Insights into stereochemical features of sulphated carbohydrates: X-ray crystallographic and modelling investigations

The three-dimensional structures of the 2-, 3-, 4- and 6-monosulphates of methyl alpha-D-galactopyranoside have been determined by X-ray crystallography; the first two as the sodium salt, the third as both the sodium and potassium salts, and the fourth as a potassium salt. These represent the principal sulphated monomers of the carrageenan polysaccharides. The results extend our knowledge of the stereochemical features, such as ring conformation, sulphate geometry, hydrogen bonding and cation co-ordination, which characterize sulphated monosaccharides. The stereochemical data have been used to derive a mean geometry of the O-sulphate group and a set of force constants for use in molecular mechanics calculations on sulphated monosaccharides. These may be used in an extrapolation of the populations of stable conformers of related oligo- and polysaccharides.