Hydration energies of deprotonated amino acids from gas phase equilibria measurements.

Singly hydrated clusters of deprotonated amino acids were studied using an electrospray high-pressure mass spectrometer equipped with a pulsed ion-beam reaction chamber. Thermochemical data, DeltaH(o), DeltaS(o), and DeltaG(o), for the hydration reaction [AA - H](-) + H(2)O = [AA - H](-).(H(2)O) were obtained from gas-phase equilibria determinations for AA = Gly, Ala, Val, Pro, Phe, Lys, Met, Trp, Gln, Arg, and Asp. The hydration free-energy changes are found to depend significantly on the side-chain substituents. The water binding energy in [AA - H](-).(H(2)O) increases with the gas-phase acidity of AA. The anionic hydrogen bond strengths in [AA - H](-).(H(2)O) are compared with those of the cationic bonds in the corresponding AAH(+).(H(2)O) systems.