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 Co 2+ , Ni 2+ , Cu 2+ , or Zn 2+ (M 2+ ) have been determined by potentiometric titrations (Table 1). Changes in stability are quantified by Δlog K M = log K   M(bipy)(UTP) M(bipy) –log K   M(UTP) M . For the Co 2+ , Ni 2+ , Cu 2+ , and Zn 2+ 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 Co 2+ , Cu 2+ , and Zn 2+ , UTP 4− binds more tightly to M (bipy) 2+ than to M 2+ . An UV. difference spectroscopic study suggests that stacked adducts between bipyridyl and the pyrimidine moiety of uridine are formed. 1 H‐NMR. studies of the bipy/uridine, bipy/UTP, and bipy/UTP/Zn 2+ 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.