Cycloaddition-promoted self-assembly of a polymer into well-defined beta sheets and hierarchical nanofibrils.

Abstract

Whereas biopolymers, such as proteins, are ubiquitous for well-defined secondary, tertiary and quaternary structures,[1] it remains a fundamental challenge to design synthetic polymers that can fold into predictable high structures. One major research thrust in our laboratory is aimed at programming weak forces into polymers to guide their assembly into well-defined molecular and nano-structures.[2] Previously we reported biomimetic multi-domain polymers following modular design of titin.[3] In this study, our attention was drawn to β-sheet polymers. Despite significant progress has been made in the area of designing discrete short peptidomimetic oligomers with β-sheet structures,[4] it remains largely illusive to materials chemists to design synthetic high polymers that can fold into well-defined β-sheets and hierarchical nanostructures. β-Sheet is not only a basic secondary structure in proteins, but also an important structural motif in many fibril biomaterials such as amyloids[5] and silks.[6] Their hierarchical nanostructures and excellent mechanical properties have inspired biomemetic material designs.[7] Despite a number of peptide-related systems were reported to form β-sheet based fibrils, most of them are short peptides[8] or peptide-polymer conjugates.[9] The self-assembly in these systems usually proceeds intermolecularly, forming relatively weak structures. Genetically engineered polypeptides via recombinant DNA technology were reported to form β-sheets and various nanostructures,[10] however, the efficiency and versatility are limited by the biosynthetic pathway. Both for fundamental interest and for advanced materials designs, it is highly desirable to develop efficient synthesis to access well-defined covalently bonded β-sheet polymers. Herein we describe a new strategy of constructing covalent synthetic polymers that fold into well-defined β-sheets and further assemble into hierarchical nanofibrils (Fig. 1).