Electron-spin polarization in the donor triplet state of bacterial photosynthetic reaction centers. I. Microsecond time-resolved EPR of Rhodopseudomonas viridis reaction centers

Abstract

The EPR transients in the three canonical transitions of the Δm = ± 1 triplet spectrum of Rhodopseudomonas viridis RC's have been measured following a 10 ns laser-flash. At microsecond time-resolution, the initial amplitude of these transitions at 100 K is inverted for the Y peaks, zero for the Z peaks, and not affected (thus firmly emissively polarized) for the X peaks as compared to the initial amplitudes at T < 10 K. This demonstrates that temperature-dependent processes in the precursor state P F affecting the populating rates of the sublevels of the primary donor triplet state (P R) are responsible for the temperature-dependent changes of the electron spin polarization (ESP) pattern. The P R decay rate constants after correcting for spin lattice relaxation (SLR) contributions for all three canonical axes do not change with temperature from 4 K to 100 K. The observed increase in decay rate constants is explained invoking a relaxation mechanism probably operating via P F, or a relaxed form of it. At T>40 K the SLR process does not affect the observed decay rate constants anymore. It is proposed that above 40 K the initial ESP (i.e., at approx. 1 μs after the laser flash) and the steady-state ESP approach each other, reducing the effect of the relaxation process. Creation of P F from P R, which is populated by TT energy transfer from the antenna triplet, is proposed as a mechanism explaining the observed spin relaxation and polarization of P R.