Density functional calculations modelling tyrosine oxidation in oxygenic photosynthetic electron transfer

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 deltaCOH 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 36 (1997) 15447] and Berthomieu et al. [Biochemistry 37 (1998) 10547]. The calculations confirm that the two tyrosines Y(D) and Y(Z) are protonated in the reduced form. On oxidation the larger 7a frequency value observed experimentally for Y(Z*) can be best explained by a greater localisation of the protonic charge released on formation of this tyrosyl free radical.