The three-dimensional structures of the pectic polysaccharides

The application of molecular modelling methods to the realistic description of cell wall polysaccharides is described. In a first section, the basic principles and tools are presented; they provide a consistent description of the levels of structural organization of polysaccharides, from the basic monomeric components to the more complex assemblies of macromolecular chains and their interactions. The second section provides a comprehensive illustration which includes the application of these methods to the elucidation of the structural features of the plant cell wall polysaccharides, pectins. The ordered and disordered states of both the `smooth' and the `hairy' regions of the pectin components are described, along with a discussion of the role of some key structural moieties such as the degree of esterification or acetylation. The stereochemical and electrostatic requirements for ionic binding to the polysaccharide chain are also presented. In an effort to provide a more global view, a `canonical' structure of pectin has been constructed and described; it corresponds to a hypothetical model having a molecular mass of 50 kDa. The main structural features of rhamnogalacturonan II, which have been determined to date, are also reported.