Luminescence Quenching in Azurin

Abstract

The luminescence of oxidized or reduced azurin, an intensely blue copper protein, has been observed. The addition of Cu 2+ or Hg 2+ reduced the luminescence of the apoprotein to one third of its value while Ag + reduced it to two thirds. This quenching is explained in terms of tryptophan distorsion produced by the different coordination of these metal ions. A singlet‐singlet internal conversion is postulated as the mechanism for luminescence quenching in all azurin derivatives. Copper is able to bind both the silver‐ and mercury‐treated protein, as shown by electron paramagnetic resonance experiments, without restoring the native blue color. Moreover addition of copper reduced the luminescence in the silver‐treated protein to that of the native protein; if NaCl is now added, the blue color is restored. This NaCl effect is absent in the mercury‐treated apoprotein. These results suggest that the blue color is related to the binding of copper to at least one of the ligands involved in the mercury or silver complexing, most probably a sulfur atom.