Tryptophan and tyrosine residues in proteins can form radical intermediates that enable long-range, multi-step electron transfer (ET) reactions.  These aromatic amino acids also undergo proton transfer, which highlights the importance of the protein environment for optimizing ET and proton transfer chemistry.  This talk describes the mechanisms of formation and quenching of a neutral tryptophan radical in azurin, a blue-copper protein.  A long-lived neutral tryptophan radical is formed upon UV-photoexcitation of an azurin mutant in the presence of an external electron acceptor.  The formation of this neutral radical involves oxidation of the indole ring, as well as proton transfer from a solvent-excluded cavity of the protein.  The quantum yield of tryptophan radical formation depends upon the presence of tyrosine residues in the protein.  A tyrosine-deficient mutant exhibits the highest quantum yield, and the presence of a single tyrosine residue reduces the quantum yield by 20%.  Wild-type azurin with two tyrosine residues exhibits a further reduction in quantum yield of 44% relative to absence of tyrosine.  These results suggest that proton-coupled electron transfer takes place from tyrosine to tryptophan in azurin, and that the local environment of the tyrosine residue is important for the efficiency of this reaction.   Our findings highlight the importance of amino acid radical intermediates in mediating electron and proton transfer reactions in proteins.

Electron and proton transfer from tyrosine and tryptophan residues in azurin

248th National Meeting (2014)

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