Density functional theory (DFT) calculations are performed to study the conformational flexibility of secondary structures in amyloid beta (A[Formula: see text]) polypeptide. In DFT, M06-2X/6-31[Formula: see text]G(d, p) method is used to optimize the secondary structures of 2LFM and 2BEG in gas phase and in solution phase. Our calculations show that the secondary structures are energetically more stable in solution phase than in gas phase. This is due to the presence of strong solvent interaction with the secondary structures considered in this study. Among the backbone [Formula: see text] and [Formula: see text] dihedral angles, [Formula: see text] varies significantly in sheet structure. This is due to the absence of intermolecular hydrogen bond (H-bond) interactions in sheets considered in this study. Our calculations show that the conformational transition of helix/coil to sheet or vice-versa is due to the floppiness of the amino acid residues. This is observed from the Ramachandran map of the studied secondary structures. Further, it is noted that the intramolecular H-bond interactions play a significant role in the conformational transition of secondary structures of A[Formula: see text].

中文翻译：

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β淀粉样蛋白二级结构的结构和构象灵活性研究——量子化学研究

进行密度泛函理论 (DFT) 计算以研究淀粉样蛋白 β (A[公式：参见文本]) 多肽中二级结构的构象灵活性。在DFT中，M06-2X/6-31[公式：见正文]G(d, p)方法用于优化2LFM和2BEG在气相和液相中的二级结构。我们的计算表明，二级结构在溶液相中比在气相中在能量上更稳定。这是由于存在与本研究中考虑的二级结构的强溶剂相互作用。在骨架[公式：见正文]和[公式：见正文]二面角中，[公式：见正文]的片状结构差异很大。这是由于本研究中考虑的片材中不存在分子间氢键（H-键）相互作用。我们的计算表明，螺旋/螺旋向薄片的构象转变或反之亦然是由于氨基酸残基的松软。这可以从所研究的二级结构的拉马钱德兰图上观察到。此外，注意到分子内氢键相互作用在A[公式：见正文]的二级结构的构象转变中起重要作用。