Monosaccharides, configurational protons

Introduction to 2-D NMR experiments The purpose of the standard 1-D H NMR experiment is to achieve structure-related information about sample protons (i.e., chemical shifts, spin-spin couplings, and integration data) describing the relative number of protons. Applied to anthocyanins, this information may help to identify the aglycone (anthocyanidin), number of monosaccharides present, and anomeric configuration of the monosaccharides. However, for most anthocyanins, the information gained by a standard 1 -D H NMR experiment is insufficient for complete structure elucidation. In recent years, various 2-D NMR experiments have evolved as the most powerful tools for complete structure elucidation of anthocyanins. 

A very useful source on various NMR techniques and chemical shifts of functionalized carbon atoms that contains tabulated data on monosaccharides is Reference [23]. Specifically correlating functional groups with chemical shifts of protons and the associated carbon is an important tool for elucidating the stmcture of the monosaccharide moiety of many complex oligosaccharides. Since monosaccharides contain one or more chiral centers, absolute configuration must be known in order to predict the conformational shape of the molecules. The outcome of many stereoselective functionalizations of free and partially functionalized monosaccharides depends on conformation. 

Analysis of the monosaccharide signals in the C-NMR spectrum (103.1, 77.9, 75.0, 71.3 and 66.9) and further assignment of all proton and carbon chemical shifts using H- H COSY and HMQC experiments indicated the presence of a xylose residue. The presence of xylose was confirmed by acid hydrolysis of 44 with aqueous 2N trifluoroacetic acid followed by GC analysis of the corresponding peracetylated alditol. The D-configuration was determined by GC analysis of the l-[(S)-A-acetyl-(2-hydroxypropylamino)]-l-deoxy alditol acetate derivative as for... 

Since the first report (5) of a pronounced configurational dependence for the anomeric protons of monosaccharides in aqueous solutions, studies in this laboratory have shown (6), (7 ), (8), (9) that proton Ri-values have a variety of stereospecific dependencies and reflect the overall steric environment of a proton. In the following discussion, a brief introduction to the phenomenon of spin-lattice relaxation and to the measurement of proton Ri-values by pulse Fourier transform methods, will be... 

It was concluded that the preferred solution conformation of a disaccharide depends on the absolute configurations of the constituent monosaccharides, the position and configuration of the glycosidic linkage, and on the orientation of the proton at the aglycon carbon associated with the glycosidic linkage emd the adjacent carbon atoms. 

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