Abstract
Low temperature flash absorption spectroscopy has been applied to elucidate the chemical nature of the secondary electron acceptor A1of photosystem I (PS‐I). The flash‐induced absorption changes measured in digitonin‐fractionated spinach PS‐I particles at 10 K between 240 and 525 nm are shown to comprise a major decay phase witht~ 150 μs which has been attributed to the recombination reaction P‐700+·A1→ P‐700·A1[(1984) Biochim. Biophys. Acta 767, 404‐414]. We present the absorption difference spectrum of this reaction and demonstrate that it contains contributions in the ultraviolet due to A1, which are characteristic of vitamin K1(phylloquinone).

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