Effect of guest structure on amylose-guest inclusion complexation

Amylose-guest inclusion complexes are a type of supramolecular host-guest assembly that can provide protection for and controlled release of guest molecules. The successful and efficient complexation between amylose and guest molecules is governed by factors including: guest structure and chemistry, and process method and parameters. Here we investigated the formation, crystalline structure, and thermal stability of amylose inclusion complexes with a total of ten guest molecules differing in alkyl chain length (C10 and C16), molecular shape (linear vs. branched), and functional groups (alcohol, aldehyde, carboxylic acid, and ester). Their ability to complex with amylose was evaluated using two complexation methods (partitioning from water after heating and partitioning from a DMSO/water solution), and two annealing temperatures (60 and 90 degrees C). The extent of complexation differed for the two methods, likely due to guest solubility and partitioning behavior in the respective solvent systems. Annealing temperature created inclusion complexes of different structure and dissociation temperature using the water approach. Here we suggest that the so-called Form I and Form II V-type amylose inclusion complexes differ in their crystal size, crystallinity and arrangement of guest molecules in the helical cavity, rather than being amorphous or crystalline as previously reported. Chain length, molecular shape, and functional groups affected the thermal stability of the inclusion complexes. Shorter chain length, unsaturation, and short branched chains formed inclusion complexes with lower dissociation temperatures. We propose the Form II as a tail-to-tail arrangement of molecules in the helices that leaves the functional groups at the helical openings. Guest compounds that either failed to form complexes from water or formed poor complexes were able to form inclusion complexes with amylose using the DMSO approach, suggesting solubility of the guest, flexibility of the amylose chain, or the partitioning of the guest between the solvent and the helix core affected complexation.