Binary and ternary complexes of metal ions, nucleoside 5′-monophosphates, and amino acids

Abstract

The aromatic-ring stacking interactions between the indole moiety of L -tryptophan and the purine or pyrimidine moieties of AMP 2− or CMP 2+, respectively, were studied by 1H NMR spectroscopy. Under the influence of increasing concentrations of L -trytophan the resonance of H-2, H-8 and H-1′ of AMP 2− of a H-5 and H-6 of CMP 2− are shifted upfield. Computer curve fitting of the shift data gave the stability constants K (AMP)(H·Trp) (AMP) = 6.83 ± 0.81 M −1 and K (CMP)(H·Trp) CMP = 0.77 ± 0.35 M −1 (in D 2O; 27°C; I = 0.1 M) which show that the purine moiety forms the more stable stacks. Repetition of the experiments under conditions where the amino acid exists as the anion, i.e. as tryptophanate, gave K (AMP)(Trp) (AMP) = 2.24 ± 0.29 M −1 and K (CMP)(Trp) (CMP) = 0.14 ± 0.05 M −1 (in D 2O; 27°C; I = 0.1–0.15 M). Hence, it is evident that the formation of an ionic bridge between the negatively charged phosphate moiety of the nucleotides and the positively charged ammonium group of L -tryptophan favors the formation of the stack between the nucleic bases and the indole residue. In the case with AMP 2− the stability constant of this ion pair formation could be estimated, i.e. K IP ≅ 0.6 M −1 (in D 2O; 27°C; I = 0.1–0.15 M), a value which compares well with K IP = 1.35 ± 0.76 M −1 for the (CH 3) 4N +/Trp − interaction. Intramolecular equilibrium constants for the ion pair interaction within the stacked complex and for the stacking interaction within the ion pair complex are also estimated and discussed. Stability constant of metal ion complexes could only be determined with Ni 2+ as a precipitate formed with Cu 2+ and Zn 2+. The stability constants of the binary systems, Ni 2+/AMP 2−, Ni 2+/CMP 2−, Ni 2+/Trp −, and Ni 2+/Ala −, were determined by potentiometric pH-titrations. The somewhat enhanced stability of Ni(Trp) 2 compared with Ni(Ala) 2, and of Ni(AMP) 2 2− compared with Ni(CMP) 2 2− (for which no evidence of formation was observed) is explained by self-stacking within these binary 1:2 complexes. Based on the data of the binary systems, the following ternary systems were studied and the corresponding stability constants determined: Ni 2+/AMP 2−/Trp −, Ni 2+/AMP 2−/Ala −, Ni 2+/CMP 2−/Trp −, and Ni 2+/CMP 2−/Ala −. A most likely intramolecular purine/indole stack in the ternary Ni(AMP)(Trp) − complex is not significantly reflected in the measured stability constants. This observation and the implications of stacking with regard to the stability of ternary complexes, as well as the general importance of “bridged” stacking adducts for biological systems are briefly discussed.