Investigation of association in purine—pyrimidine aqueous systems by ¹H NMR chemical shift

The association behaviour of purine and pyrimidine has been studied in deuterium oxide at 35°C by measuring the concentration‐dependent selective changes in the chemical shifts of the CH protons. A satisfactory interpretation of the cross‐interaction, i.e. the mutually induced upfield shifts in mixed solution, required the extension of the 1:1 hetero‐association model by taking into account the self‐association of both compounds. The self‐association and the hetero‐association parameters have been determined on the basis of two‐state and three‐state models. The equilibrium constants give a decreasing tendency for association within the series purine‐purine (0.82 ± 0.08 1 mol −1 ) > purine‐pyrimidine (0.54 ± 0.06 1 mol −1 ) > pyrimidine‐pyrimidine (0.050 ± 0.002 1 mol −1 ), and the values of the dimer upfield shifts suggest that the association processes are dominated by the interaction of the six‐membered rings.