Nickel(II) and cobalt(II) derivatives of the blue copper protein Pseudomonas aeruginosa azurin have been studied by resonance Raman (RR) spectroscopy at liquid-nitrogen temperatures. Vibrational assignments for the observed RR bands of Ni(II)-azurin have been made through a study of Ni-62-substituted azurin. A comparison of Ni(II)azurin RR spectra with those of the wild type (Cu-containing) protein showed Ni(II)-S(Cys) stretching vibrations, v(Ni-S)(Cys), at substantially lower frequencies (similar to 360 versus similar to 400 cm(-1), respectively), indicating that the Ni(II)S(Cys) bond is much weaker than the corresponding Cu(Il)-S(Cys) bond. Resonance enhanced predominantly v(Ni-N)(His) modes indicate that the metal-N(His) bond distances in the Ni(II) derivative are the same as those in native azurin. The vibrational data also confirm a tetrahedral disposition of ligands about the metal in Ni(Il)-azurin found in the protein crystallographic structures. As expected, excitation profile measurements on Ni(II)-azurin show that the v(Ni-S)(Cys) assignable modes give maxima at the 440-nm absorption band, which confirms a S(Cys) -> Ni(II) charge-transfer origin of the 440-nm electronic transition in Ni(II)-substituted azurin.