Photosynthesis Research

, Volume 92, Issue 3, pp 327–343

Time-resolved X-ray spectroscopy leads to an extension of the classical S-state cycle model of photosynthetic oxygen evolution

Authors

Original paper

DOI: 10.1007/s11120-007-9141-9

Cite this article as:
Dau, H. & Haumann, M. Photosynth Res (2007) 92: 327. doi:10.1007/s11120-007-9141-9

Abstract

In oxygenic photosynthesis, a complete water oxidation cycle requires absorption of four photons by the chlorophylls of photosystem II (PSII). The photons can be provided successively by applying short flashes of light. Already in 1970, Kok and coworkers [Photochem Photobiol 11:457–475, 1970] developed a basic model to explain the flash-number dependence of O2 formation. The third flash applied to dark-adapted PSII induces the S3→S4⇒S0 transition, which is coupled to dioxygen formation at a protein-bound Mn4Ca complex. The sequence of events leading to dioxygen formation and the role of Kok’s enigmatic S4-state are only incompletely understood. Recently we have shown by time-resolved X-ray spectroscopy that in the S3⇒S0 transition an interesting intermediate is formed, prior to the onset of O–O bond formation [Haumann et al. Science 310:1019–1021, 2005]. The experimental results of the time-resolved X-ray experiments are discussed. The identity of the reaction intermediate is considered and the question is addressed how the novel intermediate is related to the S4-state proposed in 1970 by Bessel Kok. This leads us to an extension of the classical S-state cycle towards a basic model which describes sequence and interplay of electron and proton abstraction events at the donor side of PSII [Dau and Haumann, Science 312:1471–1472, 2006].

Keywords

Manganese complex Mechanism of water oxidation Oxygenic photosynthesis Photosynthetic oxygen evolution Photosystem II X-ray absorption spectroscopy

Abbreviations

Chl

chlorophyll

EPR

electron paramagnetic resonance

EXAFS

extended X-ray absorption fine-structure

OEC

oxygen-evolving complex

PSII

photosystem II

P680

primary chlorophyll donor in PSII

Pheo

specific pheophytin acting as the primary electron acceptor in PSII

Q

‘quencher’ of PSII fluorescence and electron acceptor in PSII

QA

primary quinone acceptor in PSII

QB

secondary quinone acceptor in PSII

YZ

Tyr 160/161 of the D1 protein of PSII

Y Z •+

oxidized form of YZ

XANES

X-ray absorption near-edge structure

XAS

X-ray absorption spectroscopy

Copyright information

© Springer Science+Business Media B.V. 2007