Non-Covalent Interaction in Binary Thymidine/Polyamine Systems in Aqueous Solution

Results of equilibrium and NMR spectral studies have shown the formation of molecular complexes in the systems of thymidine with polyamines—ethylenediamine, 1,3-diaminopropane, putrescine, 3-aza-1,5-diaminopentane, 3-aza-1,6-diaminohexane, 4-aza-1,7-diaminoheptane, spermidine, 4,8-diaza-1,11-diaminoundecane or spermine. The overall stability constants of the adducts and the equilibrium constants of their formation have been determined. Relative to adenosine or cytidine, the pH range of complex formation is shifted towards higher values, which is a consequence of a significantly higher basicity of thymine and corresponds well with the assumed model of ion-ion or ion-dipole interactions. The pH range of adduct formation is found to coincide with that in which the polyamine is protonated and the thymidine deprotonated. The -NH₃⁺ groups from polyamine and the N(3) atom from thymidine have been identified as the centers of noncovalent interactions. The stability of the molecular complexes formed in the studied systems depends on the acid-base character of the substrates and on the structure of the reacting molecules.