High-potential states of blue and purple copper proteins

Electrochemical measurements show that there are high-potential states of two copper proteins, Pseudomonas aeruginosa azurin and Thermus thermophilus Cu_A domain; these perturbed states are formed in guanidine hydrochloride (GuHCl) solution in which the proteins are still blue (azurin) and purple (Cu_A). In each case, the high-potential state forms reversibly. Absorption (azurin, Cu_A), visible circular dichroism (azurin, Cu_A), resonance-Raman (Cu_A), and EPR (Cu_A) spectra indicate that the structure of the oxidized copper site of each high-potential form is very similar to that of the native protein. It is proposed that GuHCl perturbs one or more H-bonds in the blue or purple copper active site, thereby allowing Cu(I) to adopt a more favorable coordination structure than that in the rigid cavity of the native protein