Gas-Phase Acidities and Proton Affinities of Amino Acid Analogs from the Extended Kinetic Method

The gas-phase acidities and proton affinities of analogs of protein amino acids were investigated. The gas-phase acidities of the lysine homologues were measured by the extended kinetic method in a quadrupole ion trap mass spectrometer. Deprotonation entropy changes were also measured. For ornithine, 2,4-diaminobutyric acid, and 2,3-diaminopropionic acid, the gas-phase acidities measured were 1416 ± 17 kJ/mol, 1420 ± 8 kJ/mol, and 1405 ± 24 kJ/mol. Their changes in entropy were measured to be -19 J/mol K, 1 J/mol k, and -24 J/mol K, respectively. The gas-phase acidities and entropies of two structural analogs of arginine, citrulline and canavanine, were measured by the extended kinetic method. The gas-phase acidity of citrulline was measured to be 1366 ± 11 kJ/mol, and the gas-phase acidity of canavanine was measured to be 1401 ± 13 kJ/mol. The proton affinity and protonation entropy change for citrulline was measured as well. The proton affinity was determined to be 984 ± 11 kJ/mol with an entropy change of -6 J/mol K. The proton affinity and protonation entropy change of L-BMAA, a structural analogue of alanine and 2,3-diaminopropionic acid, was measured by the extended kinetic method to be 960 ± 7 kJ/mol. The entropy for the protonation reaction was found to be -4 J/mol K. Hybrid density functional theory calculations were performed on the compounds examined. Energy-optimized geometries were examined for structural trends and theoretical predictions for the gas-phase acidities and proton affinities were made. The experimental and theoretical determinations reveal the effects of structural changes on gas-phase thermochemical properties