Enthalpy of Dilution and Intermolecular Self-association of N⁶-Methyland N⁶, N⁶-Dimethyladenosines in Aqueous Solution

Abstract

Determination of the thermodynamic functions (parameters) of the intermolecular self-association of N⁶-monomethyladenosine (m⁶ Ado) and N⁶, N⁶-dimethyladenosine (m₂⁶ Ado) in water has been made by solution calorimetric dilution measurements at 298.15K. The standard enthalpy change ΔH° and standard entropy change ΔS° of self-association were as follows; ΔH°: -43∼-50 and -38±3kJ mol^-1, ΔS°: -122∼-147 and -100±11kJ K^-1 mol^-1 for m⁶ Ado and m₂⁶ Ado, respectively. From these values, it is clear that the methylation, especially the dimethylation, of N⁶-amino group of adenosine leads to a higher stacking equilibrium quotient than that of adenosine in consequence of an increased value of ΔS°. This supports that intermolecular association of N⁶-methylated adenosines is partly due to a contribution of substituted methyl groups to the formation of hydrophobic bond.