Synthesis and characterization of 2,3-di-O-alkylated amyloses: hydrophobic substitution destabilizes helical conformation.

Abstract

Amylose was selectively alkylated in the 2,3-O position of each anhydroglucose unit after trityl protection of the 6-OH groups. Alkyl iodides of varying chain length (C(2), C(5), C(8)) were coupled to amylose, and degrees of substitution (DSs) were varied between 0.3 and 1.8, as assessed by NMR analysis. Increasing amounts of methyl groups per anhydroglucose unit increased solubility in nonaqueous media, while at the same time reducing the ability of modified amylose to form a complex with iodine. The tendency to form inclusion complexes with the surfactant N-dodecyl pyridinium bromide decreased in the order beta-cyclodextrin >> amylose approximately solubilized starch, indicating that the frozen macrocycle of beta-cyclodextrin was the most efficient inclusion host. Introduction of the bulky trityl group abolished the helical amylose conformation, which is not readily reassumed in the presence of hydrophobic substitution of the C2 and C3 positions. These results indicated that a polar outer surface is necessary but not sufficient for the formation of a stable amylose helix.