Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States.
J Phys Chem A. 2020 Feb 27;124(8):1593-1602. doi: 10.1021/acs.jpca.9b11977. Epub 2020 Feb 14.
Infrared predissociation (IRPD) spectroscopy is used to reveal and compare the microsolvation motifs of GlyH(HO) and β-AlaH(HO). The chemical structure of these amino acids differ only in the length of the carbon chain connecting the amine and carboxyl terminals, which nonetheless leads to a significant difference in the strength of the intramolecular C═H-N hydrogen bond in the unsolvated ions. This difference makes them useful in our studies of the competition between solvation and internal hydrogen bonding interactions. Analysis of the IRPD results reveals that the sequential addition of water molecules leads to similar effects on the intramolecular interaction in both GlyH(HO) and β-AlaH(HO). Solvation of the -NH group leads to a weakening of the C═O···H-N hydrogen bond, while solvation of the carboxyl -OH leads to a strengthening of this bond. Additionally, we have found that for β-AlaH, the addition of a HO to the second solvation shell can still influence the strength of the C═O···H-N hydrogen bonding interaction. Finally, because the C═O···H-N interaction in β-AlaH is stronger than that in GlyH, more solvent molecules are needed to sufficiently weaken the intramolecular hydrogen bond such that isomers without this bond begin to be energetically competitive; this occurs at = 5 for β-AlaH and = 1 for GlyH.
红外预解离 (IRPD) 光谱用于揭示和比较 GlyH(HO) 和 β-AlaH(HO) 的微溶剂化模式。这些氨基酸的化学结构仅在连接胺和羧基末端的碳链长度上有所不同，但这导致了未溶剂化离子中分子内 C═H-N 氢键的强度有很大差异。这种差异使它们成为我们研究溶剂化和内部氢键相互作用竞争的有用工具。IRPD 结果的分析表明，水分子的顺序添加对 GlyH(HO) 和 β-AlaH(HO) 中分子内相互作用产生相似的影响。-NH 基团的溶剂化导致 C═O···H-N 氢键的减弱，而羧基 -OH 的溶剂化导致该键的增强。此外，我们发现对于 β-AlaH，第二个溶剂化壳层中添加 HO 仍可以影响 C═O···H-N 氢键的强度。最后，由于 β-AlaH 中的 C═O···H-N 相互作用比 GlyH 中的强，因此需要更多的溶剂分子来充分削弱分子内氢键，以使没有该键的异构体开始在能量上具有竞争力；对于 β-AlaH，这发生在 = 5，对于 GlyH，这发生在 = 1。
