Mass-spectrometric investigations on hydration of nucleic acid components in vacuum.

Association reactions between water and alkylated uracils. occurring under field-ionization conditions in a mass spectrometer at the tungsten point emitter surface, were studied at several temperatures. The origin of peaks observed in the mass spectra at m/e ratios corresponding to M+H and M+H-H2O were attributed to M-H2O and M-(H2O)2 hydrates, respectively, hydrogen-bonded via carbonyl groups of the diketopyrimidines (M) investigated. The appearance of these ions is explained in terms of the field-ionization mechanism of the neutral hydrates involving intramolecular H+ transfer and concomitant release of the OH+ radical. Measurements of the relative peak intensities allowed the calculation of apparent equilibrium constants. K(ass). for the association reactions, and hence the respective van't Hoff enthalpies of hydrat'on. The latter are discussed in connection with the available quantum-mechanical hydration energies for specific groups of respective canonical nucleic acid bases and experimental enthalpies of hydration of alkylated uracils with water. Specific hydration is estimated to contribute about 15-20% to the total enthalpy of interaction of the solutes with their hydration shells.