| Abstract | The parameters characterizing the base-stacking self-association of adenosine, inosine, and guanosine 5′-monophosphate have been obtained from ¹H nmr dilution studies. The thermodynamic parameters for the formation of adenosine 5′-monophosphate stacks are ΔH° = −14.5 kJ mol⁻¹ and ΔS° = −42.3 J K⁻¹ mol⁻¹, with an apparent equilibrium constant of Kc = 1.92 M⁻¹ at 30 °C. The corresponding equilibrium constants for the self-association of inosine and guanosine 5′-monophosphate are 1.36 M⁻¹ and 1.29 M⁻¹, respectively. The negative enthalpy and entropy changes cannot be explained by the concept of classical hydrophobic interactions; however, they strongly support the conclusion that dipole induced dipole forces play a major role for base-stacking in aqueous solution. The sequence of the equilibrium constants for the purine nucleoside 5′-monophosphates can be well explained by the concept of mutual polarization. The stacking geometries for adenosine and inosine 5′-monophosphate are presented as obtained from fitting the experimental shift data to refined isoshielding contours. It is concluded that the stacking pattern is not restricted to a unique geometry. |
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