Conformation and circular dichroism of oligosaccharides and model glycosides containing neuraminic acid (5-acetamido-3,5-dideoxy-D-glycero-D-galacto-nonulopyranosonic acid) residues
Abstract
C.d. spectra have been recorded for N-acetylneuraminic acid, a widely occurring component of glycoproteins and glycolipids of animal cells and tissues, as well as for some of its synthetic and naturally occurring glycosides and related compounds. Principal spectral features are systemised in terms of the accessible chromophores, and the geometry and flexibility of glycosidic linkages. The main c.d. band, centred below 200 nm, is attributed to the acetamido group, and shows little structural or conformational sensitivity. Smaller spectral features at higher wavelength (around 225 nm) arise from the n→π* transition of the carboxy chromophore, and provide a convenient index of glycosidic configuration and conformation. Thus α-linked glycosides of N-acetylneuraminic acid show a negative band, while β-glycosides give rise to a positive peak. In both cases the magnitude of the band is increased by restricted mobility. The planar symmetry rule developed by Listowsky is even more successful in predicting the sign of observed optical activity in sterically unambiguous fused ring lactone systems than the earlier and more complex lactone sector rule. Application of the planar rule provides a satisfactory unified interpretation of the observed c.d. behaviour of the carboxy chromophore of N-acetylneuraminic acid and its derivatives, in terms of established conformational principles. C.d. of the tetrasaccharide glycitol derived from the major cell surface glycoprotein of human erythrocytes, shows perturbations additional to those that occur between adjacent residues in the covalent sequence, suggesting that non-bonded interactions between branches of glycoproteins can be relevant to their tertiary structure.