C-13 NMR Study of The Synergistic Anion In Transferrins

13C NMR spectra of AI(III), Ga(III), and Zn(I1) derivatives of both human serum transferrin and ovotransferrin with I3C-enriched bicarbonate or oxalate as synergistic anions have been measured at 75.4 MHz. The good quality of the spectra allows us to get detailed information on the state of the synergistic anion. The chemical shift values of the bound anion are interpreted on the basis of the effects arising from the interaction with the protein and the metal and through comparison with simple inorganic complexes. Our ”C NMR spectral data support the idea that the synergistic anion bridges the metal to some positively charged residue of the protein in agreement with the interlocking-sites model proposed by Bates. In the case of the bicarbonate derivatives, chemical shift values suggest carbonate as the form of the bound anion; the arrangements of the anion in the N-terminal and C-terminal sites appear to be identical within the resolution of the technique. The spectral data of the tripositive metal ion derivatives, with oxalate as synergistic anion, show a strong inequivalence between the two carboxylate carbons and are indicative of oxalate monodentate binding to the metal. Oxalate arrangements in the two sites are very similar but not identical. Differences in 13C NMR spectral data between tripositive and bipositive metal derivatives are also discussed.