Stability constants of metal ion complexes formed with N3-deprotonated uridine in aqueous solution

Abstract

The acidity constant for the deprotonation of the (N3)H site of uridine (Urd) and the stability constants of the M(Urd–H) + complexes with the metal ions M 2+=Mg 2+, Ca 2+, Mn 2+, Zn 2+ or Cd 2+ have been determined by potentiometric pH titrations in aqueous solution at 25 °C and I=0.1 M (NaNO 3). The stability constant of the Pb(Urd–H) + complex could not be measured due to the hydrolysis of Pb(aq) 2+. It is shown that previously determined and compiled stability constants are far too large; the reason being most likely the neglect of M(aq) 2+ hydrolysis, at least in the cases of Mn 2+, Zn 2+ and Cd 2+. Because of the low stability of the alkaline earth ion complexes, it is suggested that in these instances outersphere species are formed which most likely also involve the C2 and/or C4 carbonyl oxygens of the uracil residue.