A series of metalloprotein mutants with novel copper cysteinate coordination environments has been probed by resonance Raman (RR) spectroscopy. These include H117G, M121E, and M121K mutants of Pseudomonas aeruginosa azurin and H46C, H80C, and H120C mutants of yeast CuZn-superoxide dismutase. In each case, excitation within a (Cys)S --> Cu charge transfer band leads to the enhancement of multiple vibrational modes of the copper cysteinate moiety. The predominant Cu-S stretching vibration, v(Cu-S), located in the 300-450 cm(-1) region, can be identified by (i) its large S- and Cu-isotope shifts, (ii) its high RR intensity, and (iii) its role as the generator of combination bands. The v(Cu-S) frequency appears to be a sensitive indicator of Cu-S(Cys) bond strength and, hence, copper coordination geometry. In the case of type 1 (T1) sites, the increased influence of the weak axial ligand upon moving from a trigonal planar (axial EPR) toward a more tetrahedral (rhombic EPR) geometry is associated with a decrease in v(Cu-S) from similar to 420 to similar to 350 cm(-1). In the case of type 2 (T2) sites with four strong ligands, v(Cu-S) undergoes further decreases from similar to 350 to similar to 310 cm(-1) as the geometry becomes more tetragonal. The Cu-S bond is successively weakened by trans ligand effects as the geometry approaches square planar. The decreased strength of the Cu-S(Cys) bond is further reflected in the increased strength of the adjacent S-C bond whose stretching frequency varies from similar to 750 cm(-1) for axial T1 sites to similar to 765 cm(-1) for T2 sites. The similar to 100-cm(-1) range in v(Cu-S) corresponds to a change in Cu-S(Cys) bond distance from similar to 2.13 Angstrom for an axial T1 site to similar to 2.29 Angstrom, for a tetragonal T2 site. The overlap of T1 and T2 v(Cu-S) frequencies near 350 cm(-1) shows that both types of Cu site can have similar Cu-S(Cys) bond strengths, despite their different EPR and optical characteristics, and points to a continuum of geometries linked through a tetrahedral structure which we describe as a T1.5 intermediate Cu site.