Abstract
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–log K. 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.