Kinetic studies on the function of A1 in the photosystem I reaction center

Flash absorption measurements with nanosecond time resolution were conducted in the near‐ultraviolet with subchloroplast particles enriched in photosystem I, at 6°C. Around 370 nm, an absorption increase decays biphasically, with a fast phase, t = 15 ± 5 ns. Control experiments show that this rapid phase is due to the reoxidation of A1 (presumably phylloquinone), which thus appears to function as an electron carrier in PS I at physiological temperature. The nanosecond kinetic phase is not altered if the iron‐sulfur centers FA and FB are kept reduced.