The electronic properties of Thermus thermophilus Cu-A in the oxidized form were studied by H-1 and C-13 NMR spectroscopy. All of the H-1 and C-13 resonances from cysteine and imidazole ligands were observed and assigned in a sequence-specific fashion. The detection of net electron spin density on a peptide moiety is attributed to the presence of a H-bond to a coordinating sulfur atom. This hydrogen bond is conserved in all natural Cu-A variants and plays an important role for maintaining the electronic structure of the metal site, rendering the two Cys ligands nonequivalent. The anomalous temperature dependence of the chemical shifts is explained by the presence of a low-lying excited state located about 600 cm(-1) above the ground state. The room-temperature shifts can be described as the thermal average of a sigma(u)* ground state and a pi(u) excited state. These results provide a detailed description of the electronic structure of the Cu-A site at atomic resolution in solution at physiologically relevant temperature.