Department of Chemistry, Drexel University, 3201 Chestnut Street, Philadelphia, Pennsylvania 19104, United States.
J Am Chem Soc. 2011 Feb 2;133(4):1066-76. doi: 10.1021/ja1089827. Epub 2010 Dec 27.
Amyloid fibrils are affiliated with various human pathologies. Knowledge of their molecular architecture is necessary for a detailed understanding of the mechanism of fibril formation. Vibrational circular dichroism (VCD) spectroscopy has recently shown sensitivity to amyloid fibrils [Ma et al. J. Am. Chem. Soc. 2007, 129, 12364 and Measey et al. J. Am. Chem. Soc. 2009, 131, 18218]. In particular, amyloid fibrils give rise to an intensity enhanced signal in the amide I band region of the corresponding VCD spectrum, offering promise of utilizing such a method for probing fibrillogenesis and the chiral structure of fibrils. Herein, we further investigate this phenomenon and demonstrate the use of VCD to probe the fibril formation kinetics of a short alanine-rich peptide. To elucidate the origin of the anomalous VCD intensity enhancement, we use an excitonic coupling model to simulate the VCD spectrum of stacked β-sheets containing one (Ising-like model) and two amide I oscillators per strand, as models for the underlying amyloid-fibril secondary structure. With this simple model, we show that the VCD intensity enhancement of amyloid-like fibrils results from intrasheet and, to a more limited extent, also from intersheet vibrational coupling between stacked β-sheets. The enhancement requires helically twisted sheets and is most pronounced for arrangements with parallel-oriented strands. Both the intersheet distance and the orientation of the amide I transition dipole moments of neighboring sheets are found to modulate the intensity enhancement of the amide I VCD signal. Moreover, our simulations suggest that, depending on the three-dimensional arrangement of the β-strands, the sign of the VCD signal of amyloid-like fibrils can be used to distinguish between right- and left-handed helical twists of parallel-oriented β-sheets. We compare the results of our simulation to experimental spectra of two short peptides, GNNQQNY, the N-terminal peptide fragment of the yeast prion protein Sup35, and an amyloidogenic alanine-rich peptide, AKY8. Our results demonstrate the advantages of using VCD spectroscopy to probe the kinetics of peptide and protein aggregation as well as the chirality of the resulting supramolecular structure.
淀粉样纤维与多种人类病理学有关。为了详细了解纤维形成的机制，了解其分子结构是必要的。振动圆二色性（VCD）光谱最近已显示出对淀粉样纤维的敏感性[Ma 等人，J. Am. Chem. Soc. 2007，129，12364 和 Measey 等人，J. Am. Chem. Soc. 2009，131，18218]。特别是，淀粉样纤维在相应的 VCD 光谱酰胺 I 带区域产生强度增强的信号，为利用这种方法探测纤维形成和纤维的手性结构提供了希望。在此，我们进一步研究了这一现象，并证明了利用 VCD 探测富含丙氨酸的短肽的纤维形成动力学。为了阐明异常 VCD 强度增强的起源，我们使用激子耦合模型模拟了含有一个（Ising 样模型）和两个酰胺 I 振荡器/链的堆叠β-片层的 VCD 光谱，作为潜在的淀粉样纤维二级结构的模型。使用这个简单的模型，我们表明，类似淀粉样纤维的 VCD 强度增强是由于片内以及在更有限的程度上也来自堆叠β-片层之间的振动耦合。增强需要螺旋扭曲的片层，并且对于平行取向的链排列最为明显。发现相邻片层之间的片层间距离和酰胺 I 跃迁偶极矩的取向都调节酰胺 I VCD 信号的强度增强。此外，我们的模拟表明，取决于β-链的三维排列，类似淀粉样纤维的 VCD 信号的符号可用于区分平行取向的β-片层的右手和左手螺旋扭曲。我们将我们的模拟结果与两种短肽的实验光谱进行比较，这两种短肽分别是酵母朊病毒蛋白 Sup35 的 N 端肽片段 GNNQQNY 和富含丙氨酸的淀粉样肽 AKY8。我们的结果表明，使用 VCD 光谱探测肽和蛋白质聚集的动力学以及所得超分子结构的手性具有优势。
