Picosecond transient absorption difference spectroscopy in the blue wavelength region (380500 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 P700 and the quinone or A1 electron acceptor (P700+ A1 - P700A1). Decay curves on the millisecond time scale at different wavelengths were measured with an autoxidizable artificial electron acceptor, benzyl viologen, and the (P700+ - P700) difference spectrum was constructed. The (A1 - A1) 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, A0. The nondecaying component of the DAS at low potential was assigned to (P700 A0 - P700 A0) since the electron-transfer pathway from A0 to A1 was blocked. The [(P700+A0 - P700A0) - (P700+ - P700] subtraction gives a spectrum, interpreted as the (A0 A0) difference spectrum of a chlorophyll a molecule, consistent with previous studies. The (A1 - A1) 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.
ASJC Scopus subject areas