The molecular conformation of the carboxyl group can be crucial for its chemical properties and intermolecular interactions, especially in complex molecular environments such as polypeptides. Here, we study the conformational behaviour of the model amino acid N-acetylproline in solution at room temperature with two-dimensional infrared spectroscopy. We find that the carboxyl group of N-acetylproline adopts two distinct conformations, syn- and anti-. In the syn-conformer the O–H group is oriented at ~60^∘ with respect to the C=O and in the anti-conformer the O–H is anti-parallel to the C=O. In hydrogen-bond accepting solvents such as dimethyl sulfoxide or water, we observe that, similar to simple carboxylic acids, around 20% of the -COOH groups adopt an anti-conformation. However, when N-acetylproline is dissolved in a weakly hydrogen-bond accepting solvent (acetonitrile), we observe the formation of a strong intramolecular hydrogen bond between the carboxyl group in the anti-conformation and the amide group, which stabilizes the anti-conformer, increasing its relative abundance to ~60%.

羧基的分子构象对其化学性质和分子间相互作用至关重要，尤其是在多肽等复杂分子环境中。在这里，我们使用二维红外光谱研究了模型氨基酸 N-乙酰脯氨酸在室温下溶液中的构象行为。我们发现 N-乙酰脯氨酸的羧基采用两种不同的构象，顺式和反式。在顺式构象中，O-H 基团相对于 C=O定向在 ~60 ^∘处，而在反-构象 O–H 与 C=O 反平行。在二甲基亚砜或水等接受氢键的溶剂中，我们观察到，与简单的羧酸类似，大约 20% 的 -COOH 基团采用反构象。然而，当 N-乙酰脯氨酸溶解在弱氢键接受溶剂（乙腈）中时，我们观察到反构象中的羧基与酰胺基团之间形成了强分子内氢键，从而稳定了反构象异构体, 将其相对丰度增加到 ~60%。