On the Position of the Intramolecular Equilibrium between Opened and Aromatic‐Ring Stacked Forms in Ternary Complexes Composed of Adenosine 5′‐triphosphate, Mg²⁺ or Zn²⁺, and <scp>L</scp>‐Tryptophanate, and in Related Ternary Systems

Abstract

Abstract The intramolecular and dimensionless equilibrium constant, K ′, for the equilibrium between open and aromatic‐ring stacked isomers of ternary complexes formed between adenosine 5′‐triphosphate, inosine 5′‐triphosphate, or uridine 5′‐triphosphate, Mg 2+ or Zn 2+ , and L ‐tryptophanate, 2,2′‐bipyridyl (bipy), or 1,10‐phenanthroline (phen) have been estimated by 1 H‐NMR. shift measurements in D 2 O (I = 0.1 M , NaNO 3 ; 27°). The approximate equilibrium constants K ′ are ≥ 20, ≥ 20, 1.2, 0.9, 0.7, 0.5, and 0.3 for the ternary complexes Zn (phen) (ATP) 2− , Mg (phen) (ATP) 2− , Zn (bipy) (ATP) 2− , Zn (bipy) (ITP) 2− , Zn (bipy) (UTP) 2− , Zn (trp) (ATO) 3− , and Mg (trp) (ATP) 3− , respectively. The percentages of the stacked isomers decrease in the order Zn (phen) (ATP) 2− (≥ 95%) ≫ Zn (bipy) (ATP) 2− (55%) &gt; Zn (bipy) (ITP) 2− (48%) &gt; Zn (bipy) (UTP) 2− (∼ 40%), which coincides with the order of stability of the binary metal‐free adducts (phen) (ATP) 4− &gt; (bipy) (ATP) 4− ≃ (bipy) (ITP) 4− &gt; (bipy) (UTP) 4− ; the corresponding stability constants are K = 28.2 ± 4.7, 8.1 ± 2.6, 8.8 ± 1.8, and ∼ 1 ( M −1 ), respectively (D 2 O; I = 0.1 M , NaNO 3 ; 27°): these series reflect the decreasing size of the aromatic‐ring systems forming the stacks. The indole moiety of tryptophan resembles 2,2′‐bipyridyl rather than 1,10‐phenanthroline and K = 6.2 ± 1.8 M −1 for (trp) (ATP) 4− . Implications with respect to the stability of ternary complexes, and to biological systems are indicated briefly.