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, ΔHo, ΔSo, and ΔGo, for the hydration reaction [AA − H]− + H2O = [AA − H]− · (H2O) 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]− · (H2O) increases with the gas-phase acidity of AA. The anionic hydrogen bond strengths in [AA − H]− · (H2O) are compared with those of the cationic bonds in the corresponding AAH+ · (H2O) systems.