Systematic Estimates for Possible Regular Helix Models of Heteropolymeric Glucans,Elsinan and Lichenan,Using N-H Mapping Method

Regular helical structures of polysaccharides are most conveniently described by a set of the helix parameters; n for the number of chemical repeating units per turn and h in Å for the rise per unit along the helix axis. A two dimensional mapping of n-h values for possible helix models along with the potential energy surfaces allows one to estimate conformational accessibility of a given posaccharide.^1-3 Recently, we have adopted the method to study an acidic heteropolysaccharide⁴ and a branching glucan.⁵ These polysaccharides involve two or three sets of backbone glycosidic linkages (Φ-Ψ), each of which varies independently, and, therefore, enormous multidimensional spaces must be explored. Their n-h maps were calculated based on the low energy Φ-Ψ values derived from MM3^6-8-generated, relaxed-residue potential energy maps^{9, 10} of the component disaccharides. The present assessment of helix models for the two heteropolymeric glucans is achieved by calculating n-h maps in a similar fashion. These glucans are the two poly(disaccharide)s, poly(1→3)-α-D-maltotriose] (elsinan) and poly(1→3)-β-D-cellotriose] (lichenan). In addition to single-stranded helices, three types of mutiple helices; double-parallel, and double-antiparallel, and triple helices have also been examined.