MULTINUCLEAR NMR AND POTENTIOMETRIC STUDIES ON THE INTERACTION OF ZINC AND CADMIUM WITH CYTIDINE AND GLYCYLGLYCINE - THE EFFECT OF THE ANION

The stability constants for binary complexes formed between Zn2+ and Cd2+ With cytidine and glycylglycine and for the ternary complexes with the same ligands were determined by spectroscopic (H-1 and C-13 NMR) and, except those involving Zn2+ and cytidine, by potentiometric pH titrations in aqueous solution. The calculated values show that cadmium binds much more strongly to cytidine than zinc, and that a small but significant increase in the stability of zinc/cytidine interaction, but not for cadmium/cytidine, is observed in the presence of glycylglycine. The chemical shifts observed for cytidine with zinc show a marked dependence on the anion, being much larger for the chloride salt than for the nitrate and increase upon addition of glycylglycine, although the calculated stability constants are identical for both salts. Finally, the choice of reference chemical shifts used in the spectroscopic titrations and the effect of charged species on the values of die acidity constants are discussed.