 than for S, and S2 (1.3 vs. 2.6 ms) and τ sabout twice as fast for S0, S2, S3 than for S1 (13.7–14.5 vs. 28.6 ms). The same trend was observed at higher temperatures. Interpreting the results with relaxation enhancement theory and integrating them with the results from cw EPR, NMR and EXAFS spectroscopy the following model for the OEC is presented, (i) To explain the biphasic relaxation of D+ it is suggested that two Mn are close to D+ at different distances, enhance the relaxation of D+and are not magnetically coupled. Their oxidation state differs by 1 unit, is probably Mn3+ and Mn4+, and does not change during the S0 → S3 sequence. It is postulated that at high temperature there is a charge resonance between the two Mn ions that is frozen out when cooling to cryogenic temperature, (ii) Two of the four Mn of the OEC form an antiferromagnetically coupled binuclear cluster in the oxidation state Mn2+·Mn3+, Mn3+·Mn3+, Mn3+·Mn4+, Mn3+·Mn4+ in the S0, S1, S2 and S3-sta respectively, (iii) From the temperature dependence of the relaxation of D+ in the S0-state, it is estimated that the distance between the Mn cluster and D+ is 30–40 .