Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
College of Chemistry, University of California, Berkeley , Berkeley, California 94720, United States.
J Am Chem Soc. 2018 Jan 17;140(2):827-833. doi: 10.1021/jacs.7b11891. Epub 2018 Jan 8.
Peptoid polymers are often crystalline in the solid-state as examined by X-ray scattering, but thus far, there has been no attempt to identify a common structural motif among them. In order to probe the relationship between molecular structure and crystal structure, we synthesized and analyzed a series of crystalline peptoid copolymers, systematically varying peptoid side-chain length (S) and main-chain length (N). We also examined X-ray scattering data from 18 previously reported peptoid polymers. In all peptoids, we found that the unit cell dimensions, a, b, and c, are simple functions of S and N: a (Å) = 4.55, b (Å) = [2.98]N + 0.35, and c (Å) = [1.86]S + 5.5. These relationships, which apply to both bulk crystals and self-assembled nanosheets in water, indicate that the molecules adopt extended, planar conformations. Furthermore, we performed molecular dynamics simulations (MD) of peptoid polymer lattices, which indicate that all backbone amides adopt the cis conformation. This is a surprising conclusion, because previous studies on isolated molecules indicated an energetic preference for the trans conformer. This study demonstrates that when packed into supramolecular lattices or crystals, peptoid polymers prefer to adopt a regular, extended, all-cis secondary structure.
肽聚合物在固态下通常通过 X 射线散射来检查是结晶的，但到目前为止，还没有人试图确定它们之间的共同结构模式。为了探究分子结构和晶体结构之间的关系，我们合成并分析了一系列结晶肽共聚物，系统地改变了肽侧链长度（S）和主链长度（N）。我们还检查了 18 种先前报道的肽聚合物的 X 射线散射数据。在所有的肽中，我们发现晶胞尺寸 a、b 和 c 是 S 和 N 的简单函数：a（Å）= 4.55，b（Å）= [2.98]N + 0.35，c（Å）= [1.86]S + 5.5。这些关系适用于体相晶体和水中自组装的纳米片，表明分子采用扩展的平面构象。此外，我们对肽聚合物晶格进行了分子动力学模拟（MD），表明所有的主链酰胺都采用顺式构象。这是一个令人惊讶的结论，因为以前对孤立分子的研究表明，反式构象具有能量优势。本研究表明，当被包装成超分子晶格或晶体时，肽聚合物倾向于采用规则的、扩展的、全顺式二级结构。
