Intramolecular Stacking in Ternary Complexes Containing Uridine 5′-Triphosphate, 2,2′-Bipyridyl,and a Divalent Metal Ion

The stability constants of the ternary complexes containing UTP, 2,2′-bipyridyl (bipy), and Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺ (M²⁺) have been determined by potentiometric titrations (Table 1). Changes in stability are quantified by Δlog K_M = log K–log K. For the Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ systems Δlog K_M is 0.10, ?0.13, 0.36, and 0.15, respectively. All these ternary complexes are considerably more stable than would be expected on statistical grounds; indeed, for Co²⁺, Cu²⁺, and Zn²⁺, UTP^4? binds more tightly to M (bipy)²⁺ than to M²⁺. An UV. difference spectroscopic study suggests that stacked adducts between bipyridyl and the pyrimidine moiety of uridine are formed. ¹H-NMR. studies of the bipy/uridine, bipy/UTP, and bipy/UTP/Zn²⁺ systems (Figs. 1 and 2) confirm the presence of stacking in the binary adducts and in the ternary complex. There is also evidence for the existence of the stacked protonated complex, Zn(bipy) (HUTP)^?, with the proton at the γ-phosphate group. The acidity constant of this ternary complex has been measured (Fig. 3). The observed stability enhancement of stacked adducts by the formation of a metal ion bridge is discussed (Fig. 4) and biological implications are indicated.