A spectroelectrochemical method has been used to determine the reduction potential of the copper site in wild-type and 22 mutant forms of azurin from Pseudomonas aeruginosa at 25 degrees C and in the range pH 4-8; the effect of buffers and ionic strength on the potentials has also been studied. Amino-acid residues changed include Met121, which provides an S atom at a distance of about 0.3 nm from the metal, some amino acids in the hydrophobic patch, other residues believed to be important in electron transfer with physiological partners and some internal amino acids. The observed potentials span a range of about 300 mV. In all cases the potentials increase with decreasing pH, but the pKa values describing the pH dependence are essentially unchanged except in three mutants, where they change by pH 0.6-1.1 (up in one and down in two). The largest potential changes were found in some Met121 mutants, at which position large hydrophobic residues raise the potential, whereas negatively charged residues lower it; a decreased potential is also found in the Met121-->End mutant, which probably has H2O coordinated to the metal. Gly45 has its carbonyl group coordinated to copper, but the potential of Gly45-->Ala is close to that of the wild type. Some substitutions in the hydrophobic patch cause an increase in the potential, whereas substitutions involving His35 and Glu91 do not result in significant changes. No single mechanism for tuning the potential of the copper site can be discerned, but in many cases there are probably indirect effects of the protein conformation causing changes in metal-ligand interactions.