Interaction and coordination geometries for Ag(I) in the two metal sites of hemocyanin.

111Ag(I) perturbed angular correlations of gamma-rays (PAC) has been used to investigate the binuclear metal site of 111Ag(I)-substituted Carcinus aestuarii deoxyhemocyanin. The studies have shown that apo-hemocyanin is able to bind 2 mol of Ag(I) per mol of protein and that the binding is specific for the metal ion sites. The PAC spectra show pronounced changes when the stoichiometry of Ag(I) to protein is increased from 0.1 to 2.0. These changes have been interpreted as evidence of interactions between the two sites in terms of a structural destabilization of the first occupied site caused by the occupation of the second site. The experimental data for the Ag(I)-substituted metal sites do not agree well with the three-coordinated structure found in the Cu(I) holo-protein. However, if a water molecule is included as a coordinating ligand in the Ag(I) metal site a successful interpretation of the experimental data can be obtained.