Analysis of dipolar and exchange interactions between manganese and tyrosine Z in the S2YZ• state of acetate-inhibited photosystem II via EPR spectral simulations at X- and Q-Bands

Upon room-temperature illumination, acetate-inhibited photosystem II membranes 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 between the two paramagnetic species are simulated at X- and Q-band (similar 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 cluster are accurately simulated by including g anisotropy and four sets of axially symmetric Mn-55 hyperfine tensors. These parameters are then used to simulate the dipolar and exchange interactions giving rise to the interacting S2YZ. (formerly referred to as S3) EPR signal. Relative intensities of components of the S2YZ. EPR spectrum, at both X- and Q-band frequencies, are best reproduced with a dipolar coupling corresponding to an interspin distance of 7.7 Angstrom and an exchange coupling (J) of -280 x 10(-4) cm(-1).