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Inclusion complexes of amylose : morphogenesis, crystal structure and release of bioactive molecules

Abstract : Amylose, a mostly linear homopolymer of α(1,4)-linked glucosyl units extracted from native starch, has the remarkable property to form "V-amylose" crystalline complexes with a variety of small organic molecules. We have tested the ability of 120 compounds to induce the crystallization of amylose from dilute aqueous solutions. The morphology and structure of the resulting lamellar crystals were characterized by transmission electron microscopy as well as electron and X ray diffraction. The data revealed that the structures of the complexes could be classified into 10 families, 5 of which were described for the first time. In addition, 13C solid-state nuclear magnetic resonance spectra clearly showed that the helicity of V-amylose was correlated with the resonance of carbon C1 that was shifted downfield with increasing number of glucosyl units per turn. Tentative geometrical models were proposed for all allomorphs and the structure of V1-butanol was analyzed in more details by combining conformational and packing energy calculations with classical crystalline polymer structure refinement. All allomorphs contained 6-, 7- or 8-fold amylose single helices and the guest molecules could be located inside these helices, in-between, or both. Each allomorph could be obtained with different complexing agents and the helical conformation was found to depend on the size of the complexing agent. In addition, a given ligand could induce the formation of several allomorphs, suggesting that the polymorphism of V-amylose crystals is a more general characteristic than what was previously reported. The propensity for polymorphism is not only related to the nature of the complexing agent but also to the crystallization conditions. The degree of polymerization of amylose, its concentration and that of the complexing agent, the temperature of mixing and crystallization, and the solvent composition have a significant impact on the formation and crystal structure of V-amylose. In addition, crystalline complexes prepared with ibuprofen were used as a model to evaluate the potential of V-amylose as a delivery system of bioactive molecules. Different fractions of ibuprofen, likely correlated with the different locations of the guest in the crystal, were selectively released by varying the pH of the dissolution medium. Since the release mainly occurred at high pH, these inclusion complexes appear to be potentially interesting for intestinal targeting and would thus improve the therapeutic effect of ibuprofen.
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Cong Anh Khanh Le. Inclusion complexes of amylose : morphogenesis, crystal structure and release of bioactive molecules. Analytical chemistry. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAV033⟩. ⟨tel-02018878⟩

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