title: 18O isotope effect in the photosynthetic water splitting process.
abstract: In mass spectroscopic experiments of oxygen evolution in Photosystem II at 50% enrichment of H(2)18O, one expects equal signals of 18O(2) and 16O(2) unless one of the isotopes is favored by the oxygen evolving complex (OEC). We have observed a deviation from this expectation, being a clear indication of an isotope effect. We have measured the effect to be 1.14-1.30, which is higher than the theoretically predicted value of 1.014-1.06. This together with the strong temperature variation of the measured effect with a discontinuity at 11 degrees C observed for wild-type tobacco and at 9 degrees C for a yellow-green tobacco mutant suggest that an additional mechanism is responsible for the observed high isotope effect. The entry of a finite size of water clusters to the cleavage site of the OEC can explain the observation.
authors.affiliation: Institute of Nuclear Physics, ul. Radzikowskiego 152, 31-342 Cracow, Poland.
authors.forename: Kvetoslava
authors.initials: K
authors.lastname: Burda
authors.forename: Klaus P
authors.initials: KP
authors.lastname: Bader
authors.forename: Georg H
authors.initials: GH
authors.lastname: Schmid
mesh_terms: D013058:Mass Spectrometry; D010100:Oxygen / Q000737:chemistry* / Q000378:metabolism; D010103:Oxygen Isotopes; D010788:Photosynthesis; D013696:Temperature; D020524:Thylakoids / Q000737:chemistry; D013997:Time Factors; D014026:Nicotiana / Q000737:chemistry / Q000378:metabolism*; D014867:Water / Q000737:chemistry* / Q000378:metabolism
chemical_list: D010103:Oxygen Isotopes; D014867:Water; D010100:Oxygen