Temperature Dependence of the Reduction Kinetics of P680002B; in Oxygen-Evolving PS II Complexes Throughout the Range from 320 to 80 K
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 TyrZ 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 PD1 and TyrZ 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 TyrZ is increasingly replaced by the charge recombination between P680+ and QA −. It is proposed that reorganization processes which are required for the stabilization of the state P680TyrZ ox become arrested around 200 K. Freezing out of the electron donation from TyrZ to P680+ is the result. The yield of TyrZ oxidation at low temperature is <5%.
KeywordsPhotosystem II electron transfer TyrZ P680 transient absorption spectroscopy
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