Hydration of Potassiated Amino Acids in the Gas Phase

The thermochemistry of stepwise hydration of several potassiated amino acids was studied by measuring the gas-phase equilibria, AAK+(H2O)n−1 + H2O = AAK+(H2O)n (AA = Gly, AL, Val, Met, Pro, and Phe), using a high-pressure mass spectrometer. The AAK+ ions were obtained by electrospray and the equilibrium constants Kn−1,n were measured in a pulsed reaction chamber at 10 mbar bath gas, N2, containing a known partial pressure of water vapor. Determination of the equilibrium constants at different temperatures was used to obtain the ΔHno, ΔSno, and ΔGno values. The results indicate that the water binding energy in AAK+(H2O) decreases as the K+ affinity to AA increases. This trend in binding energies is explained in terms of changes in the side-chain substituent, which delocalize the positive charge from K+ to AA in AAK+ complexes, varying the AAK+-H2O electrostatic interaction.