Evidence for direct metal-nitrogen binding in aromatic sulfonamide complexes of cadmium (II)-substituted carbonic anhydrases by cadmium-113 nuclear magnetic resonance.

113Cd NMR has been used to study the nature of the metal-sulfonamide interaction in complexes of 113Cd-substituted carbonic anhydrases and the aromatic sulfonamides benzenesulfonamide and Neoprontosil. The 113Cd chemical shifts of the complexes exhibit negligible dependence on the isotopic composition of the sulfonamide nitrogen. However, benzenesulfonamide and Neoprontosil, when 90% 15N-enriched at the sulfonamide nitrogen, each split the 113Cd resonance into a doublet (J113Cd-15N approximately or equal to 200 Hz). This constitutes evidence for metal-nitrogen bonds in these complexes. The chemical shifts of the complexes (approximately 390 ppm) and their pH independence from pH 7.0 to 10.0 suggest the sulfonamides are bound in the anionic form. The resonance Raman (RR) spectra of 15N-labeled and unlabeled Neoprontosil have been obtained to clarify the report of anomalous Neoprontosil binding in the nonionized form [Petersen, R. L., Li, T.-Y., McFarland, J. T., & Watters, K. L. (1977) Biochemistry 16, 726]-a report based on the assignment of a band at approximately 900 cm-1 to a sulfonamide S-N stretching vibration. We find the frequencies of Raman bands observed in the range 800-1700 cm-1 to be virtually identical for the 15N-labeled and unlabeled molecules, indicating that none of the bands can be assigned to a S-N stretching vibration. The RR data unambiguously show the report of Petersen et al. (1977) is based on the misassignment of the band at approximately 900 cm-1.