title: Observation of the reduction and reoxidation of the primary electron acceptor in photosystem I.
abstract: Femtosecond transient absorption spectroscopy has been used to investigate the primary charge separation in a photosystem II deletion mutant from the cyanobacterium Synechocystis sp. PCC 6803. These cells contain only the photosystem I reaction center and have a pigment content of approximately 100 chlorophylls per P700. Utilizing relatively high excitation intensities, the difference spectrum for the reduction of primary electron acceptor [(A0(-)-A0) difference spectrum] was obtained from experiments performed under both reducing and oxidizing conditions. Both approaches yield very similar results with the (A0(-)-A0) difference spectrum displaying a maximum bleaching at 687 nm. The shape of the difference spectrum suggests that the primary electron acceptor in photosystem I may be a chlorophyll a molecule. The observed rate of primary radical pair formation depends on the overall rate of decay of excitations in the antenna; the radical pair state forms as the antenna decays. The decay of the primary radical pair state is characterized by a 21-ps time constant. Under conditions that avoid annihilation effects, the mean lifetime for excitations in the antenna is 28 ps [Hastings, G., Kleinherenbrink, F.A.M., Lin, S., & Blankenship, R.E. (1994) Biochemistry (preceding paper in this issue)]. This indicates that the reduced acceptor decays faster than it forms. Therefore, only a low concentration of the reduced acceptor will accumulate under most conditions.
authors.affiliation: Department of Chemistry and Biochemistry, Arizona State University, Tempe 85287-1604.
authors.forename: G
authors.initials: G
authors.lastname: Hastings
authors.forename: F A
authors.initials: FA
authors.lastname: Kleinherenbrink
authors.forename: S
authors.initials: S
authors.lastname: Lin
authors.forename: T J
authors.initials: TJ
authors.lastname: McHugh
authors.forename: R E
authors.initials: RE
authors.lastname: Blankenship
mesh_terms: D002734:Chlorophyll / Q000737:chemistry / Q000378:metabolism; D000458:Cyanobacteria / Q000737:chemistry*; D004563:Electrochemistry; D004579:Electron Transport; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes; D009154:Mutation; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D013053:Spectrophotometry
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; C001785:chlorophyll P 700