Department of Food Science, Cornell University, Ithaca, NY 14853, United States.
Carbohydr Res. 2010 Sep 23;345(14):2060-6. doi: 10.1016/j.carres.2010.06.017. Epub 2010 Jul 6.
Molecular dynamics simulations have been used to calculate the potentials of mean force for separating short cellooligomers in aqueous solution as a means of estimating the contributions of hydrophobic stacking and hydrogen bonding to the insolubility of crystalline cellulose. A series of four potential of mean force (pmf) calculations for glucose, cellobiose, cellotriose, and cellotetraose in aqueous solution were performed for situations in which the molecules were initially placed with their hydrophobic faces stacked against one another, and another for the cases where the molecules were initially placed adjacent to one another in a co-planar, hydrogen-bonded arrangement, as they would be in cellulose Ibeta. From these calculations, it was found that hydrophobic association does indeed favor a crystal-like structure over solution, as might be expected. Somewhat more surprisingly, hydrogen bonding also favored the crystal packing, possibly in part because of the high entropic cost for hydrating glucose hydroxyl groups, which significantly restricts the configurational freedom of the hydrogen-bonded waters. The crystal was also favored by the observation that there was no increase in chain configurational entropy upon dissolution, because the free chain adopts only one conformation, as previously observed, but against intuitive expectations, apparently due to the persistence of the intramolecular O3-O5 hydrogen bond.
分子动力学模拟已被用于计算短纤维素寡聚物在水溶液中分离的平均力势，以估计疏水性堆积和氢键对结晶纤维素不溶性的贡献。对葡萄糖、纤维二糖、纤维三糖和纤维四糖在水溶液中的四种平均力势（pmf）计算进行了一系列计算，其中分子最初被放置在疏水面彼此堆叠的位置，另一种情况是分子最初以共面、氢键排列的方式相邻放置，就像它们在纤维素 Ibeta 中一样。从这些计算中可以发现，疏水性缔合确实有利于形成类似于晶体的结构，这可能是意料之中的。更令人惊讶的是，氢键也有利于晶体堆积，部分原因可能是葡萄糖羟基水合的高熵成本，这极大地限制了氢键水的构象自由度。观察到溶解时链构象熵没有增加，这也有利于晶体，因为游离链仅采用一种构象，如前所述，但与直观的预期相反，显然是由于分子内 O3-O5 氢键的存在。
