Elucidating l-tyrosine crystal phase transitions by Raman spectroscopy and ab initio calculations

Abstract

We report here the analysis of the vibrational properties of the l-tyrosine crystal by means of Raman spectroscopy, as well as DFT calculations. The structural stability of the sample was also investigated under high pressure. Single crystals were obtained by the slow solvent evaporation technique. X-ray diffraction experiments were performed at ambient pressure and confirmed the crystalline structure. Simulations of tyrosine dimers allowed us to know some lattice modes. Raman spectra were acquired under ambient conditions and confirmed by ab initio calculations. The classification of the Raman modes is presented and as will be shown, the theoretical results agree with the experimental data. In addition, high pressure Raman spectra were measured from ambient pressure up to 5.9 GPa. The pressure-dependent studies showed that this sample undergoes two phase transitions, at approximately 2 GPa and 3.6 GPa. The transition at 2 GPa is characterized by change of bands in the low-wavenumber region of the spectrum and splitting of a mode associated with torsion of NCCC and rocking of NH3. In the phase transition at 3.6 GPa, changes in the lattice modes and variation of dω/dP for modes between 300 and 450 cm−1 are observed. The observed transitions were reversible when the system brought back to ambient pressure. This study is the first on the crystal of the amino acid l-tyrosine subjected to high pressures, allowing the observation and understanding of the structural and vibrational behavior under these conditions, possibly expanding the possibilities of using this material.