Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556-5670, USA.
Molecular Structure Facility, University of Notre Dame, Notre Dame, IN 46556-5670, USA.
Phys Chem Chem Phys. 2021 Oct 20;23(40):22912-22922. doi: 10.1039/d1cp03320d.
Seven doubly C-labeled isotopomers of methyl β-D-glucopyranoside, methyl β-D-xylopyranoside, methyl β-D-galactopyranoside, methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside and methyl β-D-galactopyranosyl-(1→4)-β-D-xylopyranoside were prepared, crystallized, and studied by single-crystal X-ray crystallography and solid-state C NMR spectroscopy to determine experimentally the dependence of values in aldopyranosyl rings on the C1-C2-O2-H torsion angle, , involving the C2 carbon of the C1-C2-C3 coupling pathway. Using X-ray crystal structures to determine in crystalline samples and by selecting compounds that exhibit a relatively wide range of values in the crystalline state, values measured in crystalline samples were plotted against and the resulting plot compared to that obtained from density functional theory (DFT) calculations. For values ranging from ∼90° to ∼240°, very good agreement was observed between the experimental and theoretical plots, providing strong validation of DFT-calculated spin-coupling dependencies on exocyclic C-O bond conformation involving the central carbon of geminal C-C-C coupling pathways. These findings provide new experimental evidence supporting the use of values as non-conventional spin-coupling constraints in MA'AT conformational modeling of saccharides in solution, and the use of NMR spin-couplings not involving coupled hydroxyl hydrogens as indirect probes of C-O bond conformation. Solvomorphism was observed in crystalline βGal-(1→4)-βGlcOCH wherein the previously-reported methanol solvate form was found to spontaneously convert to a monohydrate upon air-drying, leading to small but discernible conformational changes in, and a new crystalline form of, this disaccharide.
制备并结晶了 7 种双 C 标记的甲基 β-D-吡喃葡萄糖苷、甲基 β-D-吡喃木糖苷、甲基 β-D-吡喃半乳糖苷、甲基 β-D-吡喃半乳糖基-(1→4)-β-D-吡喃葡萄糖苷和甲基 β-D-吡喃半乳糖基-(1→4)-β-D-吡喃木糖苷的同位素异构体，通过单晶 X 射线晶体学和固态 C NMR 光谱学进行了研究，以实验确定涉及 C1-C2-C3 偶联途径的 C2 碳原子的 aldopyranosyl 环中 值对 C1-C2-O2-H 扭转角， 的依赖性。使用 X 射线晶体结构来确定晶态样品中的 值，并选择在晶态下表现出相对较宽 值范围的化合物，将在晶态样品中测量的值与 值和从密度泛函理论 (DFT) 计算得到的 值进行作图比较。对于从 ∼90°到 ∼240°的值范围，观察到实验和理论图之间非常好的一致性，为 DFT 计算的自旋偶合对涉及偕二 C-C-C 偶联途径中心碳原子的外环 C-O 键构象的依赖性提供了强有力的验证。这些发现为使用 值作为糖在溶液中的 MA'AT 构象建模中的非常规自旋偶合约束以及不涉及偶合羟基氢的 NMR 自旋偶合作为 C-O 键构象的间接探针提供了新的实验证据。在结晶 βGal-(1→4)-βGlcOCH 中观察到溶剂化作用，其中先前报道的甲醇溶剂化物形式在空气干燥时被发现自发转化为一水合物，导致该二糖的 和新的结晶形式发生微小但可辨别的构象变化。
