Poly-l-alanine (PLA) sequences are a key element in the structure of the crystalline domains of spider dragline silks, wild silkworm silks, antifreeze proteins, and amyloids. To date, no atomic-level structures of antiparallel (AP)-PLA longer than Ala₄ have been reported using the single-crystal X-ray diffraction analysis. In this work, dipolar-assisted rotational resonance solid-state NMR spectra were observed to determine the effective internuclear distances of ¹³C uniformly labeled alanine tetramer with antiparallel (AP) β-sheet structure whose atomic coordinates are determined from the X-ray crystallographic analysis. Initial build-up rates, R_j,k, were obtained from the build-up curves of the cross peaks by considering the internuclear distances arising in the master equation. Subsequently, experimentally obtained effective internuclear distances, r^eff_j,k(obs), were compared with the calculated r^eff_j,k(calc) values obtained from the X-ray crystallographic data. Fairly good correlation between r^eff_j,k(obs) and r^eff_j,k(calc) was obtained in the range of 1.0–6.0 Å, with the standard deviation of 0.244 Å, without considering the zero-quantum line-shape functions. It was further noted that the internuclear distances of intermolecular contributions provide details relating to the molecular packing in solid-state samples. Thus, the present data agree well with AP-β-sheet packing but do not agree with P-β-sheet packing.

中文翻译：

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反平行β-Sheet丙氨酸四聚体固态同核相关光谱的定量分析

聚-1-丙氨酸（PLA）序列是蜘蛛拉铲丝，野生蚕丝，抗冻蛋白和淀粉样蛋白的晶体结构域中的关键元素。迄今为止，使用单晶X射线衍射分析尚未报道出比Ala ₄长的反平行（AP）-PLA原子级结构。在这项工作中，观察到偶极辅助旋转共振固态NMR光谱，以确定¹³ C均匀标记的具有反平行（AP）β-折叠结构的丙氨酸四聚体的有效核间距，该原子的位置由X射线晶体学分析确定。初始堆积率R _j，k通过考虑主方程式中出现的核间距离，从交叉峰的累积曲线获得。随后，将实验获得的有效核间距离r ^eff _j，k（ob）与从X射线晶体学数据获得的计算出的r ^eff _j，k（calc）值进行比较。r ^eff _j，k（obs）与r ^eff _j，k之间的相关性相当好（calc）在1.0–6.0Å的范围内获得，标准偏差为0.244Å，而没有考虑零量子线形函数。还应注意的是，分子间作用的核间距离提供了有关固态样品中分子堆积的详细信息。因此，本数据与AP-β-片材包装非常吻合，但与P-β-片材包装不同。