Conformational analysis of natural products

Hetero- and homonuclear coupling constant calculation

Measurement of long-range carbon-carbon coupling constants in a uniformly enriched complex polysaccharide.

Xu Q, Bush CA; Carbohydr Res 306 (1998) 335-339
PubMed 9648243


A quantitative coherence transfer scheme for 1H-detected measurement of long-range carbon-carbon coupling constants in NMR spectra of complex carbohydrates is described. It is applied to a uniformly highly 13C-enriched monosaccharide and to a complex cell wall polysaccharide from Streptococcus mitis J22 having seven distinct sugars in the repeating subunit. Coupling values within the ring were compared to published values for monosaccharides to demonstrate the validity of the method. An attempt was made to relate coupling constants between carbon atoms across the glycosidic linkage to the dihedral angles of a recently published flexible model for the polysaccharide which is based on 3JCH data. The experimental coupling constants do not agree with any single conformation demonstrating that the repeating subunit of the polysaccharide must be flexible. This conclusion is in accord with results of molecular modeling nuclear Overhauser effect and 3JCH data.


Karplus curve

Coupling constant calculation

Karplus-type equations are frequently used to relate vicinal coupling constants, i. e. 3J, to torsion angles. To calculate a coupling constant for a given dihedral angle (θ) enter a value in the form and press Calculate.
To calculate a torsion angle from a coupling constant enter the coupling constant in the 3J field press Calculate. There may be up to four solutions.
The results are shown below!
Torsion angle (θ): °
Coupling constant (3J): Hz