Karle I L, Flippen-Anderson J L, Uma K, Balaram P
Laboratory for the Structure of Matter, Naval Research Laboratory Washington, DC 20375-5320.
Biopolymers. 1993 May;33(5):827-37. doi: 10.1002/bip.360330511.
The structure of the peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe has been determined in crystals obtained from a dimethylsulfoxide-isopropanol mixture. Crystal parameters are as follows: C38H69N7O10.H2O.2C3H7OH, space group P2(1), a = 10.350 (2) A b = 26.084 (4) A, c = 10.395 (2) A, beta = 96.87 (12), Z = 2, R = 8.7% for 2686 reflections observed > 3.0 sigma (F). A single 5-->1 hydrogen bond is observed at the N-terminus, while two 4-->1 hydrogen bonds characteristic of a 3(10)-helix are seen in the central segment. The C-terminus residues, Ala(6) and Leu(7) are extended, while Val(5) is considerably distorted from a helical conformation. Two isopropanol molecules make hydrogen bonds to the C-terminal segment, while a water molecule interacts with the N-terminus. The structure is in contrast to that obtained for the same peptide in crystals from methanol-water [I. L. Karle, J. L. Flippen-Anderson, K. Uma, and P. Balaram (1990) Proteins: Structure, Function and Genetics, Vol. 7, pp. 62-73] in which two independent molecules reveal an almost perfect alpha-helix and a helix penetrated by a water molecule. A comparison of the three structures provides a snapshot of the progressive effects of solvation leading to helix unwinding. The fragility of the heptapeptide helix in solution is demonstrated by nmr studies in CDCl3 and (CD3)2SO. A helical conformation is supported in the apolar solvent CDCl3, whereas almost complete unfolding is observed in the strongly solvating medium (CD3)2SO.
已在由二甲亚砜 - 异丙醇混合物获得的晶体中确定了肽Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe的结构。晶体参数如下：C38H69N7O10.H2O.2C3H7OH，空间群P2(1)，a = 10.350 (2) Å，b = 26.084 (4) Å，c = 10.395 (2) Å，β = 96.87 (12)，Z = 2，对于2686个观察到的I > 3.0σ(F)的反射，R = 8.7%。在N端观察到一个单一的5→1氢键，而在中间段看到两个3(10)-螺旋特有的4→1氢键。C端残基Ala(6)和Leu(7)是伸展的，而Val(5)从螺旋构象有相当大的扭曲。两个异丙醇分子与C端段形成氢键，而一个水分子与N端相互作用。该结构与在甲醇 - 水晶体中获得的相同肽的结构[I. L. 卡尔勒、J. L. 弗利彭 - 安德森、K. 乌玛和P. 巴拉拉姆(1990年)《蛋白质：结构、功能与遗传学》，第7卷，第62 - 73页]形成对比，在后者中两个独立分子呈现出几乎完美的α - 螺旋以及一个被水分子穿透的螺旋。这三种结构的比较提供了溶剂化导致螺旋解旋的渐进效应的快照。七肽螺旋在溶液中的脆弱性通过在CDCl3和(CD3)2SO中的核磁共振研究得到证明。在非极性溶剂CDCl3中支持螺旋构象，而在强溶剂化介质(CD3)2SO中观察到几乎完全展开。
