Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I

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.