On the stability of amino acid zwitterions in the gas phase: The influence of derivatization, proton affinity, and alkali ion addition

Collision cross sections have been measured for a series of N- and C-methylated glycines cationized by alkali ions using ion mobility methods. In all cases the measured cross sections are in excellent agreement with model structures obtained from a number of different theoretical approaches. Unfortunately both charge solvation and zwitterion structures are predicted to have nearly identical cross sections. On the basis of a conformational search by molecular mechanics methods and density functional theory calculations at the B3LYP/DZVP level it is found that the lowest energy forms of alkali cationized glycine and alanine are charge solvation structures, whereas lowest energy singly and doubly N-methylated glycines are salt bridges independent of metal ion. ol-Amino isobutyric acid forms a salt bridge when sodiated and a charge solvation structure when rubidiated. In the most stable charge solvation structures rubidium is bound to one or both carboxyl oxygens, while sodium is bound to both the N- and the C-terminus. The stability of stilt bridge structures relative to charge solvation structures is found to be nearly proportional to the amino acid proton affinity (PA). For sodiated molecules a PA of >217 kcal/mol results in salt bridge formation, for rubidiated a PA of >219. Predictions are made for the structural preferences of all the common amino acids as a function of cationizing metal ion.