The relation between the electron spin polarization of the donor triplet state of the photosynthetic reaction center fromRhodopseudomonas viridisand the redox state of the primary acceptor

Abstract

The relation between the electron spin polarization of the donor triplet state of the photosynthetic reaction center from Rhodopseudomonas viridis and the redox state of the primary acceptor The hypothesis [( 1986) Photobiochem. Photobiophys. 11,95 -1001 that the temperature dependence of the electron spin polarization (ESP) pattern of the Am = k 1 EPR spectrum of the triplet state PR of the Rhodopseudomonas viridis reaction center is caused by magnetic interaction between the reduced menaquinoneiron complex Q-Fe*+ and the electron spin on I-(reduced bacteriopheophytin b), which is part of the radical pair P+I-(P' is the oxidized primary electron donor P960) has been investigated. It was found that the AEAEAE ESP pattern of the EPR spectrum detected at T> 20 K changes into the usual AEEAAE pattern, when Q-Fe*+ is photochemically converted into Q*-Fez+. This demonstrates that the presence of Q-in Q-Fe*+ is a necessary condition to obtain the AEAEAE ESP pattern.

THE ELECTRON SPIN POLARIZATION OF THE DONOR‐TRIPLET STATE IN NATIVE and MODIFIED PHOTOSYNTHETIC REACTION CENTERS FROM

“…When the ubiquinone is replaced by menaquinone, which couples magnetically much stronger to I; than UQ- (Okamura et al, 1979) the observed fraction is even lower: 0.5 at 100 K. Although the ESP pattern of PR for Rb. sphaeroides R-26 does not change with temperature, as previously reported (Van Wijk, 1986b), our present results demonstrate that its F-value does change with temperature.…”

“…sphaeroides R-26 has the same effect as a double reduction of MQ in Rps. viridis (Van Wijk et al, 1986b), as expected.…”

“…In previous papers (Van Wijk et al, 1986a, 1986b we have reported that the temperature dependence of the Y-amplitudes (and -signs) of the steady state triplet EPR spectra of PR can be explained by the Fez+ acting as a relaxation source and the singly reduced primary quinone as a relaxation carrier during the lifetime of PF. The present experiments allow an experimental verification of this model.…”

“…sphaeroides R-26 RC's with an intact acceptor complex is similar to the observations reported earlier for Rps. viridis and C. vinosum (Van Wijk et al, 1985;Van Wijk et al, 1986a;Van Wijk et al,. 1986b).…”

“…However, recently we reported deviating ESP patterns at temperatures above -20 K in RC preparations from Rhodopseudomonas viridis and chromatophores from Chromatium vinosum (Van Wijk et al, 1985;Van Wijk et al, 1986a). We demonstrated that the observed effect depends on the presence of a (singly) reduced primary acceptor (a,) in the native quinone-iron complex (Van Wijk et al, 1986b) indicating that the magnetic coupling between Iand Q z is involved. At T > 20 K TI processes on the high spin Fe2+ (S = 2) in the QzFe2+ complex (Butler et al, 1984) are believed to induce rapid spin transitions on Q-.…”