An investigation of the light-induced spin polarization in reaction centres 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 sphaeroides Y are presented. A strong component with a spectral width of similar to 0.4 mT appears near g = 2 and a weak component with a width of similar to 570 mT is observed between g approximate to 1 - 6. The narrow spectrum is primarily due to P+ and the broad component is assigned 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+)). At times < 200 ns after the laser the P+ contribution has an AEA (A = absorption, E = emission) polarization pattern at both X- and K-band, The K-band spectrum has a somewhat higher spectral resolution and three distinct time constants in the polarization decay. At later times, the initial AEA pattern relaxes to a purely absorptive spectrum. The decay of the spin polarized (Q(A)(-.)Mn(2+)) signal is about two orders of magnitude slower than that of P-854(+.). However, its rise time is roughly the same as the decay of the AEA pattern near g = 2. Both contributions to the spectra can be observed up to temperatures above 200 K with little or no change in form. However, the transition from the early to late signal near g = 2 is faster at higher temperatures as a result of faster spin relaxation. The dominant relaxation rate follows Arrhenius behaviour. However, the fast component observed at K-band is independent of temperature. The effect of the magnetic properties of the 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 discussed.