Direct demonstration of electron transfer between tryptophan and tyrosine in proteins

With several proteins it has been shown that electrons can be transferred intramolecularly from tyrosine to electron-deficient tryptophan units. Rates vary from ∼ 10 2s −1 (in lysozyme) to ∼ 2×10 4 s −1 (in trypsin). For β-lactoglobulin the activation energy is 45kJ mol −1. This is incompatible with charge conduction along the polypeptide chain and rules out any mechanism involving temperature-labile hydrogen bonds as the main pathway. It seems likely that the electron transfer proceeds directly between the aromatic groups, while they are maintained at a distance from each other.