Spectroscopic studies of nickel(II) carbonic anhydrase and its adducts with inorganic anions

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Abstract
Nickel(II) carbonic anhydrase, NiBCA II, and its adducts with nitrate, acetate, cyanate and azide, have been investigated through 1H NMR and electronic absorption spectroscopies. From the pH dependence of the molar absorbance the acidity constants of NiBCA were determined. The anions bind to the metal ion forming 1 : 1 adducts, and the corresponding affinity constants have been determined. The 1H NMR spectra of NiBCA and its adducts have been recorded, and the signals corresponding to the meta-like protons of the co-ordinated histidines followed by 1H NMR titration. The T1 values of these signals were measured and resonance assignments made based on nuclear Overhauser enhancement experiments. The co-ordination geometry of the metal ion in NiBCA and its adducts is discussed on the basis of the temperature dependence of the isotropic shifts, molar absorbance, and longitudinal relaxation times.