Adenosine and Inosine 5′‐triphosphates

Abstract

The chemical shift of the protons, H(2) and H(8), in the proton magnetic resonance spectra of ATP, ITP, and inosine was measured in aqueous solution in dependence on pH (2 to 12). Based on these data the sites of protonation or deprotonation are identified and the acidity constants determined. A comparison of the constants due to the nucleotides with the corresponding ones of the nucleosides evidences that the basicity of the nucleic base increases somewhat under the influence of the triphosphate chain. The possibility of an intramolecular hydrogen bound between N(7) and the proton of the γ‐phosphate group is discussed. From the line broadening effects in the presence of Cu 2+ it is concluded that this metal ion interacts mainly with N(7) of ATP, but with the N(1), O(6)‐site of ITP. With inosine as ligand N(7) is the predominate binding site at pH <6, while at pH >7 N(1), O(6) is preferred. Experiments with ITP in the presence of [Cu 2+ ] = [2,2′‐bipyridyl] (under conditions where Cu(Bipy) 2+ is nearly completely formed) reveal the surprising fact that in the pH range 2 to 6 the line broadening of the signal due to H(8), and H(2) as well, increases significantly compared with a system containing only ITP and Cu 2+ . This observation is easily explained by the assumption of a chargetransfer interaction within the ternary complex, Cu(Bipy) (ITP) 2‐ , i.e. between the purine part of the nucleotide and 2,2′‐bipyridyl. This interaction could indeed be confirmed by spectrophotometric measurements in the ultraviolet (λ max = 313 nm). Similar results were obtained for the mixed‐ligand complex, Cu(Bipy) (ATP) 2‐ . The distance between the planes of the aromatic rings in these systems is estimated and a structure for these metal‐ion‐bridged charge‐transfer complexes is suggested. The biological implications of such ternary, metal‐ion‐bridged, charge‐transfer complexes are briefly indicated.