多肽在去折叠状态下的构象及其机制研究

A full understanding of the mechanism of the protein folding process requires a detailed structural, dynamic, and thermodynamic characterization of both the starting and the final states. The final states have been characterized at high resolution by X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy, as demonstrated by the structures of a large set of proteins in the protein data bank (PDB). The starting states have evaded detailed structural and energetic characterization due to their heterogeneous nature, solubility problems, and complex dynamics. According to calculations by Flory, there should be no single dominant backbone conformation in an unfolded polypeptide chain. The so-called random coil model is still used as a framework for describing unfolded proteins despite persistent doubts. The model assumes that unfolded proteins represent an ensemble of featureless random chain molecules with very large associated chain entropy. Recent results suggest that polyproline II (PII) is a dominant backbone conformation in unfolded peptides. Recently, we have shown that the backbone conformation of a seven residue alanine peptide (XAO) predominantly adopts PII based on CD and NMR, as well as resonance Raman and Raman optical activity (ROA) measurements. Later, we studied 19 amino acids in the context of the host-guest peptide model AcGGXGGNH2, with X=G excepted. Our results showed that most amino acids favor PII and PII contents of different residues anticorrelate with their beta propensities. Why is PII a dominant conformation in unfolded states of proteins? Regarding the bias of backbone conformation to PII, solvent hydration, steric effects, side chain-backbone interaction and n-pi* interactions, respectively, are suggested responsible by different studies. In this proposal, we attempt to answer why PII is the dominant conformation in unfolded states of peptides through characterizing the above-mentioned interactions plus dipole-dipole interactions. Perturbation of each interaction will be realized through designing and synthesis series of model peptides. To understand the underlying physical mechanism for why PII is dominant, conformational distribution of each peptide will be characterized by NMR and CD.

)：蛋白质在折叠状态下的结构可以通过晶体衍射和NMR及其它低分辨率方法获得；然而蛋白质在去折叠状态下的结构一直以来都无法得出明确的结论。较公认的猜想是：各种二级结构（α、β及其它结构）并存于去折叠状态下，其中没有任何一种结构占绝对多数。在前期工作中，我们发现模型短肽在去折叠状态下可能以Polyproline II螺旋(PII)为主的结构存在；随后发现, 19种氨基酸形成PII的倾向性与它们形成β结构的倾向性反相关。我们将继续研究多肽在去折叠状态下为什么以PII 结构为主存在。我们曾提出阐述其物理化学机制可从以下作用力开始：溶剂化作用、空间位阻效应、主链侧链相互作用、偶极偶极相互作用及n-pi*超共轭效应。我们将对各作用力的研究简化还原成对一系列设计并合成的多肽衍生物进行物理化学研究。

蛋白质在折叠状态下的结构可以通过晶体衍射和 NMR及其它低分辨率方法获得；然而蛋白质在去折叠状态下的结构一直以来都无法得出明确的结论。较公认的猜想是： 各种二级结构（α 、β 及其它结构）并存于去折叠状态下，其中没有任何一种结构占绝对多数。在前期工作中，我们发现模型短肽在去折叠状态下可能以 Polyproline II 螺旋(PII)为主的结构存在。本项目研究多肽在去折叠状态下为什么以 PII 结构为主存在。我们研究以下作用力对多肽构象的影响：溶剂化作用、氨基酸侧链电子诱导效应和 n-pi*超共轭效应。