Photosystem II single crystals studied by EPR spectroscopy at 94 GHz: The tyrosine radical Y [Formula: see text]

Electron paramagnetic resonance (EPR) spectroscopy at 94 GHz is used to study the dark-stable tyrosine radical Y [Formula: see text] in single crystals of photosystem II core complexes (cc) isolated from the thermophilic cyanobacterium Synechococcus elongatus. These complexes contain at least 17 subunits, including the water-oxidizing complex (WOC), and 32 chlorophyll a molecules/PS II; they are active in light-induced electron transfer and water oxidation. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with four PS II dimers per unit cell. High-frequency EPR is used for enhancing the sensitivity of experiments performed on small single crystals as well as for increasing the spectral resolution of the g tensor components and of the different crystal sites. Magnitude and orientation of the g tensor of Y [Formula: see text] and related information on several proton hyperfine tensors are deduced from analysis of angular-dependent EPR spectra. The precise orientation of tyrosine Y [Formula: see text] in PS II is obtained as a first step in the EPR characterization of paramagnetic species in these single crystals.