Determinations of Proton Affinities of Methylated Cysteine and Serine Homologs

Non-protein amino acids (NPAAs) are of interest to study for their potential to be incorporated into peptides and proteins, as well as for understanding their structure-energetics relationships. Studying these amino acids’ thermochemical properties such as their acidity and basicity allow for elucidation of the structure and bonding characteristics of these molecules. By examining these molecules in the gas-phase in mass spectrometers, we are able to determine their intrinsic thermodynamic properties without solvation effects. These thermochemical properties, in part, determine the structure, function and bonding of the molecules. This study focused on determining the proton affinities of methylated cysteine and serine homologs by using Cook’s extended kinetic method with orthogonal distance regression analysis in a triple quadrupole mass spectrometer. The NPAAs selected have been shown to mis-incorporate into small peptides, making it of interest to determine how these changes affect the structures and energetics of the peptide.1The NPAAs studied included α-methylcysteine, L-penicillamine (gem-dimethyl cysteine), α-methylserine, and 3-methylthrenonine (gem-dimethyl serine), and their proton affinities were determined to be 923.7 ±11 kJ/mol, 925 ±15 kJ/mol, 932 ±20 kJ/mol and 924 ±15 kJ/mol, respectively. These experimental proton affinities are in excellent agreement with Boltmann-weighted computational proton affinities determined for these molecules. All of the methylated homologs had larger proton affinities than their respective protein amino acid, serine and cysteine, which have proton affinities of with proton affinities of 903 kJ/mol and 912 kJ/mol respectively.