Methanol modification of the electron paramagnetic resonance signals from the S₀ and S₂ states of the water-oxidizing complex of Photosystem II

The Mn-derived electron paramagnetic resonance (EPR) multiline signal from the S₀ state of the water-oxidizing complex is observable only in the presence methanol. In the present study, we have characterized the effect of methanol on the EPR signals from the S₀ and S₂ states as well as on the EPR Signal II(slow) originating from the Tyrosine(D)(ox) radical. The amplitudes of the S₀ and S₂ multiline signals increase with the methanol concentration in a similar way, whereas the S₂ g=4.1 excited state signal amplitude shows a concomitant decrease. The methanol concentration at which half of the spectral change has occurred is ~0.2% and the effect is saturating around 5%. Methanol has an effect on the microwave power saturation of the S₂ multiline signal, as well. The microwave power at half saturation (P(1/2)) is 85 mW in the presence of methanol, whereas the signal relaxes much slower (P(1/2)~27 mW) without. The relaxation of Signal II(slow) in the presence of methanol has also been investigated. The P(1/2) value of Signal II(slow) oscillates with the S cycle in a similar way as without methanol, but the P(1/2) values are consistently lower in the methanol-containing samples. From the results, we conclude that methanol modifies the magnetic properties of the S₀ and S₂ states in a similar way. The possible site and nature of methanol binding is discussed. Copyright (C) 1999 Elsevier Science B.V.