title: Near-IR absorbance changes and electrogenic reactions in the microsecond-to-second time domain in Photosystem I.
abstract: The back-reaction kinetics in Photosystem I (PS I) were studied on the microsecond-to-s time scale in cyanobacterial preparations, which differed in the number of iron-sulfur clusters to assess the contributions of particular components to the reduction of P700+. In membrane fragments and in trimeric P700-FA/FB complexes, the major contribution to the absorbance change at 820 nm (delta A820) was the back-reaction of FA- and/or FB- with lifetimes of approximately 10 and 80 ms (approximately 10% and 40% relative amplitude). The decay of photoinduced electric potential (delta psi) across a membrane with directionally incorporated P700-FA/FB complexes had similar kinetics. HgCl2-treated PS I complexes, which contain FA but no FB, retain both of these kinetic components, indicating that neither can be assigned uniquely to a specific acceptor. These results suggest that FA- reduces P700+ directly and argue for a rapid electron equilibration between FA and FB, which would eliminate their kinetic distinction in a back-reaction. In PsaC-depleted P700-Fx cores, as well as in P700-FA/FB complexes with chemically reduced FA and FB, the major contribution to the delta A820 and the delta psi decay is a biphasic back-reaction of F-X (approximately 400 microseconds and 1.5 ms) with some contribution from A-1 (approximately 10 microseconds and 100 microseconds), the latter of which is variable depending on experimental conditions. The delta A820 decay in a P700-A1 core devoid of all iron-sulfur clusters comprises two phases with lifetimes of 10 microseconds and 130 microseconds (2.7:1 ratio). The biexponential back-reaction kinetics found for each of the electron acceptors may be related to existence of different conformational states of the PS I complex. In all preparations studied, excitation at 532 nm with flash energies exceeding 10 mJ gives rise to formation of antenna 3Chl, which also contributes to delta A820 decay on the tens-of-microsecond time scale. A distinction between delta A820 components related to back-reactions and to 3Chl decay can be made by analysis of flash saturation dependencies and by measurements of kinetics with preoxidized P700.
authors.affiliation: Department of Biochemistry, G. W. Beadle Center, University of Nebraska, Lincoln 68588-0664, USA.
authors.forename: I R
authors.initials: IR
authors.lastname: Vassiliev
authors.forename: Y S
authors.initials: YS
authors.lastname: Jung
authors.forename: M D
authors.initials: MD
authors.lastname: Mamedov
authors.lastname: Semenov AYu
authors.forename: J H
authors.initials: JH
authors.lastname: Golbeck
mesh_terms: D015086:2,6-Dichloroindophenol; D002734:Chlorophyll; D000458:Cyanobacteria; D004563:Electrochemistry; D007425:Intracellular Membranes / Q000502:physiology; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes; D008564:Membrane Potentials; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045331:Photosystem I Protein Complex; D013055:Spectrophotometry, Infrared; D013997:Time Factors
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D002734:Chlorophyll; D015086:2,6-Dichloroindophenol