Controlled Assembly of Chiral Structure of Diphenylalanine Peptide

Chirality is ubiquitous in nature and it plays an important role in both biological and material sciences. Inspired by nature, scientists have prepared various chiral structures or hybrid materials by self-assembly of polypeptides, amino acids, carbohydrates and their derivatives. These studies provide a good model for understanding of supramolecular chirality and mimicking the self-assembly of organisms. In the past decade, diphenylalanine (FF) and its derivatives have attracted great attentions and have been substantially studied. FF is derived from the core recognition motif of the Alzheimer's disease beta-amyloid polypeptide, and it could readily self-assemble into nanotubes, nanowires, nanovesicles, nanofibers and microtubes. Moreover, the polymorphisms of FF-based assemblies can be easily manipulated by controlling the experimental conditions such as concentrations, solvents, pH and temperatures. However, there is few report on the chiral structures obtained from the self-assembly of FF and its derivatives. In this paper, we selected cationic diphenylalanine peptide (CDP) as the assembly units and have obtained CDP nanofibers and helical fibers in ethanol solution by controlling the aging time. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used to characterize the morphologies of CDP assemblies. The mechanism for the formation of CDP nanofibers and helical fibers in ethanol solution was studied by infrared spectroscopy and circular dichroism spectroscopy. It was found that CDP was first assembled into nanofibers. With the increase of aging time, CDP nanofibers twisted and finally assembled into helical fibers similar to the ropes. Spectral data analysis showed that the transformation of nanofibers into helical fibers was mainly due to the strong electrostatic repulsion between positive charges in adjacent peptide molecules and the beta-sheet secondary structure controlled by hydrogen bonding between peptide segments. This work realizes the regulation of supramolecular assembly structure by simply controlling the ripening time, and provides a simple and feasible method for the controlled preparation of supramolecular chiral assembly.