Conformational analysis of natural products

Hetero- and homonuclear coupling constant calculation

Density Functional Calculations on Disaccharide Mimics: Studies of Molecular Geometries and Trans-O-glycosidic 3JCOCH and 3JCOCC Spin-Couplings

Cloran F, Carmichael I, Serianni AS; J Am Chem Soc 121 (1999) 9843-9851


Density functional theory (DFT) using the B3LYP functional and the split-valence 6-31G* basis set has been used to investigate the structures and conformations of four b-[1-4]-linked disaccharide mimics, 1-4. Systematic functional group substitution at sites near the glycosidic linkage was used to evaluate the effects of sterics and interresidue hydrogen bonding on the preferred glycosidic torsion angles phi and psi. Using DFT-optimized geometries, the same hybrid functional, and a specially designed basis set, vicinal NMR scalar coupling constants involving carbon (3JCOCH, 3JCOCC) across the glycosidic linkages of 1-4 were calculated as a function of the phi and psi torsion angles, and the resulting torsional dependencies were compared to recently reported experimental Karplus curves for these coupling pathways (Bose, B.; et al. J. Am. Chem. Soc. 1998, 120, 11158-11173). The new computational data are in excellent agreement with experimental results and confirm the general shape of the experimental curves. For 3JCOCH, however, small discrepancies were observed at the extreme dihedral angles, suggesting some deficiencies in the theory and/or experimental data. For 3JCOCC, the new computed couplings confirm the existence of terminal electronegative substituent effects on coupling magnitude, and computed couplings in the 0-100 range of dihedral angles lead to an improved Karplus curve for the interpretation of 3JCOCC values across the O-glycosidic linkages of oligosaccharides.


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