Division of Chemistry, Naval Research Laboratory, Washington, DC 20375-5342, USA.
Chemistry. 2009 Nov 9;15(44):11867-77. doi: 10.1002/chem.200901129.
Beta helices--helices formed by alternating D,L-peptides and stabilized by beta-sheet hydrogen bonding--are found naturally in only a handful of highly hydrophobic peptides. This paper explores the scope of beta-helical structure by presenting the first design and biophysical characterization of a hydrophilic D,L-peptide, 1, that forms a beta helix in methanol. The design of 1 is based on the beta-hairpin/beta helix--a new supersecondary that had been characterized previously only for hydrophobic peptides in nonpolar solvents. Incorporating polar residues in 1 provided solubility in methanol, in which the peptide adopts the expected beta-hairpin/beta-helical structure, as evidenced by CD, analytical ultracentrifugation (AUC), NMR spectroscopy, and NMR-based structure calculations. Upon titration with water (at constant peptide concentration), the structure in methanol (1 m) transitions cooperatively to an extended conformation (1 w) resembling a cyclic beta-hairpin; observation of an isodichroic point in the solvent-dependent CD spectra indicates that this transition is a two-state process. In contrast, neither 1 m nor 1 w show cooperative thermal melting; instead, their structures appear intact at temperatures as high as 65 degrees C; this observation suggests that steric constraint is dominant in stabilizing these structures. Finally, the (1)H NMR C alphaH spectroscopic resonances of 1 m are downfield-shifted with respect to random-coil values, a hitherto unreported property for beta helices that appears to be a general feature of these structures. These results show for the first time that an appropriately designed beta-helical peptide can fold stably in a polar solvent; furthermore, the structural and spectroscopic data reported should prove useful in the future design and characterization of water-soluble beta helices.
β 螺旋——由交替的 D，L-肽形成并通过β-折叠氢键稳定的螺旋——仅在少数高度疏水性肽中天然存在。本文通过介绍第一个设计和生物物理特性表征亲水性 D，L-肽 1，来探索β-螺旋结构的范围，1 在甲醇中形成β 螺旋。1 的设计基于β-发夹/β-螺旋——这是一个以前仅在非极性溶剂中对疏水性肽进行了特征描述的新超二级结构。在 1 中加入极性残基可提供在甲醇中的溶解度，在甲醇中，该肽采用预期的β-发夹/β-螺旋结构，这一点通过 CD、分析超速离心（AUC）、NMR 光谱和基于 NMR 的结构计算得到证实。当用（在恒定肽浓度下）水滴定时，甲醇（1 m）中的结构协同地转变为伸展构象（1 w），类似于环状β-发夹；溶剂依赖性 CD 光谱中观察到等色点表明该转变是一个两态过程。相比之下，1 m 和 1 w 都没有表现出协同热融解；相反，它们的结构在高达 65°C 的温度下似乎保持完整；这一观察表明，空间位阻在稳定这些结构中起主导作用。最后，1 m 的（1）H NMR CαH 光谱共振相对于无规卷曲值向高场位移，这是迄今为止报道的β 螺旋的一个未报道的特性，似乎是这些结构的一个普遍特征。这些结果首次表明，适当设计的β-螺旋肽可以在极性溶剂中稳定折叠；此外，报告的结构和光谱数据将来应该对水溶性β 螺旋的设计和特性描述有用。
