Photosystem II single crystals studied by EPR spectroscopy at 94 GHz:: The tyrosine radical YD•

Electron paramagnetic resonance (EPR) spectroscopy at 94 GHz is used to study the dark-stable tyrosine radical Y-D(.) in single crystals of photosystem H 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 tenser components and of the different crystal sites. Magnitude and orientation of the g tenser of Y-D(.) and related information on several proton hyperfine tensors are deduced from analysis of angular-dependent EPR spectra. The precise orientation of tyrosine Y-D(.) in PS II is obtained as a first step in the EPR characterization of paramagnetic species in these single crystals.