Electron paramagnetic resonance signals from the S3 state of the oxygen-evolving complex.: A broadened radical signal induced by low-temperature near-infrared light illumination

The tetranuclear manganese cluster responsible for the oxidation of water in photosystem II cycles through five redox states denoted S-i (i = 0, 1, 2, 3, 4). Progress has been made recently in the detection of weak low-field EPR absorptions in both the perpendicular and parallel modes, associated with the integer spin state S-3 [Matsukawa, T., Mine, H., Yoneda, D., and Kawamori, A. (1999) Biochemistry 38, 4072-4077]. We confirm observation of these signals and have obtained them in high yield by illumination of photosystem II membranes, in which the non-heme iron was chemically preoxidized. It is shown that a split g = 4 signal accompanies the S-3 State signals. The signals diminish in the presence of ethanol and vanish in the presence of methanol. This effect is similar to that exerted by these alcohols to the high-spin component (g = 4.1) of the S-2 State and suggests that the latter spin configuration is the precursor of the Sg State low-field signals. The Sg State shows similar sensitivity to infrared illumination as has been observed previously in the S-2 State [Boussac, A., Un, S., Horner, O., and Rutherford, A. W. (1998) Biochemistry 37, 4001-4007]. Illumination of the Sg State with near-infrared light (700-900 nm), at temperatures around 50 K, results in the modification of the low-field signals and most notably to the appearance of a broad (Delta H similar to 200 G) radical-type signal centered at g = 2. The signal is tentatively assigned to the interaction of the Mn cluster in a modified S-2 State with a radical.