Bekiroglu Somer, Sandström Anders, Kenne Lennart, Sandström Corine
Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, SE-750 07 Uppsala, Sweden.
Org Biomol Chem. 2004 Jan 21;2(2):200-5. doi: 10.1039/b311852e. Epub 2003 Dec 10.
Density functional theory (DFT) and Hartree-Fock (HF) quantum mechanical calculations have been performed on the disaccharides, [small beta]-l-Fucp-(1[rightward arrow]4)-[small alpha]-d-Galp-OMe, [small beta]-l-Fucp-(1[rightward arrow]4)-[small alpha]-d-Glcp-OMe, and [small beta]-l-Fucp-(1[rightward arrow]3)-[small alpha]-d-Glcp-OMe. The [capital Delta][small delta]-values (difference between the chemical shift in the disaccharide and the corresponding monosaccharide methyl glycoside) for the exchangeable hydroxy protons have been calculated and compared to experimental values previously measured by NMR spectroscopy for samples in aqueous solutions. The calculations performed on molecules in vacuum showed that hydroxy protons hydrogen bonded to the neighboring ring oxygens have large positive [capital Delta][small delta]-values, indicating that they are deshielded relative to those in the corresponding methyl glycoside. The NMR experiments showed instead that these hydroxy protons close to the neighboring ring oxygens were shielded. This discrepancy between calculated and experimental data was attributed to solvent effects, and this hypothesis has been confirmed in this work by monitoring the chemical shift of the hydroxy proton of methanol in water, ethers and water/ether solutions. Shielding of the hydroxy proton of methanol is observed for increased ether concentrations, whereas deshielding is observed for increased concentration of water. The shielding observed for hydroxy protons in disaccharides is a consequence of reduced hydration due to intermolecular hydrogen bonding or steric effects. In strongly hydrated systems such as carbohydrates, the hydration state of a hydroxy proton is the key factor determining the value of the chemical shift of its NMR signal, and the [capital Delta][small delta] will be a direct measure of the change in hydration state.
已对二糖β-L-岩藻糖基-(1→4)-α-D-半乳糖基-OMe、β-L-岩藻糖基-(1→4)-α-D-葡萄糖基-OMe和β-L-岩藻糖基-(1→3)-α-D-葡萄糖基-OMe进行了密度泛函理论（DFT）和哈特里-福克（HF）量子力学计算。计算了可交换羟基质子的Δδ值（二糖与相应单糖甲基糖苷化学位移之差），并与先前通过核磁共振光谱法测量的水溶液样品的实验值进行了比较。在真空中对分子进行的计算表明，与相邻环氧形成氢键的羟基质子具有较大的正Δδ值，表明它们相对于相应甲基糖苷中的质子去屏蔽。相反，核磁共振实验表明，这些靠近相邻环氧的羟基质子被屏蔽。计算数据与实验数据之间的这种差异归因于溶剂效应，并且在这项工作中通过监测甲醇羟基质子在水、醚和水/醚溶液中的化学位移证实了这一假设。随着醚浓度的增加，观察到甲醇羟基质子的屏蔽，而随着水浓度的增加，观察到去屏蔽。在二糖中观察到的羟基质子屏蔽是由于分子间氢键或空间效应导致水合作用降低的结果。在碳水化合物等强水合体系中，羟基质子的水合状态是决定其核磁共振信号化学位移值的关键因素，而Δδ将直接衡量水合状态的变化。
