Temperature Dependence of the Reduction Kinetics of P680^002B; in Oxygen-Evolving PS II Complexes Throughout the Range from 320 to 80 K

Abstract

Transient absorbance difference spectroscopy has been used to study the reduction kinetics of P680⁺ after the first flash given to darkadapted oxygen-evolving PS II complexes from Thermosynechococcus elongatus as a function of temperature between 80 and 320 K. The half-life of P680⁺ reduction by Tyr_Z increases from 20 ns at 300 K to about 4 μs at 150 K corresponding to an activation energy of (122 ± 3) meV. Analyzing the temperature dependence of the rate in terms of nonadiabatic electron transfer theory, one obtains a reorganization energy of about 700 meV and an edge-to-edge distance of about 8.5 Å which is in good agreement with the distance between P_D1 and Tyr_Z in the recent structural model of PS II at 3.0 Å resolution (Loll et al. 2005). In the range from 260 to 150 K, the re-reduction of P680⁺ by Tyr_Z is increasingly replaced by the charge recombination between P680⁺ and Q_A ⁻. It is proposed that reorganization processes which are required for the stabilization of the state P680Tyr_Z ^ox become arrested around 200 K. Freezing out of the electron donation from Tyr_Z to P680⁺ is the result. The yield of Tyr_Z oxidation at low temperature is <5%.