A comparison of electron transfer in ribonucleotide reductase and the bacterial photosynthetic reaction center

The energy requirements for electron transfer in two different proteins are compared, based on quantum chemical calculations. The methods used are the hybrid density functional B3LYP combined with dielectric cavity models to account for the effects of the polarizable protein. The experimental exothermicities for the electron transfer steps from the chlorophyll special pair both to the pheophytin and to the quinone are well reproduced. For ribonucleotide reductase (RNR), the same methods in contrast predict a large endothermicity for the electron transfer from the cysteine at the substrate site to the tyrosyl radical in the interior of the protein. This indicates that another type of process is active in RNR. Previous work has suggested that this process is hydrogen atom transfer.