Osmometric studies on self-association of pyrimidines in aqueous solutions: evidence for involvement of hydrophobic interactions.

Vapour pressure osmometric studies were performed on stacking self-association of 25 uracil derivatives variously C- and N-substituted with polar and alkyl groups in aqueous solution at various temperatures. The respective equilibrium association constants Kst were computed on the assumption of the isodesmic model of self-association (K2=K3=...=Kn=Kst). Enthalpies of association for most of the compounds studied were obtained from the temperature-dependence of Kst, according to the van 't Hoff equation. Analysis of the equilibrium and thermodynamic parameters in terms of the association mechanism demonstrated the involvement of classical hydrophobic interactiors in the stabilization of complexes of di-and higher alkylated uracils. Data for the derivatives substituted with polar groups proved consistent with the predominant involvement of dipole-induced dipole forces in the association.