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Normal coordinate analysis that utilizes a general valence force field and the Wilson FG matrix method has been applied to several structural models representing the active site of the blue copper protein, azurin. The models included tetrahedral and square planar CuN2SS', trigonal CuN2S, and trigonal bipyramidal CuN2SS'O structures in which the Ns are imidazole nitrogens of histidines, S is the thiolate sulfur ofcysteine, S' is the thioether sulfur ofmethionine, and 0 is a peptide carbonyl oxygen. For constant Cu-ligand bond lengths and initial force constants, the force field was refined against the most intense ofthe observed frequencies (424, 404, 369, and 261 cm-) in the resonance Raman spectrum of Pseudomonas aeruginosa azurin. The most satisfactory fit between observed and calculated frequencies occurs for tetrahedral and trigonal structures. The calculations provide detailed assignments for the resonance Raman spectrum of azurin and reveal considerable mixing of Cu-S(Cys) and Cu-N(His) vibrational modes. The trigonal model is favored because it is shown that the =260-cm-vibration is an invariant feature in the resonance Raman spectra ofblue copper proteins, even those lacking a methionine in the vicinity of the copper atom. The present analysis ascribes the high frequencies ofthe Cu-ligand stretching modes and the resonance enhancement to the coupled nature of their vibrations and the Franck-Condon overlaps with predominant (Cys)S--Cu(II) charge transfer bands in the visible region.

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