Gas-Phase Thermochemistry of Non-Protein Amino Acids by the Extended Kinetic Method

In this study, we have been examining the gas-phase acidities of two amino acid homologues, homocysteine and homoserine, and the proton affinity of another amino acid homologue, homohomoserine. Homocysteine, homoserine, and homohomoserine are non-protein amino acids (NPAA) that have very similar structures to the protein amino acids (PAA) cysteine and serine. The gas-phase acidities of both cysteine and serine are well known and an analysis of the gas-phase basicity of their homologues has yielded information regarding the effects of small changes in structure on a basic physical property. Similarly, the proton affinities of serine and homoserine are well-known, so investigating the proton affinity of the next homologue has yielded more information as well. To do this experiment, we used a triple quadrupole mass spectrometer with an electrospray ionization source and utilized the kinetic method. In this method, the gas-phase acidity of the compound of interest is determined by comparing it to other compounds of known gas-phase acidity, called reference acids, inside the mass spectrometer. A proton-bound dimer is formed and is collided with an inert gas at various energies to obtain a kinetic plot. This is repeated for multiple reference acids and an intersection is found. This intersection is the gas-phase acidity of the compound of interest. A similar analysis was done for the proton affinity of homohomoserine. The final values obtained were 1394.6 ± 10 kJ/mol for the gas-phase acidity of homoserine, 1392.8 ± 10 kJ/mol for the gas-phase acidity of homocysteine, and 968.7 ± 10 kJ/mol for the proton affinity of homohomoserine