Phase transitions in DL-norvaline crystal under high pressure by Raman spectroscopy

Abstract

Amino acids are essential for the formation of proteins and the functioning of living organisms. Among them, DL-norvaline (2-amino-pentanoic acid) stands out for its similarity to valine and its importance in the application of several fields, becoming a valuable model for studying stability and phase behaviors under different thermodynamic conditions. In this study, we investigated the vibrational properties and structural stability of DL-norvaline single crystals under high-pressure conditions using Raman spectroscopy. The crystals exhibit monoclinic symmetry with eight molecules per unit cell (Z = 8), belonging to the C2/c space group, as confirmed by powder X-ray diffraction. Density Functional Theory calculations were performed on a single molecule in its zwitterionic form to support the experimental findings. The Raman spectra of DL-norvaline under extreme pressure conditions, between 10⁻⁴ GPa and 8.2 GPa, were obtained by using a Membrane Diamond Anvil Cell. We observed changes in three pressure ranges: (i) at 0.1 GPa, (ii) at 1.7 GPa and (iii) around 4 GPa. In both (i) and (ii), changes in the region of the external modes, relating to the crystal lattice, and internal modes suggest phase transition related to the conformations of the molecule. In (iii) the presence of disorder or loss of crystalline structure is evident. These findings provide an understanding of the behavior of DL-norvaline under high pressures, expanding knowledge about amino acid polymorphism and stability, which are fundamental for its technological applications.