Ordering of Polypeptides in Liquid Crystals, Gels and Micelles

Ordered structures assembled from polypeptides have attracted a great deal of attention over the past few decades. Both alpha-helix and beta-sheet conformations of polypeptides support the formation of ordered structures during the assembly process. For polypeptides with alpha-helix conformation, the ordered structures are formed mainly by side-by-side packing of alpha-helix rods. For polypeptides with beta-sheet conformation, ordering of the chains can be achieved by parallel or antiparallel packing. The ordering characteristic of polypeptide chains gives rise to fascinating assembly behaviors of polypeptide homopolymers and copolymers in solution. Usually, a decrease in polymer concentration is accompanied by the assembly of polypeptides into liquid crystals (LCs), gels, and micelles. This review describes the ordering structures of polypeptides in these assemblies. In LC structures, polypeptide homopolymer chains are packed in a highly ordered fashion with smectic, nematic, and cholesteric phases. Both polypeptide homopolymers and copolymers support the formation of gels in solution. The dislocated side-by-side packing of polypeptide helices is the basic ordering characteristic of the polypeptides in gels. Compared with the alpha-helix conformation, gels formed from polypeptides with beta-sheet conformation show higher stability. In dilute solutions, amphiphilic polypeptide copolymers can self-assemble into micelles that include cylinders, vesicles, and complex hierarchical structures. The ordering nature of the polypeptide chains can be observed in the assemblies. The close relationship with proteins makes polypeptides and their assembly structures ideal models for protein research and promising candidates in biorelated applications.