Analysis of Dipolar and Exchange Interactions between Manganese and Tyrosine Z in the SY State of Acetate-Inhibited Photosystem Ⅱ via EPR Spectral Simulations at X- and Q-Bands

摘要

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 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 # and an exchange coupling (J) of -280 × 10↑(-4) cm↑(-1).