Acid–base thermochemistry of gaseous aliphatic α-aminoacids

Abstract

Acid–base thermochemistry of isolated aliphatic amino acids (denoted AAA): glycine, alanine, valine, leucine, isoleucine and proline has been examined theoretically by quantum chemical computations at the G3MP2B3 level. Conformational analysis on neutral, protonated and deprotonated species has been used to identify the lowest energy conformers and to estimate the population of conformers expected to be present at thermal equilibrium at 298 K. Comparison of the G3MP2B3 theoretical proton affinities, PA, and ΔH_acid with experimental results is shown to be correct if experimental thermochemistry is re-evaluated and adapted to the most recent acidity–basicity scales. From this point of view, a set of evaluated proton affinities of 887, 902, 915, 916, 919 and 941 kJ mol⁻¹, and a set of evaluated ΔH_acid of 1433, 1430, 1423, 1423, 1422 and 1426 kJ mol⁻¹, is proposed for glycine, alanine, valine, leucine, isoleucine and proline, respectively. Correlations with structural parameters (Taft's σ_α polarizability parameter and molecular size) suggest that polarizability of the side chain is the major origin of the increase in PA and decrease in ΔH_acid along the homologous series glycine, alanine, valine and leucine/isoleucine. Heats of formation of gaseous species AAA, AAAH⁺ and [AAA−H]⁻ were computed at the G3MP2B3 level. The present study provides previously unavailable Δ_fH°₂₉₈ for the ionized species AAAH⁺ and [AAA−H]⁻. Comparison with Benson's estimate, and correlation with molecular size, show that several experimental Δ_fH°₂₉₈ values of neutral or gaseous AAA might be erroneous.