Crystallography of carbohydrates

Anomeric and exoanomeric effects were studied with X-ray diffraction techniques and with quantum-chemical computational methods. To investigate the anomeric effect, the electron distribution in trans-2,5-dichloro-1,4-dioxane was studied by X-ray and neutron diffraction at 97 K. Electron-deformation-density maps were obtained by Fourier and multipole-expansion techniques. For comparison, ab initio calculations were carried out with the quantum-chemical program GAUSSIAN-82. Qualitatively, the theoretical maps agree well with the experimental findings. The predominance of the exoanomeric effect was found in an investigation of blood-group-antigenic-determinant oligosaccharides of the A B O system. The semiempirical quantum-chemical method PCILO (perturbation configuration interaction using localized orbitals) was used. The exact agreement for these two approaches shows that flexible substituents on sugar rings have practically no effect on the overall conformation of the clycosidic linkage, but in all cases the exoanomeric effect is operative in the lowest energy conformations. It is concluded that the antigenic determinant activity is only dependent on the configuration at the active site, but this structural difference does not influence the conformation of the ring system