Sohn Woon Yong, Habka Sana, Gloaguen Eric, Mons Michel
LIDYL, CEA, CNRS, Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France.
Phys Chem Chem Phys. 2017 Jul 14;19(26):17128-17142. doi: 10.1039/c7cp03058d. Epub 2017 Jun 21.
The presence in crystallized proteins of a local anchoring between the side chain of a His residue, located in the central position of a γ- or β-turn, and its local main chain environment, was assessed by the comparison of protein structures with relevant isolated model peptides. Gas phase laser spectroscopy, combined with relevant quantum chemistry methods, was used to characterize the γ- and β-turn structures in these model peptides. A conformer-selective NH stretch infrared study provided evidence for the formation in vacuo of two types of short-range H-bonded motifs, labelled ε-6 and δ-7/π, bridging the His side chain (in its gauche+ rotamer) to the neighbouring NH(i) and CO(i) sites of the backbone; each side chain-backbone motif was found to be specific of the tautomer (ε or δ) adopted by the His side chain in its neutral form. A close comparison between β- and γ-turns, selected from the Protein Data Bank, and the gas phase models demonstrated that a significant proportion of the gauche+ His rotamer distribution of proteins was well described by the corresponding gas phase H-bonded structures. This is consistent with the persistence of local 6 and 7/π intramolecular interactions in proteins, emphasizing the relevance of gas phase data to secondary structures that are poorly accessible to solvents, e.g., in the case of a specific compact topology (Xxx-His β-turns). Deviations from the gas phase structures were also observed, mainly in His-Xxx β-turns, and assigned to solvent accessible turn structures. They were well accounted for by theoretical models of microhydrated turns, in which a few solvent molecules take over the gas phase motifs, constituting a water-mediated local anchoring of the His side chain to the backbone. Finally, the present gas phase benchmark models also pinpointed weaknesses in the protein structure determination by X-ray diffraction analysis; in particular, besides the lack of tautomer information, inaccuracies in the description of imidazole ring flip rotamerism were identified.
通过将蛋白质结构与相关的分离模型肽进行比较，评估位于γ-或β-转角中心位置的组氨酸（His）残基侧链与其局部主链环境之间是否存在局部锚定。气相激光光谱结合相关量子化学方法，用于表征这些模型肽中的γ-和β-转角结构。一项构象选择性的NH伸缩红外研究提供了证据，证明在真空中形成了两种类型的短程氢键基序，标记为ε-6和δ-7/π，它们将His侧链（处于gauche+构象异构体）与主链相邻的NH(i)和CO(i)位点连接起来；发现每个侧链-主链基序都特定于His侧链以中性形式采用的互变异构体（ε或δ）。从蛋白质数据库中选择的β-和γ-转角与气相模型之间的密切比较表明，蛋白质中gauche+ His构象异构体分布的很大一部分可以由相应的气相氢键结构很好地描述。这与蛋白质中局部6和7/π分子内相互作用的持久性一致，强调了气相数据对于溶剂难以接近的二级结构的相关性，例如在特定紧密拓扑结构（Xxx-His β-转角）的情况下。也观察到与气相结构的偏差，主要在His-Xxx β-转角中，并归因于溶剂可及的转角结构。它们可以由微水合转角的理论模型很好地解释，其中一些溶剂分子取代了气相基序，构成了His侧链与主链的水介导局部锚定。最后，目前的气相基准模型还指出了X射线衍射分析在蛋白质结构测定中的弱点；特别是，除了缺乏互变异构体信息外，还发现了咪唑环翻转构象异构体描述中的不准确之处。
