Almond A, Brass A, Sheehan J K
Division of Biochemistry, School of Biological Sciences, University of Manchester, Manchester M13 9PT, UK.
Glycobiology. 1998 Oct;8(10):973-80. doi: 10.1093/glycob/8.10.973.
Studies of the hyaluronan (HA) tetrasaccharides are important for understanding hydrogen-bonding in the HA polymer, as they are probably the smallest oligomers in which characteristics of the constituent monosaccharides and the polymer are simultaneously exhibited. Here we present extensive molecular dynamics simulations of the two tetrasaccharides of HA in dilute aqueous solution. These simulations have confirmed the existence of intramolecular hydrogen-bonds between the neighboring sugar residues of HA in solution, as proposed by Scott (1989). However, our simulations predict that these intramolecular hydrogen-bonds are not static as previously proposed, but are in constant dynamic exchange on the sub-nanosecond time-scale. This process results in discrete internal motion of the HA tetrasaccharides where they rapidly move between low energy conformations. Specific interactions between water and intramolecular hydrogen-bonds involving the hydroxymethyl group were found to result in differing conformations and dynamics for the two alternative tetrasaccharides of HA. This new observation suggests that this residue may play a key role in the entropy and stability of HA in solution, allowing it to stay soluble up to high concentration. The vicinal coupling constants3 J NHCH of the acetamido groups have been calculated from our aqueous simulations of HA. We found that high values of 3J NHCH approximately 8 Hz, as experimentally measured for HA, are consistent with mixtures of both trans and cis conformations, and thus3 J NHCH cannot be used to imply a purely trans conformation of the acetamido. The rapid exchange of intramolecular hydrogen-bonds indicates that although the structure is at any moment stabilized by these hydrogen-bonds, no one hydrogen-bond exists for an extended period of time. This could explain why NMR often fails to provide evidence for intramolecular hydrogen-bonds in HA and other aqueous carbohydrate structures.
对透明质酸（HA）四糖的研究对于理解HA聚合物中的氢键很重要，因为它们可能是同时展现出组成单糖和聚合物特征的最小寡聚物。在此，我们展示了HA的两种四糖在稀水溶液中的广泛分子动力学模拟。这些模拟证实了溶液中HA相邻糖残基之间存在分子内氢键，正如Scott（1989年）所提出的那样。然而，我们的模拟预测这些分子内氢键并非如先前提出的那样是静态的，而是在亚纳秒时间尺度上处于持续的动态交换中。这个过程导致了HA四糖的离散内部运动，它们在低能量构象之间快速移动。发现水与涉及羟甲基的分子内氢键之间的特定相互作用导致了HA的两种替代四糖具有不同的构象和动力学。这一新观察结果表明，该残基可能在HA在溶液中的熵和稳定性方面起关键作用，使其能够在高浓度下保持可溶。通过我们对HA的水溶液模拟计算了乙酰氨基的邻位耦合常数3J NHCH。我们发现，如对HA实验测量的约8Hz的高3J NHCH值与反式和顺式构象的混合物一致，因此3J NHCH不能用于暗示乙酰氨基的纯反式构象。分子内氢键的快速交换表明，尽管结构在任何时刻都由这些氢键稳定，但没有一个氢键会长时间存在。这可以解释为什么NMR常常无法为HA和其他水性碳水化合物结构中的分子内氢键提供证据。
