Spectral resolution of the primary electron acceptor A₀ in photosystem i

The reduced state of the primary electron acceptor of Photosystem I, A ₀, was resolved spectroscopically in its lowest energy Q_y region for the first time without the addition of chemical reducing agents and without extensive data manipulation. To carry this out, we used the menB mutant of Synechocystis sp. PCC 6803 in which phylloquinone is replaced by plastoquinone-9 in the A₁ sites of Photosystem I. The presence of plastoquinone-9 slows electron transfer from A₀ to A₁, leading to a long-lived A₀^- state. This allows its spectral signature to be readily detected in a time-resolved optical pump-probe experiment. The maximum bleaching (A₀^- - A₀) was found to occur at 684 nm with a corresponding extinction coefficient of 43 mM^-1 cm^-1. The data show evidence for an electrochromic shift of an accessory chlorophyll pigment, suggesting that the latter Q _y absorption band is centered around 682 nm.