Conformational analysis of natural products

13C glycosylation shifts and conformation

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observation that chemical shift changes (ΔδC) are related to the proton-proton distance between interacting carbons was originally made in hydrocarbons.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.

rH,H(Å)=2.961-0.0776Δδ (ppm)
Where rH,H is the distance between H-1 of the non-reducing residue and the proton at the substituted position in the reducing residue.

Since the 1H-1H distances are related to the glycosidic torsion angles the following relationships can be derived (assuming that φH remains constant):

ψH(°)=-54.24+5.296ΔδC-1
ψH(°)=-56.93+5.133ΔδC-n

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.
glycosylation shifts:
non-reducing end, ΔδC-1 ppm
reducing end, ΔδC-n ppm
ψH °
rH,H Å
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°.
Results for some α-linked disaccharides
disaccharide ΔδC1' ΔδCn ψH rH,H
α-D-Glc-(→2)-α-D-GlcOMe4.584.63-32°2.60Å
α-D-Glc-(→2)-β-D-GlcOMe5.654.68-29°2.56Å
α-D-Glc-(→2)-α-D-ManOMe8.709.33-9°2.26Å
α-D-Glc-(→2)-β-D-ManOMe8.737.70-13°2.32Å
α-D-Glc-(→3)-α-D-GlcOMe6.967.41-18°2.40Å
α-D-Glc-(→3)-β-D-GlcOMe6.957.15-19°2.41Å
α-D-Glc-(→3)-α-D-ManOMe8.657.98-12°2.32Å
α-D-Glc-(→3)-α-D-GalOMe3.055.14-34°2.64Å
α-D-Glc-(→4)-α-D-GalOMe8.079.51-10°2.28Å
α-D-Glc-(→4)-α-D-GlcOMe7.667.76-15°2.36Å
α-D-Glc-(→4)-β-D-GlcOMe7.597.61-16°2.37Å
α-D-Man-(→2)-α-D-GlcOMe3.523.71-37°2.68Å
α-D-Man-(→2)-β-D-GlcOMe5.654.98-28°2.55Å
α-D-Man-(→2)-α-D-ManOMe8.058.24-13°2.33Å
α-D-Man-(→3)-α-D-ManOMe8.037.60-15°2.35Å

"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.