Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I.
Picosecond transient absorption difference spectroscopy in the blue wavelength region (380-500 nm) was used to study the early electron acceptors in photosystem I. Samples were photosystem I core particles with about 100 chlorophylls per reaction center isolated from the cyanobacterium Synechocystis sp. PCC 6803. After excitation at 590 nm at room temperature, decay-associated spectra (DAS) were determined from global analysis in the blue region, yielding two transient components and one nondecaying component. A 3 ps decay phase is interpreted as primarily due to antenna excited-state redistribution. A 28 ps decay phase is interpreted as due to overall excited-state decay by electron transfer. The nondecaying component is ascribed to the difference spectrum of P(700) and the quinone or A(1) electron acceptor (P(700)(+)A(1)(-) - P(700)A(1)). Decay curves on the millisecond time scale at different wavelengths were measured with an autoxidizable artificial electron acceptor, benzyl viologen, and the (P(700)(+) - P(700)) difference spectrum was constructed. The (A(1)(-) - A(1)) difference spectrum was obtained by taking the difference between the above two difference spectra. A parallel picosecond experiment under strongly reducing conditions was also done as a control experiment. These conditions stabilize the electron on an earlier acceptor, A(0). The nondecaying component of the DAS at low potential was assigned to (P(700)(+)A(0)(-) - P(700)A(0)) since the electron-transfer pathway from A(0) to A(1) was blocked. The [(P(700)(+)A(0)(-) - P(700)A(0)) - (P(700)(+) - P(700))] subtraction gives a spectrum, interpreted as the (A(0)(-) - A(0)) difference spectrum of a chlorophyll a molecule, consistent with previous studies. The (A(1)(-) - A(1)) spectrum resolved on the picosecond time scale shows significant differences with similar spectra measured on longer time scales. These differences may be due to electrochromic effects and spectral evolution.
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Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe 85287-1604, USA.
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D001594:Benzyl Viologen / Q000737:chemistry; D002734:Chlorophyll / Q000737:chemistry; D000077194:Chlorophyll A; D000458:Cyanobacteria / Q000737:chemistry / Q000235:genetics; D007834:Lasers; D010084:Oxidation-Reduction; D010782:Photolysis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics; D017384:Sequence Deletion; D013053:Spectrophotometry / Q000379:methods
D045322:Photosynthetic Reaction Center Complex Proteins; D001594:Benzyl Viologen; D002734:Chlorophyll; C001785:chlorophyll P 700; D000077194:Chlorophyll A