Amyloid-Like Hierarchical Helical Fibrils and Conformational Reversibility in Functional Polyesters Based on l-Amino Acids

Abstract

The present investigation reports one of the first examples of synthetic polymers that capable of undergoing reversible conformation transformation and also self-assembled to hierarchical helical amyloid-like fibrils. A new temperature selective melt polycondensation reaction was developed for amino acid monomers L-aspartic acid and L-glutamic acid to produce high molecular weight linear functional polyesters. These new polyesters have hydrogen bonded urethane (or carbamate) units that are in-built in each repeating unit. The polymer chains have adapted expanded chain conformation through β-sheet hydrogen bonding interactions and produced twisted ribbon-like assemblies. These twisted ribbons have subsequently undergone interchain folding for making double helical structures. The double helical fibrils aligned together to produce amyloid-like fibrils of few micrometer in length. Upon chemical deprotection of the pendent urethane units; the resultant cationic functional polyester adapted coil-like conformation and exhibited spherical charged nanoparticles of 200 ± 20 nm in size. Dynamic light scattering and zeta potential measurements revealed that both the charge and size of the spherical structures could be varied by altering the diol segment length in the polymer backbone. The coil-like chains in the charged spherical particles could be reversibly expanded into amyloid-like fibrils via fluorophore chemical substitution using dansyl chloride. The dansyl-substituted polymer exhibited helical fibrils and strong fluorescence. Thus, the L-amino acid based polyesters exhibited complete reversible conformational changes from hierarchical helical amyloid-like fibrils to charged nanoparticles in a single polymer system. These new nonpeptide polyester analogues, their amyloid fibrils, cationic polymer assemblies and fluorescent fibrils are very new based on l-amino acids, which may be useful for a wide range of biomedical applications.