Time-resolved X-ray spectroscopy tracks photosynthetic O2 formation

Natural photosynthesis can convert solar energy into chemical energy with almost 100% efficiency. During photosynthesis, O₂ is evolved at a tetra manganese-calcium complex bound to the proteins of photosystem II. As the details of the mechanism have not been fully understood, artificial solar systems still capture only a minute amount of energy. Very recently, a German research group has succeeded in providing new insights into the mechanism by means of time-resolved X-ray absorption spectroscopy, which measures Mn K X-ray fluorescence after laser-flash illumination with a time resolution of 10 μs. The model of the so-called S-cycle treats the manganese complex cycles through five oxidation states, but only four intermediates have been identified experimentally (S0 through S3). Dioxygen is formed during the transition from S3 to S0, but the expected S4 intermediate in this transition has been elusive. Real-time X-ray monitoring of photosynthetic O₂ production has identified the S4 intermediate and, in contrast to previous proposals, the research group concluded that it is formed by a deprotonation process rather than by electron transfer. The experiments were done at beamline ID26, European Synchrotron Radiation Facility (ESRF), Grenoble, France. For more information, see the paper, "Photosynthetic O₂ Formation Tracked by Time-Resolved X-ray Experiments", M. Haumann et al., Science, 310, 1019-1021 (2005).