The blue-copper proteins azurin from Pseudomonas aeruginosa, azurin from Alcaligenes denitrificans, and the mutant Met121Gln of azurin from Alcaligenes denitrificans have been investigated by resonance Raman spectroscopy. Large deuterium isotope shifts up to 6 cm(-1) have been observed for bands in the 350-460 cm-1 range after incubation of the apoproteins in D2O and subsequent reconstitution with copper. The shifts derive from the deuteration of the amide hydrogens of Asn47 and Phe114 that form a hydrogen bond with the sulfur of the copper-coordinated cysteine. This observation reveals the coupling of the copper-sulfur and sulfur-hydrogen vibrations and indicates that these amide groups have to be taken into account to properly describe the electronic structure of the blue-copper site.