Proton affinities of the commonly occuring L-amino acids by using electrospray ionization-ion trap mass spectrometry

In this study, the use of an electrospray ionization (ESI)-ion trap mass spectrometry to perform thermochemical determinations using the kinetic method is shown. In this method, competitive dissociations of selected proton-bound heterodimeric [BiHB](+) ions are investigated and particularly, the product ion abundance [BiH+] / [BH+] ratio is accurately measured and compared to the proton affinity of each neutral partner (i.e. the B-i and B), The proton affinities of the amino acids are well known and these compounds were chosen to investigate the ion trap potentiality for such purpose, particularly, when externally prepared ions were injected. It appears that, in spite of the low abundance of home and heterodimer ions, constant and reproducible [BiH+] / [BH+] ratios are obtained when sequential tandem mass spectrometry (MS/MS) experiments are performed within similar ion trapping conditions. Such conditions are reached for a constant ion excitation q(z) value. The resulting ln([BiH+] / [BH+]) dependence upon the PA(Bi) and PA(B) values (from the literature) is linear and yields a reproducible slope after several measurements are made to determine the accuracy of this approach. Under various excitation conditions, in spite of the change in effective temperature, neither the scale order nor the PA values are changed. Furthermore, entropy variation (S-0) between leucine and other amino acids was evaluated by several methods.