Protonation thermochemistry of α-aminoacids bearing a basic residue

Eur. J. Mass Spectrom. 10, 977–992 (2004)
DOI: 10.1255/ejms.687

Protonation thermochemistry of α-aminoacids bearing a basic residue

Guy Bouchoux,^*a David-Alexandre Buisson,^b Cyril Colas^a and Michel Sablier^a
^aLaboratoire des Mécanismes Réactionnels, UMR 7651, Ecole Polytechnique, 91128 Palaiseau cedex, France
^bCEN Saclay 91190 Gif sur Yvette cedex, France.

ABSTRACT:

The proton affinity, PA, and protonation entropy, Δ_pS°, of glycine (Gly), 1, aspartic acid (Asp), 2, asparagine (Asn), 3, histidine (His), 4, lysine (Lys), 5, glutamic acid (Glu), 6, and glutamine (Gln), 7, have been reinvestigated by the extended kinetic method, using the “isothermal point” method and the orthogonal distance regression, ODR, technique. The proton affinity values of a-aminoacids bearing a basic residue (PA = 926.8; 965.2; 996.0; 993.9; 981.8 and 988.1 kJ.mol^–1 for 2–7, respectively) show significant deviation from the tabulated values. As expected from the effect of a strong intramolecular hydrogen bond in the protonated forms of these peculiar aminoacids, negative protonation entropies are detected (Δ_pS°= –36; –43; –37; –29; –95 and –55 J mol^–1 K^–1 for for 2–7 respectively).

Keywords: protonation entropy, proton affinity, basicity, extended kinetic method, aminoacids, molecular orbital calculations

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