Resonance raman study of metal-formazans: A model for complexes of metalloenzymes with chelate inhibitors

The resonance Raman (RR) spectra of a series of triaryl formazans and several of their metal complexes have been recorded. The spectra of the free formazans display several bands of weak to medium intensity that are unaffected by physical state or metal complex formation. However, strong bands in the 1050–1200 cm⁻¹ and 1330–1430 cm⁻¹ regions are sensitive to both the physical state of the formazan and to the formation of metal complexes. This observation, coupled with the intensity of these bands, leads to the conclusion that they are fundamentals of CN, φN, CN, and NN stretching character that are diagnostic of conformation of the formazyl linkage. The bands ascribed to ν(CN) and ν(NN) in the 1330–1430 cm⁻¹ region may also be used as indicators of the formation of metal complexes.

The results of the RR studies of the free and complexed formazans are compared with spectra obtained for the inhibitor complex formed between liver alcohol dehydrogenase and 2-carboxy-2′-hydroxy-5′-sulfoformazylbenzene (zincon). Similarities between the spectra of zincon bound to the enzyme and the various model complexes lead to the conclusion that zincon is coordinated to the enzyme zinc atom but that the formazyl linkage is in an “open” rather than “closed” conformation on the enzyme surface, whereas the simple zinc complex is in the “closed” conformation.