title: Rapid loss of structural motifs in the manganese complex of oxygenic photosynthesis by X-ray irradiation at 10-300 K.
abstract: Structural changes upon photoreduction caused by x-ray irradiation of the water-oxidizing tetramanganese complex of photosystem II were investigated by x-ray absorption spectroscopy at the manganese K-edge. Photoreduction was directly proportional to the x-ray dose. It was faster in the higher oxidized S2 state than in S1; seemingly the oxidizing potential of the metal site governs the rate. X-ray irradiation of the S1 state at 15 K initially caused single-electron reduction to S0* accompanied by the conversion of one di-mu-oxo bridge between manganese atoms, previously separated by approximately 2.7 A, to a mono-mu-oxo motif. Thereafter, manganese photoreduction was 100 times slower, and the biphasic increase in its rate between 10 and 300 K with a breakpoint at approximately 200 K suggests that protein dynamics is rate-limiting the radical chemistry. For photoreduction at similar x-ray doses as applied in protein crystallography, halfway to the final Mn(II)4 state the complete loss of inter-manganese distances <3 A was observed, even at 10 K, because of the destruction of mu-oxo bridges between manganese ions. These results put into question some structural attributions from recent protein crystallography data on photosystem II. It is proposed to employ controlled x-ray photoreduction in metalloprotein research for: (i) population of distinct reduced states, (ii) estimating the redox potential of buried metal centers, and (iii) research on protein dynamics.
authors.affiliation: Freie Universität Berlin, FB Physik, Arnimallee 14, D-14195 Berlin, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Grabolle
authors.forename: Michael
authors.initials: M
authors.lastname: Haumann
authors.forename: Claudia
authors.initials: C
authors.lastname: Müller
authors.forename: Peter
authors.initials: P
authors.lastname: Liebisch
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
mesh_terms: D020816:Amino Acid Motifs; D001703:Biophysics / Q000379:methods; D002736:Chloroplasts / Q000378:metabolism; D003461:Crystallography; D004307:Dose-Response Relationship, Radiation; D004583:Electrons; D007477:Ions; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry; D008345:Manganese / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry; D045332:Photosystem II Protein Complex / Q000378:metabolism; D011506:Proteins / Q000737:chemistry; D013057:Spectrum Analysis; D018724:Spinacia oleracea / Q000378:metabolism; D017356:Synchrotrons; D013696:Temperature; D013997:Time Factors; D014965:X-Rays
chemical_list: D007477:Ions; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D011506:Proteins; D008345:Manganese