O-Acetylation of carbohydrates such as sialic acids is common in nature, but its role is not clearly understood due to the lability of O-acetyl groups. We demonstrated previously that 9-acetamido-9-deoxy-N-acetylneuraminic acid (Neu5Ac9NAc) is a chemically and biologically stable mimic of the 9-O-acetyl-N-acetylneuraminic acid (Neu5,9Ac₂) of the corresponding sialoglycans. Here, a systematic nuclear magnetic resonance (NMR) spectroscopic and molecular dynamics (MD) simulation study was undertaken for Neu5,9Ac₂-containing GM3 ganglioside glycan (GM3-glycan) and its Neu5Ac9NAc analog. GM3-glycan with Neu5Ac as the non-O-acetyl form of Neu5,9Ac₂ was used as a control. Complete ¹H and ¹³C NMR chemical shift assignments, three-bond ¹H-¹³C trans-glycosidic coupling constants (³J_CH), accurate ¹H-¹H coupling constants (³J_HH), nuclear Overhauser effects and hydrogen bonding detection were carried out. Results show that structural modification (O- or N-acetylation) on the C-9 of Neu5Ac in GM3 glycan does not cause significant conformational changes on either its glycosidic dihedral angles or its secondary structure. All structural differences are confined to the Neu5Ac glycerol chain, and minor temperature-dependent changes are seen in the aglycone portion. We also used Density Functional Theory (DFT) quantum mechanical calculations to improve currently used ³J_HH Karplus relations. Furthermore, OH chemical shifts were assigned at −10°C and no evidence of an intramolecular hydrogen bond was observed. The results provide additional evidence regarding structural similarities between sialosides containing 9-N-acetylated and 9-O-acetylated Neu5Ac and support the opportunity of using 9-N-acetylated Neu5Ac as a stable mimic to study the biochemical role of 9-O-acetylated Neu5Ac.

中文翻译：

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含 9-O-乙酰唾液酸的 GM3 神经节苷脂聚糖及其 9-N-乙酰模拟物的 NMR、MD 和 DFT 构象组合分析。

唾液酸等碳水化合物的O-乙酰化在自然界中很常见，但由于O-乙酰基团的不稳定性，其作用尚不清楚。我们之前证明了 9-乙酰氨基-9-脱氧-N-乙酰神经氨酸 (Neu5Ac9NAc ) 是相应唾液酸聚糖的 9- O-乙酰-N-乙酰神经氨酸 (Neu5,9Ac ₂ ) 的化学和生物稳定模拟物。在这里，对含有Neu5,9Ac ₂的 GM3 神经节苷脂聚糖 (GM3-聚糖) 及其 Neu5Ac9NAc 类似物进行了系统的核磁共振 (NMR) 光谱和分子动力学 (MD) 模拟研究。GM3-聚糖与 Neu5Ac 作为Neu5,9Ac的非O-乙酰基形式₂用作对照。完整的¹ H 和¹³ C NMR 化学位移分配、三键¹ H- ¹³ C 转糖苷偶联常数 ( ³ J _CH )、准确的¹ H- ¹ H 偶联常数 ( ³ J _HH )、核 Overhauser 效应和氢键进行了检测。结果表明结构修饰（O - 或N-乙酰化）在 GM3 聚糖中 Neu5Ac 的 C-9 上不会引起其糖苷二面角或其二级结构的显着构象变化。所有结构差异都仅限于 Neu5Ac 甘油链，在苷元部分可以看到轻微的温度依赖性变化。我们还使用密度泛函理论 (DFT) 量子力学计算来改进当前使用的³ J _HH Karplus 关系。此外，OH 化学位移被指定在 -10°C，并且没有观察到分子内氢键的证据。结果提供了关于含有 9- N-乙酰化和 9- O-乙酰化 Neu5Ac 的唾液酸苷之间结构相似性的额外证据，并支持使用 9-N-乙酰化 Neu5Ac 作为稳定的模拟物来研究 9- O-乙酰化 Neu5Ac的生化作用。