AN INVESTIGATION OF THE LIGHT-INDUCED SPIN POLARIZATION IN REACTION CENTERS OF RHODOBACTER-SPHAEROIDES-Y

Low temperature spin polarized X- and K-band transient EPR spectra of the state P+.(Q(A)(-.)Mn(2+)) in reaction centres of Rhodobacter sphae roides Y are presented. A strong component with a spectral width of si milar to 0.4 mT appears near g = 2 and a weak component with a width o f similar to 570 mT is observed between g approximate to 1 - 6. The na rrow spectrum is primarily due to P+ and the broad component is assign ed to (Q(A)(-.)Mn(2+)). Both signals are polarized as a result of the correlation of the unpaired spins on P-865(+.) and (Q(A)(-.)Mn(2+)). A t times < 200 ns after the laser the P+ contribution has an AEA (A = a bsorption, E = emission) polarization pattern at both X- and K-band, T he K-band spectrum has a somewhat higher spectral resolution and three distinct time constants in the polarization decay. At later times, th e initial AEA pattern relaxes to a purely absorptive spectrum. The dec ay of the spin polarized (Q(A)(-.)Mn(2+)) signal is about two orders o f magnitude slower than that of P-854(+.). However, its rise time is r oughly the same as the decay of the AEA pattern near g = 2. Both contr ibutions to the spectra can be observed up to temperatures above 200 K with little or no change in form. However, the transition from the ea rly to late signal near g = 2 is faster at higher temperatures as a re sult of faster spin relaxation. The dominant relaxation rate follows A rrhenius behaviour. However, the fast component observed at K-band is independent of temperature. The effect of the magnetic properties of t he metal on the spin polarization and the expected differences between the spectra of P+.(Q(A)(-.)Mn(2+)) and P+.(Q(A)(-.)Fe(2+)) are discus sed.