Submillimeter High-Field EPR Studies of the Primary Donor in Plant Photosystem I P700•+

In this present study we report millimeter and submillimeter high-field CW EPR spectra of P700 •+, the primary electron donor in photosystem I. The data shown are the first well-resolved chlorophyll radical spectra observed without prior deuteration. The signal was generated from isolated plant photosystem I (in both digitonin and Triton-X100 preparations) by photooxidation. At 325 GHz and higher frequencies, the spectrum was resolvable into the three principal components of its g -matrix. They were obtained to high accuracy by spectral simulation and were found to be g xx = 2.003 17(±7 × 10 -5 ), g yy = 2.002 64(±7 × 10 -5 ), g zz = 2.002 26(±7 × 10 -5 ) at 40 K and g xx = 2.003 07(±7 × 10 -5 ), g yy = 2.002 60(±7 × 10 -5 ), g zz = 2.002 26(±7 × 10 -5 ) at 200 K. These values indicate a temperature dependence of the g -values in P700 •+ that was measured over a temperature range of 5−260 K using both preparations. The results showed that g xx changed with temperature while g yy and g zz remain constant within the error margin, suggesting that the P700 •+ radical becomes less anisotropic with increasing temperature.