Photosystem II contains two symmetry related redox active tyrosine residues, YD(Tyr160) and YZ(Tyr161). YZ is thought to serve as an electron transfer intermediate between the tetranuclear Mn cluster, where water oxidation occurs, and the photo-oxidized chlorophyll moiety PYD displays well ordered hydrogen bonding in ENDOR spectra consistent with its being both oxidized and reduced by P680. Initial high field EPR studies are reported of the dark adapted stable neutral radical YD?. The principle g-values are easily resolved in the high field (140 GHz) EPR spectrum: g1=2.00785, g2=2.00455, g3=2.00235. The well defined turning points are indicative of a defined hydrogen bonding environment for YD. In contrast to a previous high field study by Un et al., 1994 the hyperfine coupling anisotropy to the b- and the 3,5- protons of YD is resolved at each of the g-value turning points. Simulations of the experimental spectrum enable the determination of the various proton hyperfine couplings as well as the correlation of the hyperfine- and g-tensor orientations. These high field EPR results are in excellent agreement with the pulsed ENDOR studies of Force et al., 1995 and Gilchrist et al., 1995. These experiments demonstrate the neccesity of quadrature detection to observe the saturated dispersion EPR signal in dilute spin cases, frequently encountered in proteins, where the absorption signal is saturated. As demonstarted in the simultions of the YD spectrum, the saturated dispersion lineshape must be accounted for to properly replicate the experimental features. The center of the dispersion EPR line is also observed to saturate with increasing power. This distortion of the EPR dispersion lineshape, due to saturation, is avoided in the echo detected EPR spectrum which displays greater resolution.
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