2015/06/29 by Katherine M. Davis, Brendan T. Sullivan, Mark Palenik +7
Physics and Astronomy · #physics.bio-ph #physics.chem-ph
paper · pdf · doi:10.1103/physrevx.8.041014
published as Phys. Rev. X 8, 041014 (2018) · 49 pages, includes main text (pgs. 1-16) and supplementary material (pgs. 17-49), 13 figures (4 main text, and 9 supplementary), 12 tables (2 main text, 10 supplementary)
arxiv created 2015/06/29 · arxiv updated 2018/10/31
Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A Mn4Ca cluster embedded in the photosystem II protein environment is responsible for the production of atmospheric oxygen. Here, time-resolved x-ray emission spectroscopy (XES) was used to observe the process of oxygen formation in real time. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within 50 μs) changes to the Mn Kβ XES spectrum. However, no oxidation of the Mn4Ca core above the all MnIV state was detected to precede O-O bond formation. A new mechanism featuring MnIV=O formation in the S3 state is proposed to explain the spectroscopic results. This chemical formulation is consistent with the unique reactivity of the S3 state and explains facilitation of the following S3 to S0 transition, resolving in part the kinetic limitations associated with O-O bond formation. In the proposed mechanism, O-O bond formation precedes transfer of the final (4th) electron from the Mn4Ca cluster, in agreement with experiment.