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 ～ 10²s^?1 (in lysozyme) to ～ 2×10⁴ 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.