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.
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