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 (∼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 (∼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 55 Mn hyperfine tensors. These parameters are then used to simulate the dipolar and exchange interactions giving rise to the interacting S 2 Y Z • (formerly referred to as S3) EPR signal. Relative intensities of components of the S 2 Y 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 .