MM3 potential energy surfaces of trisaccharides. II. Carrageenan models containing 3,6‐anhydro‐ D ‐galactose

The adiabatic potential energy surfaces (PES) of six trisaccharides—namely 3,6‐An‐α‐ D ‐Gal p ‐(1→3)‐β‐ D ‐Gal p ‐(1→4)‐3,6‐An‐α‐ D ‐Gal p , β‐ D ‐Gal p ‐(1→4)‐3,6‐An‐α‐ D ‐Gal p ‐(1→3)‐β‐ D ‐Gal p , and their derivatives sulfated on positions 2 and 4 of the β‐galactose unit—were obtained using the MM3 force field. Each PES was described by a single contour map for which the energy is plotted against the two ψ glycosidic angles, given the small variations of the ϕ glycosidic torsional angle in the low‐energy regions of disaccharide maps. In five of the six examples, the surfaces are those expected from the maps of the disaccharidic repeating units of carrageenans, with less important factors altering the additive effect of both linkages. However, when a sulfate group is present on C2 of a β‐galactose reducing end, a new low‐energy minimum in a different region is produced, originated in a hydrogen bond between the first and third monosaccharidic moieties of the trisaccharide. The flexibility of the β‐linkages is nearly identical to that in their disaccharide counterparts, while that of the α‐linkages is slightly reduced, independent of their presence closer or further away from the reducing end. A fair agreement is observed between the x‐ray fiber diffraction analysis for a κ‐carrageenan double helix and the surfaces obtained for the trisaccharide analogs of that polymer. © 2003 Wiley Periodicals, Inc. Biopolymers 70: 227–239, 2003