Binding ability of sialic acid towards biological and toxic metal ions. NMR, potentiometric and spectroscopic study

The binary complexes of 5-amino-3,5-dideoxy-D-glycero-D-galactononulosic acid (NANA), commonly called N-Acetyl Neuraminic Acid, formed with biological metal ions such as Co(II) and Cu(II) and toxic metal ions such as Cd(II) and Pb(II) are investigated in aqueous solution by means of potentiometry, UV and NMR spectroscopy. The corresponding ternary systems with 2,2’-bipyridine are studied in aqueous solution by potentiometry and UV spectroscopy.NANA co-ordinates all metal ions, in both binary and ternary systems trough carboxylic group (protonated or deprotonated according to pH condition), pyranosidic ring oxygen and glycerol chain alcoholic hydroxy groups. The prevailing species in the pH range 2-7 are [M(NANA)2] type, their stability constants are greater than those of simple carboxylate complexes. Over pH 7 the species [M(NANA)2OH]- are also formed, but they don’t prevent the precipitation of metal hydroxides. This work provides information on the solution state chemistry of NANA in presence of bivalent metal ions; its great affinity for Cd(II) and Pb(II) toxic metals, near physiological condition, and the relatively high stability of the complex species found may account also for toxicity mechanism.