Crystallographic Characterization of Helical Secondary Structures in 2:1 and 1:2 α/β-Peptides

Oligomers containing both α- and β-amino acid residues (“α/β-peptides”) are intriguing as potential foldamers. A large set of α/β-peptide backbones can be generated by combining α- and β-amino acid residues in different patterns; however, most research to date has focused on the simplest pattern, 1:1 α:β. We have begun to explore the range of variation that can be achieved with α-residue/β-residue combinations by examining the folding behavior of oligomers that contain 2:1 and 1:2 α:β patterns. The β-residues in our systems have a five-membered-ring constraint (trans-2-aminocyclopentanecarboxylic acid (ACPC) residues), because these preorganized subunits strongly promote helical folding for 1:1 α:β backbones and pure β backbones. Previously we concluded that two helical conformations are available to 2:1 and 1:2 α/β-peptides containing ACPC or analogously constrained β-residues, one helix defined by i,i+3 CO···H−N backbone hydrogen bonds and the other defined by i,i+4 CO···H−N hydrogen bonds. These deductions were based on 2D NMR analysis of a 2:1 heptamer and a 1:2 hexamer in methanol. Crystallographic analysis of a pair of analogous nonpolar α/β-peptides showed only the i,i+3 hydrogen-bonded helical conformations. We now report four new crystal structures of 2:1 α/β-peptides, ranging in length from 5 to 11 residues, and six new crystal structures of 1:2 α/β-peptides, ranging in length from 6 to 10 residues. All 10 of these new structures are fully helical, and all helices display the i,i+3 CO···H−N hydrogen bonding pattern. These crystallographic data sets, collectively, provide high structural definition for the i,i+3 hydrogen-bonded helical secondary structures available to these foldamer backbones.

同时包含α-与β-氨基酸残基的寡聚体（“α/β肽（α/β-peptide）”）作为潜在折叠体（foldamer）具有极高研究价值。通过以不同模式组合α-与β-氨基酸残基，可构建大量α/β肽骨架；但迄今为止绝大多数研究均聚焦于最简模式：1:1的α:β比例。我们已启动相关探索，旨在通过考察含2:1与1:2 α:β比例模式的寡聚体折叠行为，明确α-残基与β-残基组合所能实现的结构可变范围。
本研究体系中的β-残基带有五元环约束（反式-2-氨基环戊烷羧酸（ACPC）残基），因为这类预组织化结构单元可强力促进1:1 α:β骨架与纯β肽骨架的螺旋折叠。此前我们曾提出，含ACPC或类似约束型β-残基的2:1与1:2型α/β肽可采用两种螺旋构象：一种由i,i+3型C=O···H−N骨架氢键定义，另一种则由i,i+4型C=O···H−N氢键定义。该推论基于对甲醇溶液中2:1七聚体与1:2六聚体的二维核磁共振（2D NMR）分析。对一对类似非极性α/β肽的晶体学分析仅观测到i,i+3型氢键螺旋构象。
我们在此报道4个全新的2:1型α/β肽晶体结构，其残基长度介于5至11之间；以及6个全新的1:2型α/β肽晶体结构，残基长度介于6至10之间。上述10个全新结构均为完全螺旋构象，且所有螺旋均呈现i,i+3型C=O···H−N氢键模式。综上，这些晶体学数据集为这类折叠体骨架可形成的i,i+3型氢键螺旋二级结构提供了高清晰度的结构界定。