O-Acetyl Side-Chains in Monosaccharides: Redundant NMR Spin-Couplings and Statistical Models for Acetate Ester Conformational Analysis

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• 作者：Toby Turney ; Qingfeng Pan ; Luke Sernau ; Ian Carmichael ; Wenhui Zhang ; Xiaocong Wang ; Robert J. Woods ; Anthony S. Serianni
• 刊名：Journal of Physical Chemistry B
• 出版年：2017
• 出版时间：January 12, 2017
• 年：2017
• 卷：121
• 期：1
• 页码：66-77
• 全文大小：766K
• ISSN：1520-5207

文摘

α- and β-<span class="smallcaps">dspan>-glucopyranose monoacetates 1–3 were prepared with selective <sup>13sup>C enrichment in the O-acetyl side-chain, and ensembles of <sup>13sup>C–<sup>1sup>H and <sup>13sup>C–<sup>13sup>C NMR spin-couplings (J-couplings) were measured involving the labeled carbons. Density functional theory (DFT) was applied to a set of model structures to determine which J-couplings are sensitive to rotation of the ester bond θ. Eight J-couplings (<sup>1sup>J<sub>CCsub>, <sup>2sup>J<sub>CHsub>, <sup>2sup>J<sub>CCsub>, <sup>3sup>J<sub>CHsub>, and <sup>3sup>J<sub>CCsub>) were found to be sensitive to θ, and four equations were parametrized to allow quantitative interpretations of experimental J-values. Inspection of J-coupling ensembles in 1–3 showed that O-acetyl side-chain conformation depends on molecular context, with flanking groups playing a dominant role in determining the properties of θ in solution. To quantify these effects, ensembles of J-couplings containing four values were used to determine the precision and accuracy of several 2-parameter statistical models of rotamer distributions across θ in 1–3. The statistical method used to generate these models has been encoded in a newly developed program, MA′AT, which is available for public use. These models were compared to O-acetyl side-chain behavior observed in a representative sample of crystal structures, and in molecular dynamics (MD) simulations of O-acetylated model structures. While the functional form of the model had little effect on the precision of the calculated mean of θ in 1–3, platykurtic models were found to give more precise estimates of the width of the distribution about the mean (expressed as circular standard deviations). Validation of these 2-parameter models to interpret ensembles of redundant J-couplings using the O-acetyl system as a test case enables future extension of the approach to other flexible elements in saccharides, such as glycosidic linkage conformation.