Conformational properties of chito-oligosaccharides: titration, optical rotation, and carbon-13 N.M.R. studies of chito-oligosaccharides

Several chito-oligosaccharides [β-(1→4)-linked oligosaccharides of 2-amino-2-deoxy-d-glucose] have been studied. A semi-empirical treatment of titration and rotation data indicates that the glycosidic torsion-angles (ΔΦ and ΔΨ) are all closely similar throughout the oligomers and also in the polymer (chitosan), in their fully protonated forms. The microscopic, thermodynamic dissociation-constants are evaluated from data based on the 1H-n.m.r. titration of chitobiose at 25°. We also measured the 13C-chemical shifts in chitobiose. With the exception of the C-1 and C-4′ resonances of chitobiose, the observed chemical shifts are in good agreement with those calculated from δ (chitobiose)  δ (cellobiose) + δ (2-amino-2-deoxy-glucose)  δ(glucose). The discrepancy is utilized for further conformational analysis. On the basis of these observations and of the exoanomeric effect, we consider that these deviations can be accounted for by a change in the glycosidic torsion angle (ΔΨ) of chitobiose from that of cellobiose.