Raman spectroscopy of l,l-diphenylalanine crystal under high pressure

The l,l-diphenylalanine can self-assemble into long and stiff nanotubes with remarkable mechanical and chemical stability. In this work, Raman spectroscopy was used to investigate the vibrational properties of l,l-diphenylalanine single crystal under high-pressure conditions up to 7.4 GPa. The results show great modifications in the low-wavenumber bands accompanied by some changes in the internal mode vibrations between 3.9 and 4.6 GPa, indicating the occurrence of a structural phase transition. Since the coupling between water molecules and the nanochannels of the l,l-diphenylalanine crystal is extremely sensitive to the tubular arrangement, the vibrational changes observed in the spectral region of internal modes can be interpreted as conformational modifications in the dipeptide backbone. On decompression, we found that the phase transition is reversible since the ambient spectrum is recovered when the pressure is completely released.