Hydration of gas-phase protonated alkylamines, amino acids and dipeptides produced by electrospray

An electrospray ionization ion source is described that produces ions continuously at 1 atm and introduces them into a temperature-variable reaction chamber in the pulsed mode where the ion–molecule equilibria can be determined in the gas phase at 10 mbar. Technical aspects and operating parameters of the technique are presented. The equilibrium constants for the hydration reactions B+(H2O)n−1 + H2O = B+(H2O)n, where B+ = (CH3)2NH2+, C2H5NH3+, n-C4H9NH3+, GluH+, MetH+, ValH+, (Glu-Met)H+ and (Met-Glu)H+, were determined with a magnetic mass spectrometer, and the thermochemical properties, ΔHn−1,n∘,ΔSn−1,n∘ and ΔGn−1,n∘, for these reactions were determined and discussed. The results obtained with the present technique for protonated alkylamines are compared with previously published values, demonstrating the usefulness of this technique for quantitative measurements of the gas phase ion thermochemistry based on ion–molecule equilibria. A correlation between the free energy changes for the attachment of the first water molecule to the protonated amino acids and the corresponding gas-phase basicities is obtained.