The kinetics of reduction of P700+, the stably oxidized electron donor of Photosystem I, by plastocyanin (PC) has been investigated by pump-probe optical spectroscopy in living cells of the green alga Chlamydomonas reinhardtii, between 277 and 318 K. The reduction of P 700+ in vivo is described by two kinetic components with lifetimes of 6 ± 0.5 and 56 ± 1 μs at room temperature. The rapid reduction phase, which is attributed to reduction of P700 + by prebound PC, is thermally activated with an apparent activation barrier of 105-115 meV. The analysis of the in vivo reaction is consistent with (i) reduced PC and PS I forming a relatively tight binary complex that does not undergo kinetically limiting conformational reconfiguration and (ii) the activation barrier being determined principally by enthalpic contributions to the free energy change. Under the approximation that entropic contributions to the free energy change associated with this electron transfer reaction are negligible, a lower boundary value of the reorganization energy is estimated to be 0.54-0.63 eV, which is on the lower range of the distribution for intraprotein electron transfer reactions. This low activation barrier is discussed in terms of the optimization of primary donor reduction.
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