High‐field (94‐GHz) EPR spectroscopy on the S2 multiline signal of photosystem II

The multiline signal of the S2 state in Photosystem II was measured both in frozen‐solution and single‐crystal preparations from the cyanobacterium Thermosynechococcus elongatus. The frozen‐solution EPR spectrum shows a gaussian‐like line shape without any resolution of Mn hyperfine couplings. This line shape can be understood on the basis of the single‐crystal spectra, where a strong orientation dependence of partially resolved hyperfine structures appears. Simulation of the frozen‐solution spectrum on the basis of Mn hyperfine couplings taken from published pulse‐ENDOR data yields a fully rhombic g‐matrix for the multiline signal with principal components 1.997, 1.970, and 1.965. The resulting isotropic g‐value g iso = 1.977 is surprisingly small compared to other manganese complexes containing manganese ions in the formal oxidation states three and four.