Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud 11, Orsay, France.
J Struct Biol. 2011 Feb;173(2):197-201. doi: 10.1016/j.jsb.2010.11.018. Epub 2010 Nov 24.
A characteristic feature of the dense phases formed by fiber-shaped molecules is their organization into parallel rods packed in a hexagonal or pseudo-hexagonal lateral network. This is typically the case for the collagen triple helices inside fibrils, as confirmed by recent X-ray diffraction experiments carried out on highly crystallized fibers obtained by immersing the freshly extracted fibers in a salt-controlled medium. However such diffraction patterns also generally exhibit additional features in the form of diffuse scattering, which is a clear signature of a low degree of lateral ordering. Only few studies have analyzed and modeled the lateral packing of collagen triple helices when the structure is disordered. Some authors have used the concept of short-range order but this approach does not contain any echo of a hexagonal order. In this study, we use an analytical expression derived from the paracrystal model which retains the hexagonal symmetry information and leads to a good agreement with the experimental data in the medium-angle region. This method is quite sensitive to the degree of disorder and to the inter-object distance. One clear result is that the shift in peak positions, generally attributed to variations in intermolecular distances, can also arise from a change in the degree of ordering without any significant modification of the distances. This underlines the importance of evaluating the degree of ordering before attributing a shift in peak position to a change in the unit-cell. This method is generic and can be applied to any system composed of rod-shaped molecules.
纤维状分子形成的密集相的一个特征是它们组织成平行棒，这些棒以六边形或拟六边形的侧网状结构排列。这是纤维状内部胶原三螺旋的典型情况，这一点已被最近在将新鲜提取的纤维浸入盐控制的介质中获得的高度结晶纤维上进行的 X 射线衍射实验所证实。然而，这种衍射模式通常还以漫散射的形式表现出其他特征，这是侧序排列程度低的明显标志。只有少数研究分析和模拟了胶原三螺旋在无序时的侧堆积。一些作者使用了短程有序的概念，但这种方法没有任何六边形有序的回声。在这项研究中，我们使用了一种从准晶模型导出的分析表达式，该模型保留了六边形对称性信息，并在中角度区域与实验数据很好地吻合。这种方法对无序程度和物体间距离非常敏感。一个明显的结果是，峰位置的偏移，通常归因于分子间距离的变化，也可能是由于有序程度的变化而产生，而距离没有任何显著的变化。这强调了在将峰位置的偏移归因于单元的变化之前，评估有序程度的重要性。该方法是通用的，可以应用于任何由棒状分子组成的系统。
