Temperature dependence of biphasic forward electron transfer from the phylloquinone(s) A1 in photosystem I:: only the slower phase is activated

The temperature dependence of the biphasic electron transfer (ET) from the secondary acceptor A1 (phylloquinone) to iron-sulfur cluster F-X was investigated by flash absorption spectroscopy in photosystem I (PS 1) isolated from Synechocystis sp. PCC 6803. While the slower phase (tau = 340 ns at 295 K) slowed upon cooling according to an activation energy of 110 meV, the time constant of the faster phase (tau = 11 ns at 295 K) was virtually independent of temperature. Following a suggestion in the literature that the two phases arise from bidirectional ET involving two symmetrically arranged phylloquinones, Q(K)-A and Q(K)-B, it is concluded that energetic parameters (most likely the driving forces) rather than the electronic couplings are different for ET from Q(K)-A to F-X and from Q(K)-B to F-X. Two alternative schemes of ET in PS I are presented and discussed. (C) 2003 Elsevier Science B.V All rights reserved.