Methanol modification of the electron paramagnetic resonance signals from the S0 and S2 of the water-oxidizing complex of photosystem II
1999
Deak, Z. | Peterson, S. | Geijer, P. | Ahrling, K.A. | Styring, S.
The Mn-derived electron paramagnetic resonance (EPR) multiline signal from the S(0) 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 S(0) and S(2) states as well as on the EPR Signal II(slow) originating from the Tyrosine(D)OX radical. The amplitudes of the S(0) and S(2) multiline signals increase with the methanol concentration in a similar way, whereas the S(2) g = 4.1 excited state signal amplitude shows a concomitant decrease. The methanol concentration at which half of the spectral change has occurred is approximately equal to 0.2% and the effect is saturating around 5%. Methanol has an effect on the microwave power saturation of the S(2) 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) approximately equal to 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 S(0) and S(2) states in a similar way. The possible site and nature of methanol binding is discussed.
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