Studies on the reaction pathway from light absorption to water cleavage in photosynthesis

Photosynthetic water cleavage is the key process of biological solar energy exploitation [1]. The overall reaction takes place in a multimeric integral membrane protein complex referred to as Photosystem II (PS II) and comprises three types of reaction sequences:

1. generation of electronically excited singlet states, migration to the photochemically active chlorophyll a component P680 and trapping in the form of a stabilised radical pair [2],
2. oxidation of two water molecules to dioxygen and four protons via a sequence of four single electron abstraction steps energetically driven by P680⁺· as oxidant [3] and
3. reduction of one plastoquinone molecule to plastoquinol via a sequence of two single electron transfer steps with as reductant [4].

The present report provides a concise summary on our current state of knowledge of the functional and structural organization of these reactions. Recent data will be presented which was gathered from experiments using time resolved single photon counting of fluorescence decay, nonlinear fluorescence and absorption, flash induced changes of absorption and fluorescence yield. It is also shown that structural information can be obtained for distances between functional groups in PS II by evaluation of spectroscopic and kinetic data.

1. G.Renger (1983) Biological Energy Conservation. In: Biophysics (W. Hoppe, W. Lohmann, H. Markl and H. Ziegler, eds.) pp. 347- 371, Springer, Berlin
2. G. Renger (1992) Energy transfer and trapping in Photosystem II In: Topics in Photosynthesis, The Photosystems: Structure, Function and Molecular Biology (Barber, J., ed.), pp. 45 - 99, Elsevier, Amsterdam
3. G. Renger (1997) Mechanistic and structural aspects of photosynthetic water oxidation. Physiol. Plant. 100, 828-841
4. Lavergne, J., Briantais, J.M. (1996) In: Ort, D.R., Yocum, C.F. (eds) Oxygenic Photosynthesis: The Light Reactions, Kluwer Academic Publishers, pp 265-287