Fischer Kaitlyn C, Sherman Summer L, Voss Jonathan M, Zhou Jia, Garand Etienne
Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States.
J Phys Chem A. 2019 Apr 18;123(15):3355-3366. doi: 10.1021/acs.jpca.9b01578. Epub 2019 Apr 8.
The IR predissociation spectra of microsolvated glycine and l-alanine, GlyH(HO) and AlaH(HO) , n = 1-6, are presented. The assignments of the solvation structures are aided by HO/DO substitution, IR-IR double resonance spectroscopy, and computational efforts. The analysis reveals the water-amino acid as well as the water-water interactions, and the subtle effects of the methyl side chain in l-alanine on the solvation motif are also highlighted. The bare amino acids exhibit an intramolecular hydrogen bond between the protonated amine and carboxyl terminals. In the n = 1-2 clusters, the water molecules preferentially solvate the protonated amine group, and we observed differences in the relative isomer stabilities in the two amino acids due to electron donation from the methyl weakening the intramolecular hydrogen bond. The structures in the n = 3 clusters show a further preference for solvation of the carboxyl group in l-alanine. For n = 4-6 clusters, the solvation structure of the two amino acids is remarkably similar, with one dominant isomer present in each cluster size. The first solvation shell is completed at n = 4, evidenced by a lack of free NH and OH stretches on the amino acid, as well as the first observation of HO-HO interactions in the spectra of n = 5. Finally, we note that calculations at the density functional theory (DFT) level show excellent agreement with the experiment for the smaller clusters. However, when water-water interactions compete with water-amino acid interactions in the larger clusters, DFT results show greater disagreement with experiment when compared to MP2 results.
本文展示了微溶剂化甘氨酸和L-丙氨酸（GlyH(HO) 和AlaH(HO) ，n = 1 - 6）的红外预解离光谱。通过HO/DO取代、红外-红外双共振光谱以及计算研究辅助确定了溶剂化结构。分析揭示了水-氨基酸以及水-水之间的相互作用，同时也突出了L-丙氨酸中甲基侧链对溶剂化模式的微妙影响。裸露的氨基酸在质子化胺和羧基末端之间呈现分子内氢键。在n = 1 - 2的簇中，水分子优先溶剂化质子化胺基团，并且我们观察到由于甲基的电子给予作用削弱了分子内氢键，两种氨基酸的相对异构体稳定性存在差异。n = 3的簇结构显示出对L-丙氨酸羧基的进一步溶剂化偏好。对于n = 4 - 6的簇，两种氨基酸的溶剂化结构非常相似，每个簇尺寸都存在一种主要异构体。在n = 4时完成了第一溶剂化壳层，这通过氨基酸上缺乏游离的NH和OH伸缩振动得以证明，同时在n = 5的光谱中首次观察到HO - HO相互作用。最后，我们注意到密度泛函理论（DFT）水平的计算结果与较小簇的实验结果显示出极佳的一致性。然而，当在较大簇中水分子间相互作用与水-氨基酸相互作用相互竞争时，与MP2结果相比，DFT结果与实验结果的分歧更大。
