Environment of copper in Pseudomonas aeruginosa azurin probed by binding of exogenous ligands to Met121X (X = Gly, Ala, Val, Leu, or Asp) mutants

Electron and optical paramagnetic resonance spectroscopic analyses of the binding of exogenous ligands to the Met121X mutant of Pseudomonas aeruginosa azurin indicate that small molecules bind to copper through the cavity formed by the side chain of Met121. The strong absorption bands of azurin develop due to S-to-Cu charge transfer transitions. Small carboxylic acids react strongly with metal ions and copper attains a type 1.5 character. The EPR spectra of azurin are discussed.