Investigation of the interaction of the water oxidising manganese complex of photosystem II with the aqueous solvent environment

Abstract

Interaction of the water oxidising manganese complex of photosystem II with the aqueous environment has been investigated using electron paramagnetic resonance spectroscopy and electron spin echo envelope modulation spectroscopy to detect interaction of [ 2 H]methanol with the complex in the S 2 state. The experiments show that the classical S 2 multiline signal is associated with a manganese environment which is not exposed to the aqueous medium. An electron paramagnetic resonance spectroscopy signal, also induced by 200 K illumination, showing 2 H modulation by methanol in the medium and a modified multiline electron paramagnetic resonance spectroscopy signal formed in parallel to it, are suggested to be associated with a second manganese environment exposed to the medium.z 1999 Federation of European Biochemical Societies.

Evidence for the presence of a component of the Mn complex of the photosystem II reaction centre which is exposed to water in the S2 state of the water oxidation complex

“…The results, in agreement with our previous experiments [12], suggest that the samples contain Mn centres in two different environments which give rise to a multiline EPR spectrum. In one environment the centres decay to an EPR silent state during storage at 77 K. Centres in the second environment are stable during 77 K storage.…”

“…The results presented in this paper parallel those we have presented earlier demonstrating the interaction of methanol with photosystem II in the S 2 state [12]. The experiments show that a weak magnetic interaction between 2 H in water and a component of the photosystem II reaction centre can be detected by ESEEM in the S 2 state immediately after 200 K illumination.…”

“…They suggested the methanol was bound to the Mn through the hydroxyl group. In similar experiments we observed weak interaction, essentially at the Larmor frequency of deuterium compatible with a Mn deuterium distance of 4–6 Å [12]. However, we found that the component interacting with the 2 H was metastable, decaying over a period of weeks during storage at 77 K. This is in contrast to the behaviour of the classical component giving rise to the S 2 multiline signal in the presence of methanol which is stable for very long periods at 77 K. Samples still show strong multiline Mn signals after up to 2 years storage.…”

“…In [12] we discussed the possibility that the ESEEM signal was associated with a component of the multiline Mn signal reflecting a Mn centre in a different environment to that of the major component of the signal. The same conclusion may be drawn from the present experiments.…”

An estimation of the size of the water cluster present at the cleavage site of the water splitting enzyme

“…There are two possible scenarios: one assumes that the cleavage site of water splitting enzyme remains`dry' until a speci¢c mechanism permits water access to the catalytic site [6]; the other hypothesis insists that water molecules are present in the vicinity of the splitting center independently of the redox state of the manganese complex [7]. The latter theory is in accordance with the Yachandra et al model of oxygen evolution [8] and the ¢nding of Evans et al [9] that at least one of the two Mn dimers is exposed to an aqueous environment. The free access of water molecules to the water splitting enzyme can be summarized in the following formula of a four-step univalent oxidative pathway:…”