Proton affinity of the commonly occuring

L-amino acids by using electrospray ionization-ion trap mass spectrometry

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Eur. J. Mass Spectrom. 6, 443 - 449 (2000)

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

C. Afonso
Centre dEtudes du Bouchet, BP 3, 91710 Vert-le-Petit, France, and 2-LCSOB, UMR 7613 Université Pierre & Marie Curie, Bat. F, boite 45, 4 place Jussieu, 75252 Paris Cedex 05, France
F. Modeste and P. Breton
Centre dEtudes du Bouchet, BP 3, 91710 Vert-le-Petit, France
F. Fournier and J.-C. Tabet^*
2-LCSOB, UMR 7613 Université Pierre & Marie Curie, Bat. F, boite 45, 4 place Jussieu, 75252 Paris Cedex 05, France

ABSTRACT:

In this study, the use of ESI-ion trap mass spectrometry to perform thermochemical determinations using the Cooks kinetic method is shown. In this method, competitive dissociations of selected proton-bound heterodimeric [B_iHB]⁺ ions are investigated and particularly, the product ion abundance [B_iH⁺] / [BH⁺] ratio is accurately measured and compared to the proton affinity of each neutral partner (i.e. the B_i and B). The proton affinity 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 homo and heterodimer ions, constant and reproducible [B_iH⁺]/[BH⁺] ratios are obtained when sequential 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([B_iH⁺]/[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 (dDS⁰) between leucine and other amino acids was evaluated by several methods.

Keywords: kinetic method, ion trap, effective temperature, amino acids, proton affinity

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