Photosynthetic water oxidation at high O2 backpressure monitored by delayed chlorophyll fluorescence.

The atmospheric dioxygen is produced by photosynthetic organisms. This light-driven process culminates in what appears as one step: a four-electron abstraction from two water molecules bound to the Mn4Ca complex of photosystem II. Recently, an intermediate of the O2-producing reaction sequence was stabilized by elevated oxygen backpressure and detected by UV flash photometry [Clausen, J., and Junge, W. (2004) Nature 430, 480]. We scrutinized its properties by delayed chlorophyll fluorescence measurements. Half-suppression of oxygen evolution was observed at a similar O2 pressure of 2.3 bar, as previously, now with photosystem II membrane particles from spinach, without artificial electron acceptors, and at a high signal-to-noise ratio. The data are tentatively interpreted as the stabilization of a 2-fold oxidized state of the catalytic center (S2*) with bound peroxide and its slow conversion into the normal S2 state by the release of peroxide.