Analysis of dipolar and exchange interactions between manganese and tyrosine Z in the S₂Y_Z^. 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 (∼9.5 GHz) electron paramagnetic resonance (EPR) signal at 10 K. This EPR signal arises from an interaction between the S = 1/2 multiline S₂ 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 (∼33 GHz) frequencies utilizing second-order perturbation theory. The positions and relative intensities of the hyperfine lines in the S = 1/2 S₂ state multiline EPR signal of the noninteracting Mn₄ cluster are accurately simulated by including g anisotropy and four sets of axially symmetric ⁵⁵Mn hyperfine tensors. These parameters are then used to simulate the dipolar and exchange interactions giving rise to the interacting S₂Y_Z^. (formerly referred to as S3) EPR signal. Relative intensities of components of the S₂Y_Z. EPR spectrum, at both X- and Q-band frequencies, are best reproduced with a dipolar coupling corresponding to an interspin distance of 7.7 Å and an exchange coupling (J) of -280 × 10^-4 cm^-1.