Sequential bond energies of water to sodium proline cation

Absolute bond dissociation energies of water to sodium proline cations and proline to hydrated sodium cations are determined experimentally by collision-induced dissociation of Na+Pro(H2O)x, where x = 1–4, complexes with xenon in a guided ion beam mass spectrometer. Experimental results show that the binding energies of water and proline to the complexes decrease monotonically with increasing number of water molecules. Ab initio calculations at three different levels show reasonable agreement with the experimental bond energies of water and proline for x = 1–4 except the theoretical values are higher for losing proline from x = 3 and 4. The primary binding site for Na+ is at the C terminus of proline for x = 0–4, i.e., the solvated sodiated proline complexes are in their zwitterionic forms. Calculations suggest that the first solvent shell of Na+Pro is essentially complete at four waters.