Influence of inter‐turn hydrogen bonding on the stabilization of the helical inclusion complex of amylose

Abstract

AbstractTwo different modes of interaction between 2‐chloro‐N‐dodecylpyridinium iodide (CDPI) and randomly substituted methylamylose (MA), which is mainly composed of 2,3‐O‐methylamylose or 6‐O‐methylamylose (6‐MA), are reported. The fluorescence intensity of CDPI is enhanced and its reactivity toward alkaline hydrolysis is retarded upon the addition of MA. Both cases show a saturation‐type relationship with increasing concentration in the case of MA, indicating the formation of an inclusion complex. In addition to the contribution of hydrophobic‐lipophilic interaction, the intramolecular hydrogen bonding of the host may also play an important role in stabilizing its helical conformation, as proved by the fact that the dissociation equilibrium constant, Kd, evaluated from kinetic data, increases with increasing degree of substitution of MA. This deduction is confirmed by experiments using 6‐MA as the host instead of MA. With increasing concentration of added 6‐MA, a linear relationship for the fluorescence and a bell‐type profile for the hydrolysis of CDPI are found. These results can be interpreted by the solubilization of CDPI in 6‐MA micelles. As in the case of cyclodextrin, in each turn of the amylose helix, the primary hydroxyl groups are located on one side and the secondary groups on the other side. Therefore, in the case of 6‐MA, where the primary hydroxyl groups are completely blocked, the macromolecules lose the ability to adopt a helical conformation. In conclusion, inter‐turn hydrogen bonding is an important factor contributing to the formation and stabilization of the helical inclusion complex of amylose.