Mixed-ligand complexes of the type M(ATPXAa)3-, where M2+ = Mn2+, Cu2+, Zn2+, Cd2+, or Pb2+ and Aa- = alaninate, 2-aminopropionate (α-aminobutyrate), norvalinate, norleucinate, leucinate (leu), or isoleucinate, have been studied by potentiometric pH titrations and 1H NMR; some earlier results on M(ATP)(tryptophanate)3- have been included for comparison. The potentiometric measurements (with Mn2+, Cu2+, and Zn2+) reveal a slightly higher formation tendency, expressed as [formula-ommitted]for the systems with leucinate compared to those with alaninate. This increase in stability is attributed to an intramolecular hydrophobic ligand-ligand interaction between the purine moiety of ATP4- and the isopropyl residue of leucinate. The position of the intramolecular isomeric equilibrium between an “open” and “closed” form, in which the hydrophobic interaction occurs, was estimated: of the ternary M(ATP)(leu)3~ complexes with Mn2+, Cu2+, and Zn2+ about 41, 21, and ~5% (to ~30%) exist in the folded, i.e. closed, form. The intramolecular aromatic-ring stacking interactions between the purine moiety of ATP4~ and the indole residue of tryptophanate (trp”) in the M(ATP)(trp)3- complexes is (as expected) more pronounced: of the complexes with Mn2+, Cu2+, and Zn2+ about 52, 35, and 74% exist in the stacked form. 1H NMR shift measurements of the mentioned ATP/amino acid systems in the absence and presence of Zn2+, Cd2+, or Pb2+ confirmed that such hydrophobic ligand-ligand interactions exist and that they are effectively promoted by the formation of a metal ion bridge between the two reactants; to some extent a promotion can also occur by the polar interactions between the ammonium group of the amino acid and the phosphate chain of the nucleotide. For the metal ion facilitated interaction: the longer the side chain of the aliphatic amino acid, the larger is the upfield shift of the terminal methyl group(s) of the amino acid side chain, resulting from the interaction with the aromatic purine moiety within the ternary complex. The AG0 values calculated from the equilibrium constants agree well with the theoretical predictions for such interactions. It is also shown that mixed-ligand complexes of the mentioned kind exist in the physiological pH range and that the formation of a metal ion bridge increases the probability for “recognition” between two species; this result is fascinating regarding the specificity and selectivity observed in nature.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry