Dorofeeva Olga V, Ryzhova Oxana N
Department of Chemistry, Lomonosov Moscow State University , Moscow 119991, Russia.
J Phys Chem A. 2014 May 15;118(19):3490-502. doi: 10.1021/jp501357y. Epub 2014 May 5.
Accurate gas-phase enthalpies of formation (ΔfH298°) of 20 common α-amino acids, seven uncommon amino acids, and three small peptides were calculated by combining G4 theory calculations with an isodesmic reaction approach. The internal consistency over a set of ΔfH298°(g) values was achieved by sequential adjustment of their values through the isodesmic reactions. Four amino acids, alanine, β-alanine, sarcosine, and glycine, with reliable internally self-consistent experimental data, were chosen as the key reference compounds. These amino acids together with about 100 compounds with reliable experimental data (their accuracy was supported by G4 calculations) were used to estimate the enthalpies of formation of remaining amino acids. All of the amino acids with the previously established enthalpies of formation were later used as the reference species in the isodesmic reactions for the other amino acids. A systematic comparison was made of 14 experimentally determined enthalpies of formation with the results of calculations. The experimental enthalpies of formation for 10 amino acids were reproduced with good accuracy, but the experimental and calculated values for 4 compounds differed by 11–21 kJ/mol. For these species, the theoretical ΔfH298°(g) values were suggested as more reliable than the experimental values. On the basis of theoretical results, the recommended values for the gas-phase enthalpies of formation were also provided for amino acids for which the experimental ΔfH298°(g) were not available. The enthalpies of sublimation were evaluated for all compounds by taking into account the literature data on the solid-phase enthalpies of formation and the ΔfH298°(g) values recommended in our work. A special attention was paid to the accurate prediction of enthalpies of formation of amino acids from the atomization reactions. The problems associated with conformational flexibility of these compounds and harmonic treatment of low frequency torsional modes were discussed. The surprisingly good agreement between the ΔfH298°(g) values calculated from the atomization and isodesmic reactions is largely the result of a fortuitous mutual compensation of various corrections used in the atomization reaction procedure.
通过将G4理论计算与等键反应方法相结合，计算了20种常见α-氨基酸、7种不常见氨基酸和3种小肽的精确气相生成焓（ΔfH298°）。通过等键反应对一组ΔfH298°(g)值进行顺序调整，实现了它们之间的内部一致性。选择了四种具有可靠内部自洽实验数据的氨基酸，即丙氨酸、β-丙氨酸、肌氨酸和甘氨酸，作为关键参考化合物。这些氨基酸与约100种具有可靠实验数据的化合物（其准确性得到G4计算的支持）一起用于估算其余氨基酸的生成焓。所有先前已确定生成焓的氨基酸随后都被用作等键反应中其他氨基酸的参考物种。对14个实验测定的生成焓与计算结果进行了系统比较。10种氨基酸的实验生成焓得到了很好的重现，但4种化合物的实验值与计算值相差11 - 21 kJ/mol。对于这些物种，理论ΔfH298°(g)值被认为比实验值更可靠。基于理论结果，还为没有实验ΔfH298°(g)值的氨基酸提供了气相生成焓的推荐值。通过考虑关于固相生成焓的文献数据和我们工作中推荐的ΔfH298°(g)值，对所有化合物的升华焓进行了评估。特别关注了从原子化反应精确预测氨基酸生成焓的问题。讨论了与这些化合物的构象灵活性以及低频扭转模式的简谐处理相关的问题。从原子化反应和等键反应计算得到的ΔfH298°(g)值之间惊人的良好一致性，很大程度上是原子化反应过程中使用的各种校正偶然相互补偿的结果。
