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Conformational analysis
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The glycosylation shifts in α-linked gluco- and
galacto- residues have been related to the 1H-1H
distance across the glycosidic linkage and the ψH torsion
angle. The observation that chemical shift changes (ΔδC) are related to the proton-proton distance between interacting carbons was originally made in hydrocarbons. In carbohydrates various electronic and stereoelectronic effects play a much greater role and often obscure the effect of short 1H-1H distances. Thus, this equation is only valid for α-linkages and only for a limited range of torsion angles. There are newer attempts at correlations between the 13C chemical shifts of the anomeric and the glycosylated carbon resonances, in particular for solid state studies of starches and celluloses, but they are all rather limited in scope.
Since the 1H-1H distances are related to the glycosidic torsion angles the following relationships can be derived (assuming that φH remains constant):
Note that this relation implies that the glycosylation shifts of the two
carbon atoms on opposite sides should be almost the same. This gives an idea of how
accurate the relation, at best, can be expected to be.
This is a toy - do not rely on the results! Enter the glycosylations shifts below to get an estimate of ψH and rH,H. The relation has been derived from values in the following ranges: 2.2<rH,H<3.0 0<ΔδC<10 -55°<ψH<5° φH is assumed to be constant, somewhere in the range 40°-60°.
"Conformational Dependence of 13C Nuclear Magnetic Resonance Chemical Shifts in Oligosaccharides", K. Bock, A. Brignole and B. W. Sigurskjold; J.C.S., Perkin Trans. II (1986) 1711-1713. |