Solution conformation of a pectin fragment disaccharide using molecular modelling and nuclear magnetic resonance

In the present study, the conformational behaviour of methylated pectic disaccharide 4-O-alpha-D-galactopyranurosyl 1-O-methyl-alpha-D-galactopyranuronic 6,6'-dimethyl diester 1 has been completely characterized through combined n.m.r. and molecular modelling studies. The 1H-1H n.O.e. across the glycosidic bond was measured by both steady-state and transient 1D and 2D experiments. In parallel, the complete conformational analysis of the disaccharide has been achieved with the MM3 molecular mechanics method. The conformation of the pyranose ring is confirmed by the excellent agreement between the experimental and calculated intracyclic scalar coupling constants. The iso-energy contours displayed on the 'relaxed' map indicate an important flexibility about the glycosidic linkage. There is no significant influence of the methoxyl group on the conformational behaviour of the disaccharide. The theoretical n.m.r. data were calculated taking into account all the accessible conformations and using the averaging methods appropriate for slow internal motions. 3JC-H coupling constants were calculated using an equation suitable for C-O-C-H segments. The agreement between experimental and theoretical data is excellent. Within the potential energy surface calculated for the disaccharide, several conformers can be identified. When these conformations are extrapolated to a regular polymer structure, they generate pectins with right- and left-handed chirality along with a two-fold helix. These different types of helical structure are the result of small changes in conformation, without any drastic variation of the fibre repeat.