Athina Zouni, Holger Dobbek (project A5) and colleagues applied serial femtosecond X-ray crystallography and simultaneous X-ray emission spectroscopy with multi-flash visible laser excitation in order to analyze photosynthetic oxygen evolution and protonation. Their study concludes that one water ligand to calcium (W3) is directly involved in substrate delivery. The results support the binding of the additional oxygen Ox in the S₃ state between Ca and Mn1 through O–O bond formation mechanisms involving O5 as one substrate. The authors identify peroxo-bond formation in the S₃ state and the nucleophilic attack of W3 onto W2 as unlikely bond formation mechanisms.

Publication: Kern, J., Chatterjee, R., Young, I. D., Fuller, F. D., Lassalle, L., Ibrahim, M., Gul, S., Fransson, T., Brewster, A. S., Alonso-Mori, R., Hussein, R., Zhang, M., Douthit, L., de Lichtenberg, C., Cheah, M. H., Shevela, D., Wersig, J., Seuffert, I., Sokaras, D., Pastor, E., Weninger, C., Kroll, T., Sierra, R. G., Aller, P., Butryn, A., Orville, A. M., Liang, M., Batyuk, A., Koglin, J. E., Carbajo, S., Boutet, S., Moriarty, N. W., Holton, J. M., Dobbek, H., Adams, P. D., Bergmann, U., Sauter, N. K., Zouni, A., Messinger, J., Yano, J., and Yachandra, V. K. (2018). Structures of the intermediates of Kok's photosynthetic water oxidation clock. Nature 563, 421-425 doi:10.1038/s41586-018-0681-2.