Institut für Organische Chemie und Chemische Biologie, Johann Wolfgang Goethe-Universität , 60438 Frankfurt am Main, Germany.
J Phys Chem B. 2017 Jun 15;121(23):5744-5758. doi: 10.1021/acs.jpcb.7b02899. Epub 2017 Jun 2.
The conformational propensity of amino acid residues is determined by an intricate balance of peptide-solvent and solvent-solvent interactions. To explore how the systematic replacement of water by a cosolvent affects the solvation of both the amino acid backbone and side chains, we performed a combined vibrational spectroscopy and NMR study of cationic glycylalanylglycine (GAG) in different ethanol/water mixtures of between 0 and 42 mol percent ethanol. Classical model peptide N'-methylacetamide was used as a reference system to probe solvent-induced spectroscopic changes. The alanine residue of GAG in water is known to exhibit a very high propensity for polyproline II (pPII). Adding up to 30 mol % ethanol at room temperature leads only to minor changes in the Ramachandran distribution of alanine, which mostly changes within the individual conformational subspaces. A further increase in the ethanol fractions leads to a destabilization of pPII and a stabilization of β-strand conformations. At higher temperatures, different degrees of enthalpy-entropy compensations lead to a much stronger influence of ethanol on the peptide's conformational distribution. Ethanol-induced changes in chemical shifts and amide I wavenumbers strongly suggest that ethanol replaces water preferentially in the solvation shell of the polar C-terminal peptide group and of the alanine side chain, whereas the N-terminal group remains mostly hydrated. Furthermore, we found that ethanol interacts more strongly with the peptide if the latter adopts β-strand conformations. This leads to an unusual positive temperature coefficient for the chemical shift of the C-terminal amide proton. Our data suggests a picture in which GAG eventually accumulates at water-ethanol interfaces if the ethanol fractions exceed 0.3, which explains why the further addition of ethanol eventually causes self-aggregation and the subsequent formation of a hydrogel.
氨基酸残基的构象倾向取决于肽-溶剂和溶剂-溶剂相互作用的复杂平衡。为了探究系统地用共溶剂替代水如何影响氨基酸主链和侧链的溶剂化，我们对阳离子甘氨酰丙氨酰甘氨酸（GAG）在不同的乙醇/水混合物（乙醇摩尔分数在 0 到 42 之间）中进行了振动光谱和 NMR 的联合研究。经典模型肽 N'-甲基乙酰胺被用作参考体系来探测溶剂诱导的光谱变化。在水中，GAG 的丙氨酸残基表现出非常高的聚丙氨酸 II（pPII）倾向。在室温下添加高达 30 mol%的乙醇只会导致丙氨酸的 Ramachandran 分布发生较小的变化，这些变化主要发生在单个构象子空间内。进一步增加乙醇分数会导致 pPII 的失稳和β-折叠构象的稳定。在较高温度下，不同程度的焓熵补偿会导致乙醇对肽构象分布的影响更大。乙醇诱导的化学位移和酰胺 I 波数变化强烈表明，乙醇优先替代极性 C 末端肽基团和丙氨酸侧链的溶剂化壳中的水分子，而 N 末端基团仍然主要被水合。此外，我们发现如果肽采用β-折叠构象，乙醇与肽的相互作用更强。这导致 C 末端酰胺质子的化学位移出现不寻常的正温度系数。我们的数据表明，如果乙醇分数超过 0.3，GAG 最终会在水-乙醇界面处积累，这解释了为什么进一步添加乙醇最终会导致自聚集并随后形成水凝胶。
