Secondary structure of chondroitin sulphate in dimethyl sulphoxide

1H NMR spectra were obtained in fully deuterated dimethyl sulphoxide of sodium chondroitin 4-sulphate and of the tetrasaccharide, hexasaccharide and octasaccharide prepared from it. Two types of NH resonance were observed, at 7.6 ppm and 8.8 ppm. The latter was the only one in the polymer spectrum, whereas both were present in the oligomer spectra: N-acetylchondrosinate produced only the 7.6-ppm signal. The upfield resonance (7.6 ppm) is characteristic of "normal", monomeric acetamido sugars and disaccharides, while the downfield resonance (8.8 ppm) suggests a hydrogen-bonded NH. The elongation of disaccharide to tetrasaccharide produces a new environment, involving a hydrogen bond at the non-reducing disaccharide NH. A structure, with three hydrogen bonds per tetrasaccharide unit, is proposed which accounts for the quantitative and qualitative aspects of the NMR spectra of polymers and oligomers. A laser light-scattering study showed that sodium chondroitin 4-sulphate has the same molecular weight in water as in dimethyl sulphoxide. The observed hydrogen bonds are therefore intramolecular, as required by the proposed structure, and probably exist in water as well as in dimethyl sulphoxide.