Statistical thermodynamics for the unexpectedly large difference between disaccharide stereoisomers in terms of solubility in water.
Statistical thermodynamics for the unexpectedly large difference between disaccharide stereoisomers in terms of solubility in water.
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DOI：
10.1039/c8cp04377a
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发表时间：
2018-09
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影响因子：
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通讯作者：
Simon Hikiri;Tomohiko Hayashi;M. Ikeguchi;M. Kinoshita
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文献类型：
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作者：
Simon Hikiri;Tomohiko Hayashi;M. Ikeguchi;M. Kinoshita
我们揭示了纤维二糖和麦芽糖之间巨大差异的物理起源，纤维二糖和麦芽糖分别由两个 β-1,4 和 α-1,4 连接的 d-葡萄糖单元组成，在水中的溶解度方面。我们构建了一个热力学理论，其中水和真空中二糖之间的化学势差被确定为关键的自由能函数。通过溶质水合的统计力学理论计算纤维二糖和麦芽糖的能量和熵成分。在原子水平上考虑二糖结构，并对水采用分子模型。分子动力学模拟用于解释二糖分子的构象波动，这也使我们能够估计构象熵。我们表明计算的纤维二糖/麦芽糖溶解度比与实验值非常吻合。由于构象熵和水熵效应，纤维二糖的溶解度变得更低。前一种效应与六元环中氧原子和邻近羟基之间的分子内氢键的更高稳定性有关：水合将分子构象的波动改变为更大或更不规则的波动，但这种改变的程度较小。后一种效应归因于分子中两个羟甲基基团的更多分离，导致水分子基团产生的排除体积重叠的可能性较低。我们认为，水中二糖的物理化学性质会根据水合效应的立体异构性而变化，并且该品种的起源是熵的。
We unravel the physical origins of the large difference between cellobiose and maltose, which consist of two β-1,4 and α-1,4 linked d-glucose units, respectively, in terms of the solubility in water. We construct a thermodynamic theory where the chemical-potential difference between disaccharides in water and in vacuum is identified as the key free-energy function. Its energetic and entropic components are calculated for cellobiose and maltose by statistical-mechanical theories for solute hydration. The disaccharide structures are taken into account at the atomic level and a molecular model is adopted for water. Molecular dynamics simulations are used to account for the conformational fluctuation of a disaccharide molecule, which also enables us to estimate the conformational entropy. We show that the cellobiose/maltose solubility ratio calculated is in good agreement with the experimental value. The solubility becomes much lower for cellobiose due to conformational-entropy and water-entropy effects. The former effect is relevant to higher stability of the intramolecular hydrogen bond between oxygen atoms in the six-membered ring and in the neighboring hydroxyl group: the hydration alters the fluctuation of a molecular conformation to a larger or less regular one, but the degree of this alteration is smaller. The latter effect is attributed to more separation of two hydroxymethyl groups in a molecule, causing lower probability of the overlap of excluded volumes generated by the groups for water molecules. We suggest that physicochemical properties of disaccharides in water become variable depending on the stereoisomerism through hydration effects and the origins of the variety are entropic.