The self-association of naturally occurring purine nucleoside 5′-monophosphates in aqueous solution

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 ΔH0 = −14.5 kJ mol−1 and ΔS0 = −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.