Hydration energies of protonated amino acids

Sequential hydration thermochemical data for the protonated amino acids, AAH+ (AA = Gly, Ala, Phe and Pro), were obtained using a pulsed ion-beam high-pressure mass spectrometer. The hydration bond energies for AAH+(H2O)n decrease with n. For the complexes, GlyH+(H2O)n, AlaH+(H2O)n and PheH+(H2O)n with n ⩽ 4, the measured enthalpy and entropy changes show that the first three H2O molecules bind to the hydrogen atoms of the – NH 3 + group, while the fourth H2O forms the second hydration shell. In the case of ProH+(H2O)nwith n ⩽ 3, two H2O molecules bind to the – NH 2 + group, and the third one is in a second shell.