Probing the Bioinorganic Chemistry of Cu(I) with ¹¹¹Ag Perturbed Angular Correlation (PAC) Spectroscopy

The two most common oxidation states of copper in biochemistry are Cu(II) and Cu(I), and while Cu(II) lends itself to spectroscopic interrogation, Cu(I) is silent in most techniques. Ag(I) and Cu(I) are both closed-shell d(10) monovalent ions, and to some extent share ligand and coordination geometry preferences. Therefore, Ag(I) may be applied to explore Cu(I) binding sites in biomolecules. Here, we review applications of Ag-111 perturbed angular correlation (PAC) of gamma-ray spectroscopy aimed to elucidate the chemistry of Cu(I) in biological systems. Examples span from small blue copper proteins such as plastocyanin and azurin (electron transport) over hemocyanin (oxygen transport) to CueR and BxmR (metal-ion-sensing proteins). Finally, possible future applications are discussed. Ag-111 is a radionuclide which undergoes beta-decay to Cd-111, and it is a gamma-gamma cascade of the Cd-111 daughter nucleus, which is used in PAC measurements. Ag-111 PAC spectroscopy may provide information on the coordination environment of Ag(I) and on the structural relaxation occurring upon the essentially instantaneous change from Ag(I) to Cd(II).