Density Functional Calculations Modelling Tyrosine Oxidation in Oxygenic Photosynthetic Electron Transfer

Abstract

Hybrid density functional calculations are used to model tyrosine oxidation during electron transfer reactions of photosystem II. The predicted frequency values for the 7a and d COH modes of the reduced form and the 7a mode of the oxidised radical form are in excellent agreement with experimental data obtained for Mn and Ca depleted systems by Hienerwadel et al. Biochemistry, 1997, 36, 15447 and Berthomieu et al. Biochemistry, 1998, 37, 10547. The calculations confirm that the two tyrosines YD and YZ are protonated in the reduced form. On oxidation the larger 7a frequency value observed experimentally for YZ· can be best explained by a greater localisation of the protonic charge released on formation of this tyrosyl free radical.