Cryogenic-temperature (77 K) resonance Raman (RR) spectroscopy and 15N and 13C isotope substitutions were used to determine vibrational mode wavenumbers associated with CuIIÈCN-and CuIIÈSCN-units CuIIÈN 3 -, for azido, cyano and thiocyanato adducts of Pseudomonas aeruginosa Met121X (where X = Gly, Ala, Val or Leu) azurins, and to evaluate the structural inÑuences of Met121 mutation and binding of exogenous ligands on the type 1 Cu site. The spectra of Met121X azurins indicate that the Cu site retains its planar trigonal geometry upon Met121 mutations to Leu, Val and Ala, but undergoes a tetrahedral distortion with a slightly extended CuÈS(Cys) bond upon mutation to Gly. The protein-anion adducts displayed RR spectra characteristic of the strongly tetrahedrally distorted Cu centers with dramatically reduced (by 50-70 cm-1) stretching wavenumbers and, m(Cu-S Cys ) consequently, greatly diminished Cu-S(Cys) interactions when compared with the uncomplexed proteins. Two azide-related modes in the adduct are observed at 375 and 2024 cm-1 and assigned as the M121A-N 3 m(Cu-N 3 ) and stretching modes based upon isotopic shifts (Ô8 and Ô67 cm-1, respectively). Their counterm as (N 3 ) 15¿14N 3 parts in the Gly, Val and Leu variants occur near 371 and 2045 cm-1, suggesting a weaker azide interaction with the Cu site than in the Ala variant. A nearly complete set of the thiocyanato-bound vibrational wavenumbers a †orded by RR spectra of M121V-SCN-permitted accurate band assignments based on the observed S 13CNand SC 15Nisotope shifts and the results of normal coordinate analysis. In particular, the characteristic m(S-C) (ca. 720 cm-1) and m(CyN) (ca. 2100 cm-1) stretching wavenumbers establish that SCN-binds to the metal via the S atom to give a planar CuÈSÈCyN moiety. We also Ðnd that CN-coordinates to the copper via its carbon atom giving rise to two 13CN-sensitive weak bands at approximately 277 and 398 cm-1, assigned as the m(Cu-C) stretch and the d(Cu-CyN) bend, respectively.