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Recent:
See New York Times article highlighting a 2015 paper (Cardona et al., 2015) on early evolution of PS II.

Photosynthetic water splitting is one of the most unique biological reactions in Nature in which the visible energy of the sun provides the driving force. In the quest to find clean, non-polluting energy sources to support human society in the future, we need to learn and take advantage of the natural system.

Photosystem II (PSII) water oxidation is the main source of oxygen on earth. This most oxidizing chemistry in all of biology (>1 volt oxidation) has evolved over billions of years catalyzes rapid oxidation (>1000 turnovers. s-1) with low thermodynamic barriers, and minimizes the generation of toxic reactive oxygen intermediates. To split water into O2, protons and electrons, large thermodynamic barriers have to be surmounted, making this one of the most difficult chemical reactions to take place in Nature. The Gibbs free energy is at least 3.2 eV (~ 135 kJ mole-1). In contemporary oxygenic photosynthesis the large thermodynamic barriers to water splitting are overcome by the unique properties of the PSII chlorophyll/protein complex. Remarkably, PSII has not changed significantly over the 2.2-2.5 billion years since when it first evolved, yet it is currently responsible for the annual release of ~1011 tons of O2 into the atmosphere and is essential for all aerobic life on Earth.

Bessel Kok (1) provided a major insight into the mechanism of water splitting by PSII with his finding that the reaction cycled through five intermediate states upon excitation with single turnover light flashes. The final S4 state, being metastable, regenerates the S0 state upon releasing O2 (1). Detailed spectroscopic EPR and EXAFS studies have revealed that the S-state cycle and the actual water spitting chemistry occurs at a Mn4Ca inorganic core, which undergoes redox cycling and accumulates the oxidizing equivalents generated by the PSII photochemical reaction centre (2). More recently the static structure of the PSII complex at atomic resolution is being deciphered from X-ray crystallographic analysis, and has provided the location and orientation of the Mn4Ca core and other essential cofactors with respect to the arrangement of the protein backbone (3). Nevertheless, to completely understand the water splitting reaction and to develop bio-mimetic systems for biotechnological purposes, the dynamics of the reaction have to be known. Two powerful tools to address this aspect are mass spectrometric isotope exchange measurements of the substrate water at the catalytic binding site (4) and vibrational FTIR spectroscopic measurements of chemical bond changes at the Mn4Ca core and the associated protein ligands (5).

(1) Kok, B., Forbush, B., and McGloin, M. (1970) Photochem. Photobiol. 11, 457-475.
(2) Hillier, W., and Messinger, J. (2005) The Water: Plastoquinone Oxidoreductase in Photosynthesis (Wydrzynski, T., and Satoh, K., Eds.) pp 567-608, Springer, Printed in The Netherlands.
(3) Ferreira, K. N., Iverson, T., Maghlaoui, K., Barber, J., and Iwata, S. (2004) Science 303, 1831-1838.
(4) Hillier, W., and Wydrzynski, T. (2004) Phys. Chem. Chem. Phys. 6, 4882-4889.
(5) Debus, R. J., Strickler, M. A., Walker, L. M., and Hillier, W. (2005) Biochemistry 44, 1367-1374.

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