Conformation of the complex oligosaccharides of glycoproteins. A vacuum ultraviolet circular dichroism study.
Circular dichroism spectroscopy in the 170-220-nm range of the ultraviolet and measurements of the amide proton coupling constants in nmr have been used to investigate the conformation of asparagine-linked glycopeptides having oligosaccharide chains of the complex antenna type. The CD spectra can be explained as the sum of three contributions, the first of which is a pair of large bands of opposite sign resulting from coupling of the adjacent chromophores of the 2-acetamido-1-N-(4-aspartyl)-2-deoxy-beta-D-glucopyranosylamine linkage. Secondly the amide chromophore of the core N-acetylglucosamine residue substituted at carbon 4 by a beta-mannosyl residue contributes a negative band at 210 nm along with a small negative signal in the 180-190-nm region. The amides of the antenna N-acetylglucosamine residues, which in some cases are substituted by beta-galactosyl residues at carbon 4, contribute a negative band at 210 and a strong positive band at 185-190 nm. Since these three contributions are approximately independent, we can detect no long range interactions between nonreducing termini and the glycopeptide linkage region. Both CD and nmr data imply that the glucosaminyl-asparagine linkage is rigidly fixed in a conformation having the amide protons trans to the sugar ring protons. These results are consistent with an extended conformation in the shape of a "Y" or a "T" for complex type asialo-oligosaccharide chains.