Laboratoire des Mécanismes Réactionnels, UMR CNRS 7651, Département de Chimie, Ecole Polytechnique, F-91128 Palaiseau cedex, France.
Phys Chem Chem Phys. 2011 Jan 14;13(2):651-68. doi: 10.1039/c0cp00775g. Epub 2010 Oct 29.
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(f)H°(298) for the ionized species AAAH(+) and AAA-H. Comparison with Benson's estimate, and correlation with molecular size, show that several experimental Δ(f)H°(298) values of neutral or gaseous AAA might be erroneous.
已通过量子化学计算在 G3MP2B3 水平上研究了分离的脂肪族氨基酸（表示为 AAA）：甘氨酸、丙氨酸、缬氨酸、亮氨酸、异亮氨酸和脯氨酸的酸碱热化学。对中性、质子化和去质子化物种进行构象分析，以确定最低能量构象，并估计在 298 K 热平衡时存在的构象的比例。如果重新评估实验热化学并使其适应最新的酸碱尺度，则将 G3MP2B3 理论质子亲和力（PA）和ΔH（acid）与实验结果进行比较是正确的。从这个角度来看，提出了一组评估的质子亲和力为 887、902、915、916、919 和 941 kJ mol(-1)，以及一组评估的ΔH（acid）为 1433、1430、1423、1423、1422 和 1426 kJ mol(-1)，分别用于甘氨酸、丙氨酸、缬氨酸、亮氨酸、异亮氨酸和脯氨酸。与结构参数（Taft 的 σ(α)极化率参数和分子大小）的相关性表明，侧链的极化率是 PA 增加和ΔH（acid）沿同系物甘氨酸、丙氨酸、缬氨酸和亮氨酸/异亮氨酸降低的主要原因。通过 G3MP2B3 水平计算了气态 AAA、AAAH(+)和AAA-H物种的生成热。本研究提供了以前不可用的离子化物种 AAAH(+)和AAA-H的Δ(f)H°(298)。与 Benson 的估计值进行比较，并与分子大小相关联，表明几个中性或气态 AAA 的实验Δ(f)H°(298)值可能是错误的。
