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) decrease with n. For the complexes. GlyH(+)(H2O), AlaH(+)(H2O)(n) and PheH+(H2O) with n <= 4, the measured enthalpy and entropy changes show that the first three H2O molecules bind to the hydrogen atoms of the - NH3+ group, while the fourth H2O forms the second hydration shell. In the case of ProH(+)(H2O) with n <= 3, two H2O molecules bind to the - NH2+ group, and the third one is in a second shell. (C) 2007 Elsevier B.V. All rights reserved.