Charge recombination in photosystem I at low temperatures kinetics of electron tunneling

Abstract

Absorption changes accompanying light-induced P-700 oxidation and the decay of P-700+ in the dark were measured in the temperature range 294-5 K over a broad time scale (three to four orders of magnitude). Two qualitatively different types of kinetics for the dark decay of P-700 + were observed. In the 294-240K region, a usual exponential kinetics is observed with the rate constant κ = 1 · 10 10 · exp(-16 000/RT) s −1, with R in cal/mol per degree. Below 220 K, a rather unusual logarithmic or near-logarithmic kinetics are observed. These kinetics can be explained quantitatively if one assumes for the various ( P-700+ ··· X -) pairs a broad rectangular or near-rectangular distribution over the values of the rate constant. The following kinetic equation corresponding to this model was obtained: n t / n o = [In( κ max/ κ min)]-1 - [In(1/ κ min)− In t] where no and nt are respectively the initial concentration of P-700 + and its concentration at time t, and kmax and kmin the maximum and minimum values of the rate constant, respectively. The decay processes observed can be ascribed to electron tunneling. Distribution over the values of k can be accounted for by different environments or different mutual orientations of P-700 + and X −, or by different distances between them in the various reacting pairs. The corresponding distribution function was reconstructed from the experimentally measured P-700+-decay curves. The rate of tunneling was found to be temperature dependent. In the 160-80-K region, the temperature dependence corresponds to an activation energy of 2.9 kcal/mol. Below 80 K, new modes of P-700+ decay with lower activation energy become operative. The tunneling distance for the majority of the ( P-700+ ··· X −) pairs was estimated from the EPR linewidth of P-700+ to exceed 13.2 A.