Oxalate as synergistic anion for Cd(II) binding to ovotransferrin

The Cd(II) derivative of ovotransferrin containing oxalate as synergistic anion was investigated through 113Cd- and 13C-NMR spectroscopy and compared with the analogous derivative obtained in the presence of bicarbonate. Cadmium loaded in the C site gives rise to a 113Cd-NMR signal at 54 ppm, while that bound to the N site is broadened beyond detection. The two resonances at 168.5 and 169.9 ppm observed in the 13C-NMR spectrum of the Cd2 derivative obtained with [13C]oxalate each correspond to slowly exchanging oxalate specifically bound to a single site. No splitting of these resonances due to 113Cd-13C magnetic coupling is observed upon insertion of 113Cd-enriched Cd(II) ion, unlike previous observations for the corresponding derivative obtained with bicarbonate as synergistic anion. It was found that the metal sites in the present derivative are inequivalent, as observed for other metal-transferrin-oxalate adducts. The C site is found to be sensitive to a residue ionization with a pKa of 9.5.