Photogeneration and Quenching of Tryptophan Radical in Azurin

J Phys Chem B. 2015 Jul 23;119(29):9438-49. doi: 10.1021/jp511523z. Epub 2015 Feb 17.


Tryptophan and tyrosine can form radical intermediates that enable long-range, multistep electron transfer (ET) reactions in proteins. This report describes the mechanisms of formation and quenching of a neutral tryptophan radical in azurin, a blue-copper protein that contains native tyrosine (Y108 and Y72) and tryptophan (W48) residues. A long-lived neutral tryptophan radical W48• is formed upon UV-photoexcitation of a zinc(II)-substituted azurin mutant in the presence of an external electron acceptor. The quantum yield of W48• formation (Φ) depends upon the tyrosine residues in the protein. A tyrosine-deficient mutant, Zn(II)Az48W, exhibited a value of Φ = 0.080 with a Co(III) electron acceptor. A nearly identical quantum yield was observed when the electron acceptor was the analogous tyrosine-free, copper(II) mutant; this result for the Zn(II)Az48W:Cu(II)Az48W mixture suggests there is an interprotein ET path. A single tyrosine residue at one of the native positions reduced the quantum yield to 0.062 (Y108) or 0.067 (Y72). Wild-type azurin with two tyrosine residues exhibited a quantum yield of Φ = 0.045. These data indicate that tyrosine is able to quench the tryptophan radical in azurin.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Azurin / chemistry*
  • Azurin / genetics
  • Cobalt / chemistry
  • Electrons
  • Mutation
  • Photolysis
  • Pseudomonas aeruginosa
  • Spectrum Analysis
  • Tryptophan / chemistry*
  • Tyrosine / chemistry


  • Azurin
  • Cobalt
  • Tyrosine
  • Tryptophan