Hadden Jodi A, French Alfred D, Woods Robert J
Complex Carbohydrate Center, University of Georgia, 315 Riverbend Road, Athens, GA 30602.
Southern Regional Research Center, U.S. Department of Agriculture, 1100 Robert E. Lee Boulevard, New Orleans, LA 70124.
Cellulose (Lond). 2014 Apr 1;21(2):879-884. doi: 10.1007/s10570-013-0051-z.
Previous studies of calculated diffraction patterns for cellulose crystallites suggest that distortions that arise once models have been subjected to MD simulation are the result of both microfibril twisting and changes in unit cell dimensions induced by the empirical force field; to date, it has not been possible to separate the individual contributions of these effects. To provide a better understanding of how twisting manifests in diffraction data, the present study demonstrates a method for generating twisted and linear cellulose structures that can be compared without the bias of dimensional changes, allowing assessment of the impact of twisting alone. Analysis of unit cell dimensions, microfibril volume, hydrogen bond patterns, glycosidic torsion angles, and hydroxymethyl group orientations confirmed that the twisted and linear structures collected with this method were internally consistent, and theoretical powder diffraction patterns for the two were shown to be effectively indistinguishable. These results indicate that differences between calculated patterns for the crystal coordinates and twisted structures from MD simulation can result entirely from changes in unit cell dimensions, and not from microfibril twisting alone. Although powder diffraction patterns for models in the 81-chain size regime were shown to be unaffected by twisting, suggesting that a modest degree of twist is not inconsistent with experimental data, it may be that other diffraction techniques are capable of detecting this structural difference. Until such time as definitive experimental evidence comes to light, the results of this study suggest that both twisted and linear microfibrils may represent an appropriate model for cellulose Iβ.
先前对纤维素微晶计算衍射图谱的研究表明，模型经过分子动力学（MD）模拟后出现的畸变是微纤丝扭曲和经验力场引起的晶胞尺寸变化共同作用的结果；迄今为止，尚无法区分这些效应各自的贡献。为了更好地理解扭曲在衍射数据中的表现，本研究展示了一种生成扭曲和线性纤维素结构的方法，这种方法可以在不考虑尺寸变化偏差的情况下进行比较，从而能够单独评估扭曲的影响。对晶胞尺寸、微纤丝体积、氢键模式、糖苷扭转角和羟甲基基团取向的分析证实，用这种方法收集的扭曲和线性结构在内部是一致的，并且两者的理论粉末衍射图谱显示实际上无法区分。这些结果表明，晶体坐标的计算图谱与MD模拟得到的扭曲结构之间的差异可能完全是由晶胞尺寸的变化导致的，而不仅仅是微纤丝扭曲造成的。尽管在81链尺寸范围内的模型的粉末衍射图谱显示不受扭曲影响，这表明适度程度的扭曲与实验数据并不矛盾，但可能其他衍射技术能够检测到这种结构差异。在有确凿的实验证据出现之前，本研究结果表明，扭曲和线性微纤丝都可能代表纤维素Iβ的合适模型。
