Abstract
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 I) isolated from Synechocystis sp. PCC 6803. While the slower phase ( τ=340 ns at 295 K) slowed upon cooling according to an activation energy of 110 meV, the time constant of the faster phase ( τ=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.
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