Microhydration of protonated nucleic acid bases and protonated nucleosides in the gas phase.

Thermochemical data, DeltaH(o)(n), DeltaS(o)(n), and DeltaG(o)(n), for the hydration of protonated nucleic acid bases and protonated nucleosides have been experimentally studied by equilibrium measurements using an electrospray high-pressure mass spectrometer equipped with a pulsed ion-beam reaction chamber. For protonated nucleobases the hydration enthalpies were found to be similar for all studied systems and varied between 12.4-13.1 kcal/mol for the first and 11.2-11.5 kcal/mol for the second water molecule. While for protonated nucleosides the water binding enthalpies (11.7-13.3 kcal/mol) are very close to those for protonated nucleobases, the entropy values are "more negative." The structural and energetic aspects of hydrated ions are discussed in conjunction with the available theoretical data.