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 −. Mn2+) in reaction centres of Rhodobacter sphaeroides Y are presented. A strong component with a spectral width of ∼0.4 mT appears near g = 2 and a weak component with a width of ∼570 mT is observed between g ≈ 1 – 6. The narrow spectrum is primarily due to P+. and the broad component is assigned to (Q A −. Mn2+). Both signals are polarized as a result of the correlation of the unpaired spins on P 865 +. and (Q A −. Mn2+). 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 −. Mn2+) 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 −. Mn2+) and P+.(Q A −. Fe2+) are discussed.