The Lithium Cation Binding Energies of Gaseous Amino Acids

Abstract

The lithium cation binding energies of 15 of the common amino acids were determined via the kinetic method in a quadrupole ion trap mass spectrometer. Values were obtained in two ways. First, a ladder of relative lithium cation binding energies was developed from pairwise comparisons of the amino acids. Second, values were determined by comparison to a pair of simple reference compounds, dimethoxyethane and diethoxyethane. The values from the two approaches are in good accord. The scale from glycine to glutamic acid spans a range from 41.6 to 52.9 kcal/mol. The present values for lithium cations have been compared to those obtained by others previously for sodium, copper, and silver cations. These comparisons suggest that the alkali metals have exalted binding energies for amino acids with side chains that include oxygen-bearing functional groups (i.e., alcohols and carboxylic acids) whereas the transition metals have enhanced binding energies for amino acids with side chains that include sulfur-bearing or aromatic functional groups. This analysis is in accord with the principles of hard−soft acid/base behavior.