Synthesis And Characterization Of Alpha-1,3-Alt-Alpha-1,4-Glucan (Nigeran) Ester Derivatives

Nigeran is a linear alpha-D-glucan with alternating alpha-1,3- and alpha-1,4-glycosidic linkages. Nigeran (alpha-1,3-alt-alpha-1,4-glucan) esters with different ester chain lengths were synthesized from high molecular weight nigeran extracted from mycelium of Aspergillus oryzae over-expressing a nigeran synthase gene derived from Aspergillus luchuensis. All prepared nigeran esters were crystalline polymers with high melting point, including those with ester groups longer than octanoate. This unique crystallization behavior is not reported in other polysaccharide esters. Both solvent-cast and melt-quenched films were self-standing and high transparency. Solvent-cast films were stretched up to four times the initial length and showed well-oriented X-ray fiber diagrams. Nigeran propionate had twofold screw symmetry along the molecular axis (same as neat nigeran), suggesting that the molecular conformation remained unchanged despite esterification. This result indicates that the molecular conformation of nigeran with its alpha-1,3-alt-alpha-1,4-glycosidic linkage is very stable despite chemical modification and retains high crystallinity.