Molecular Structure of the S₂ State with a g = 5 Signal in the Oxygen Evolving Complex of Photosystem II

The g-factor shift of the g = 4.1 EPR signal was detected in spinach PsbO/P/Q-depleted PS II. The effective g-factor of the signal shifts up to ∼4.9, depending on the Ca²⁺ concentration. Hyperfine structure spacing with about 3 mT was detected in this g = 5 (4.9) signal. The shift to g = 5 (4.9) was related to the distortion of the manganese cluster, derived from the modification of the chemical bond or the crystalline field of the Mn4(III) in the manganese cluster. Based on the EPR analysis of the g = 5 (4.9) spin state, another molecular structure of the S₂ state, a "distant Mn" structure, was discussed as an intermediate state between the S₂ and S₃ states.