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carbohydrate anomer differences by NMR


Bubb, William A. 2003 Concepts in Magnetic Resonance, Part A 19A(1) 1–19. DOI 10.1002/cmr.a.10080
“A value of  170 Hz for 1JC-1,H-1 indicates an equatorial proton at C-1 (e.g.,  α-D-glucose), while 1JC-1,H-1  160 Hz indicates an axial proton (e.g.,  β-D-glucose); a number of mechanisms that rationalize this difference in terms of stereoelectronic factors have been advanced (74).”
 74. Tvaroska I, Taravel FR. 1995. Carbon–proton coupling constants in the conformational analysis of sugar molecules. Adv Carbohydr Chem Biochem 51:15–61.

The configuration of the anomeric carbon is obviously a key question in structure determination of a glycoside.  Besides the difference above, there are also differences in 3JHH between carbons-1 and 2 of glucosides and I would imagine other glycosides.  The chemical shift of H-1 is different for the two anomers of free glucose.  Yet, I thought that the difference in one bond coupling magnitudes was worth highlighting because it was surprising, at least to me.  With respect to the glucose group within lactose, the chemical shift of the C-1 carbon differs by about 4 ppm.

The alpha and beta isomers of pentaacetylglucose are also quite different by H-1 NMR.  The alpha isomer is found at 6.32 ppm, with J = 3.4 Hz.  The beta isomer is found at 5.71 ppm, with J = 6.7 Hz
see also

beta is 9-11 hertz for a chair of glucose. Alpha is 3-7 Hz. See Karplus curve for approximation of J values and dihedral angle. Did not read paper.

It was a little surprising to me that the values of 3J in the β-configurations of glucose and its derivatives are as small as they are, given that in a chair they would have an anti-relationship.  One of the papers that I consulted mentioned that the presence of electronegative groups lowers the value of 3JHH, but I do not recall the details.  A glance at Figure 1 of the reference below indicates that whether the electronegative group is axial or equatorial also affects the magnitude of the coupling constant.

Another passage in the reference below might be helpful if one is working with derivatives of mannose, which is the epimer of glucose at C-2.

"Indeed, the use of 1JCH is better in some cases than the use of 3JH-1,H-2, for example in α- and β-mannopyranose derivatives, where the 3J1,2 values are very similar."


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