Neoprontosil binding to carbonic anhydrase. Resonance Raman and other studies on the ionization behavior of the sulfonamide

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Abstract
Alkalimetric, spectrophotometric, NMR and resonance Raman titrations are reported for the sulfonamide Neoprontosil (azosulfamide) in aqueous solution. An assignment of the magnetic resonance peaks for each of the Neoprontosil protons was made. Neoprontosil has 2 coupled ionizable groups, -OH and -SO2NH2, in the pH range 10.5-11.5. The close proximity of the microscopic pK for these 2 groups precludes spectroscopic characterization of the separate -SO2NH2, -O- or -SO2NH-, -OH species. No conclusion about the ionization state of the drug when bound to carbonic anhydrase [EC 4.2.1.1] can be made. The resonance Raman spectrum of Neoprontosil bound to human carbonic anhydrase B at pH 9.5 shows a shift in the intense -N.dbd.N- stretching mode from 1414 (free) to 1407 cm-1 (bound). Apparently a slight conformational change about the -N.dbd.N- single bond linkages occurs upon binding.