ANALYSIS OF DIPOLAR AND EXCHANGE INTERACTIONS BETWEEN MANGANESE AND TYROSINE-Z IN THE S2YZ-CENTER-DOT STATE OF ACETATE-INHIBITED PHOTOSYSTEM-II VIA EPR SPECTRAL SIMULATIONS AT X-BANDS AND Q-BANDS

Upon room-temperature illumination, acetate-inhibited photosystem II m embranes are known to exhibit a 240 G wide X-band (similar to 9.5 GHz) electron paramagnetic resonance (EPR) signal at 10 K. This EPR signal arises from an interaction between the S = 1/2 multiline S-2 state of the tetranuclear manganese cluster and an oxidized tyrosine residue, Y-Z(.). In the present study, the exchange and dipolar interactions be tween the two paramagnetic species are simulated at X- and Q-band (sim ilar to 33 GHz) frequencies utilizing second-order perturbation theory . The positions and relative intensities of the hyperfine lines in the S = 1/2 S-2 state multiline EPR signal of the noninteracting Mn-4 clu ster are accurately simulated by including g anisotropy and four sets of axially symmetric Mn-55 hyperfine tensors. These parameters are the n used to simulate the dipolar and exchange interactions giving rise t o the interacting S2YZ. (formerly referred to as S3) EPR signal. Relat ive intensities of components of the S2YZ. EPR spectrum, at both X- an d Q-band frequencies, are best reproduced with a dipolar coupling corr esponding to an interspin distance of 7.7 Angstrom and an exchange cou pling (J) of -280 x 10(-4) cm(-1).