title: Photosynthetic O2 formation tracked by time-resolved x-ray experiments.
abstract: Plants and cyanobacteria produce atmospheric dioxygen from water, powered by sunlight and catalyzed by a manganese complex in photosystem II. A classic S-cycle model for oxygen evolution involves five states, but only four have been identified. The missing S4 state is particularly important because it is directly involved in dioxygen formation. Now progress comes from an x-ray technique that can monitor redox and structural changes in metal centers in real time with 10-microsecond resolution. We show that in the O2-formation step, an intermediate is formed--the enigmatic S4 state. Its creation is identified with a deprotonation process rather than the expected electron-transfer mechanism. Subsequent electron transfer would give an additional S4' state, thus extending the fundamental S-state cycle of dioxygen formation.
authors.affiliation: Freie Universität Berlin, FB Physik, Arnimallee 14, D-14195 Berlin, Germany. haumann@physik.fu-berlin.de
authors.forename: M
authors.initials: M
authors.lastname: Haumann
authors.forename: P
authors.initials: P
authors.lastname: Liebisch
authors.forename: C
authors.initials: C
authors.lastname: Müller
authors.forename: M
authors.initials: M
authors.lastname: Barra
authors.forename: M
authors.initials: M
authors.lastname: Grabolle
authors.forename: H
authors.initials: H
authors.lastname: Dau
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004583:Electrons; D019277:Entropy; D007700:Kinetics; D007834:Lasers; D008345:Manganese / Q000737:chemistry; D008954:Models, Biological; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism*; D010788:Photosynthesis; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism*; D011522:Protons; D013057:Spectrum Analysis; D018724:Spinacia oleracea; D014867:Water / Q000378:metabolism; D014965:X-Rays
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D008345:Manganese; D010100:Oxygen