Potential energy surfaces of carrageenan models: carrabiose, beta-(1 --> 4)-linked D-galactobiose, and their sulfated derivatives.
The adiabatic conformational surfaces of several beta-linked disaccharides, which correspond to the repeating structures of carrageenans, were calculated using the MM3 force-field. The studies were carried out on the disaccharide beta-D-Galp-(1 --> 4)-alpha-D-Galp and eight sulfated derivatives, as well as on carrabiose (beta-D-Galp-(1 --> 4)-3,6-An-alpha-D-Galp) and five sulfated derivatives. The presence of 3,6-anhydrogalactose does not change the main features of the maps, although it increases the flexibility of the glycosidic linkage. Sulfation neither produces a striking effect on the map shape, nor a shift on the global minimum, which always remains with psi (theta(C-1'-O-4-C-4C-5)) in trans orientation, and phi (theta(O-5'-C-1'-O-4-C-4)) with a value close to -80 degrees. This effect differs from that occurring on the alpha linkage of equivalent disaccharides, for which the sulfation pattern on the beta-galactose unit shifts the global minima to different positions. A reduction in the flexibility (originated in a deepening of the global minimum well) is observed by sulfation on position 2 of the beta-D-galactose unit, and by sulfation of position 6 of the alpha-D-galactose unit (when the beta-D-galactose unit is 4-sulfated). Within the compounds containing 3,6-anhydrogalactose, the effect of sulfation is even less noticeable. The calculated low-energy regions on carrabiose derivatives agree with X-ray diffraction data on carrageenan fibers and on peracetylated carrabiose dimethyl acetal, and with NOE calculations carried out on kappa-carrabiose.
Stortz
Carlos A
CA
Departamento de Química Orgánica-CIHIDECAR, Facultad de Ciencias Exactas y Naturales, UBA, Pab 2 Ciudad Universitaria, 1428 Buenos Aires, Argentina. stortz@qo.fcen.uba.ar
D002236:Carbohydrate Conformation; D002351:Carrageenan / Q000737:chemistry*; D004187:Disaccharides / Q000737:chemistry; D008958:Models, Molecular; D013463:Sulfuric Acid Esters
D004187:Disaccharides; D013463:Sulfuric Acid Esters; C080061:carrabiose; C056853:6-O-galactopyranosylgalactose; D002351:Carrageenan