Polysaccharides are biologically important as structural, energy storage and gelling macromolecules; their rheological properties and abundance in the biomass give them technological significance. Exploiting their biological and physical properties will require better methods of conformational analysis. Molecular modeling techniques are rapidly improving, but their further development requires guidance from experiment. Excessive line broadening at times precludes the use of nmr methods. The objective of the proposed research is to apply a chiroptical method of conformational analysis, based on vacuum uv circular dichroism (CD) measurements, to determine polysaccharide conformations in systems which have resisted analysis by other means. A corollary objective is to provide experimental guidance for the further development of carbohydrate force fields used in molecular modeling methods. The long term goal is to help establish a definitive means of carbohydrate conformational analysis through the combined use of x-ray and neutron diffraction, molecular modeling, nmr and chiroptical methods. In the proposed research the linkage conformations of xylobioside and (1 to 4)-Beta-D-xylan will be determined; they have not been well established by molecular modeling and nmr. The linkage conformations, and extent of flexibility, of (1 to 2)-and (1 to 3)-alpha-mannose linkages will be determined. Those linkages are prevalent in the carbohydrate moieties of cell surface active glycoproteins. Some recent work indicates they are particularly inflexible in aqueous solution, a picture which, if confirmed, provides a mechanism for protein control of various carbohydrate-mediated cell surface phenomena. Experimental measurements on agarose gels will allow a comparison of the competing double-helix and single-helix models of gel formation, thereby clarifying an important aspect of polysaccharide gel microstructure. Conformations of the structurally related microbial polysaccharides, gellan and welan, will be compared to test a current hypothesis aimed to explain their different rheological properties. Conformations of Beta- trehalose and its C-disaccharide analog will be compared; C- disaccharides are potential glycosidase inhibitors and can serve as models for evaluating the role of exo-anomeric effects in determining carbohydrate conformational preferences. Measurement on model compound anhydrosugars will extend the applicability of chiroptical methods to algal polysaccharides. The chiroptical method is based on the fact that vacuum uv transitions are sensitive to conformational change. It has only recently been developed, through advances in instrumentation and the development of interpretive models. The method has previously been applied to the disaccharides maltose, cellobiose, alpha-trehalose and lactose, and to the polysaccharide galactan. Full use of the method requires vacuum uv cd measurements as well as NaD rotations. Experimental spectra will be correlated with spectra calculated as a function of conformation, and will lead to conformational assignments. Conclusions will be compared with those based on x-ray and neutron diffraction, molecular modeling and nmr.
