Picosecond Photochemistry of P700-Enriched and Vitamin K_1-Depleted Photosystem I Particles Isolated from Spinach :

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Picosecond transient absorption changes, with a laser intensity as low as one photon absorbed per single reaction center, were measured with vitamin K_1-depleted and P700-enriched particles which were obtained by ether treatment of spinach PS-I particles. When P700 was in the oxidized state, a bleaching that corresponded to about one-seventh of the ground state absorption was observed just after a laser flash (0 picosecond delay). A major part of the bleaching decayed with a lifetime of about 35 picoseconds, which corresponds to the relaxation of the excited antenna chl-a to the ground state. By contrast, when P700 was in the reduced state, the bleaching observed at a 0 ps delay was broader, especially on the longer wavelength side than the ground state absorption, probably because of the generation of the excited state of P700. About one half of the bleaching decayed within 35 ps and the remaining half, which had a broad spectrum and a peak around 682 nm, was conserved up to 2 ns. This long-lived bleaching probes no picosecond decay of the radical pair P700^+-A_0^-, because electrons were not transferred from A_0^- to A_1 in vitamin K_1-depleted particles. After addition of vitamin K_3, an analog of vitamin K_1, to the reduced particles, the bleaching around 685 nm decayed successively with an apparent rate of about 150 picosecond, while the bleaching around 700 nm was conserved for up to 2 nanosecond. Thus, the bleaching remaining at 2 ns resembled the difference spectrum of P700, suggesting a subnanosecond quenching of A_0^- by the externally added vitamin K_3. These observations support a recent proposal that the secondary electron acceptor A_1, in photosystem I, is vitamin K_1.

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