Temperature Dependence of the Primary Donor Triplet State g-Tensor in Photosynthetic Reaction Centers of Rhodobacter sphaeroides R-26 Observed by Transient 240 GHz Electron Paramagnetic Resonance

We report time-resolved 240 GHz EPR spectra of the primary donor triplet state 3P from photosynthetic reaction centers of Rhodobacter sphaeroides R26.1 as a function of temperature in the range 10−230 K. The data allow the determination of the principal g-tensor values and the principal axes directions of the 3P g-tensor with respect to its zero-field axes. The g-tensor measured at 240 GHz differs appreciably from previous measurements of 3P at lower frequencies and also differs from that of the cation radical state P+, which has previously been characterized at high frequencies. In contrast to P+, the 3P state exhibits significant temperature dependence in its g-tensor, particularly in the directions of the principal axes, which appear to rotate by about 30° around the x principal axis over the temperature range studied. The 3P yield anisotropy first observed by Boxer and co-workers at high field using photoselection methods is also evident in the high-field EPR spectrum as a significant variation of intensity across the spectrum. This variation is analyzed in terms of two models. The first model explicitly includes the evolution of the precursor radical pair but leads to significant ambiguities in the assignment of radical pair structural parameters. The second model utilizes only two ad hoc parameters to account for the yield anisotropy, which greatly reduces fitting parameter correlations and improves the accuracy and reliability with which the 3P magnetic parameters are determined.