In aqueous solutions containing equimolar concentrations of cytidine and glycylglycine, the presence of zinc chloride results in the formation of a ternary complex, in which cytidine is bound to the metal ion through N₃, and the metal ion in turn is chelated to glycylglycinate through the amino and amide groups, leaving the carboxylate anion uncomplexed. When the zinc salt is replaced by mercury(II) nitrate, no ternary complex is formed; only the binary complexes mercury(II)-cytidine and mercury(II)-glycylglycinate. In the absence of glycylglycine, the cytidine H₅ and H₆ signals are shifted in opposite directions on the addition of mercury(II) nitrate, and this is attributed to an increase in restriction of rotation of the amino group because of mercury(II) complexation. In the presence of glycylglycine, the extent of mercuration of cytidine decreases because of the competition for mercury(II) by glycylglycinate to form the peptide complex.