The conformations of 14 threonine-derived pseudoproline-containing dipeptides (including four d-allo-Thr derivatives) have been investigated by NMR. In solution, the major conformer observed for all dipeptides is that in which the amide bond between the pseudoproline and the preceding amino acid is cis. For dipeptides in which the N-terminus is protected, the ratio of cis- to trans-conformers does not depend significantly on the side chain of the N-terminal amino acid, or the stereochemistry of the Thr residue. However, for dipeptides bearing a free N-terminus, there are significant differences in the ratios of cis- to trans-conformers depending on the side chain present. Three dipeptides were crystallized and their X-ray structures determined. In two cases, (benzyloxycarbonyl (Cbz)-Val-Thr(ΨMe,Mepro)-OMe and Cbz-Val-Thr(ΨMe,Mepro)-OH), the dipeptides adopt a trans-conformation in the solid state, in contrast to the structures observed in solution. In the third case, (9-fluorenylmethoxycarbonyl (Fmoc)-Val-d-allo-Thr(ΨMe,Mepro)-OH), a cis-amide geometry is observed. These structural differences are attributed to crystal-packing interactions.
核磁共振研究了 14 种源自苏氨酸的含假脯氨酸二肽(包括四种 d-allo-Thr 衍生物)的构象。在溶液中,所有二肽的主要构象都是假脯氨酸与前一个氨基酸之间的酰胺键为顺式。对于 N 端受到保护的二肽,顺式构象与反式构象的比例与 N 端氨基酸的侧链或 Thr 残基的立体化学关系不大。然而,对于具有自由 N 端的二肽,顺式转化物和反式转化物的比例因侧链的存在而存在显著差异。我们对三种二肽进行了结晶,并测定了它们的 X 射线结构。在两种情况下(苄氧羰基(Cbz)-Val-Thr(ΨMe,Mepro)-OMe 和 Cbz-Val-Thr(ΨMe,Mepro)-OH ),二肽在固态下采用了反式构象,这与在溶液中观察到的结构截然不同。在第三种情况(9-芴甲氧羰基(Fmoc)-Val-d-allo-Thr(ΨMe,Mepro)-OH)中,观察到的是顺式酰胺几何结构。这些结构差异归因于晶体-堆积相互作用。