Raman study of structural transformations in self-assembled diphenylalanine nanotubes at elevated temperatures

Self-assembled peptide diphenylalanine (NH2-Phe-Phe-COOH, FF) is one of the most important emergent functional materials, demonstrating wide range of fascinating physical and chemical properties including biocompatibility, chemical variability, exceptional rigidity, outstanding piezoelectric, pyroelectric and ferroelectric responses. Up to now, the phase transitions in FF nanotubes were investigated by scanning and transmission electron microscopy, atomic force microscopy, differential scanning calorimetry, thermogravimetry and mass spectroscopy. The Raman spectroscopy was implemented at room temperature only, and the microscopic details of these transitions remained unknown. In this work, the structural transformations in FF nanotubes at elevated temperatures (up to tubes destruction at 160 degrees C) were studied by Raman spectroscopy. Two structural transformations were observed in this region: hexagonal to orthorhombic transition at about 100 degrees C and the cyclization of FF molecules at about 150 degrees C. For the first time, these transformations were considered in the context of reconstruction of the water subsystem in the nanotubes, thus demonstrating the strong relation between the peptide tube and the state of the water inside. Using an effective frequency of nanotubes' lattice vibrations, we found that many effects observed earlier by other methods are induced by the variation of the water subsystem. The analysis of certain lines in the middle part of the Raman spectrum allowed us to describe the microscopic details of FF molecule cyclization. These results improve the understanding of the role of water in the origin of outstanding properties of FF nanotubes and thus promote developing new functional devices on their basis. Copyright (c) 2017 John Wiley & Sons, Ltd.