L-tyrosine methyl ester hydrochloride crystal under high pressure and DFT calculations

The methylated organic salt L-tyrosine methyl ester hydrochloride (LTMEHCl) crystal was synthesized by the slow solvent evaporation method. The crystal structure was verified through Powder X-ray Diffraction. Three-dimensional periodic Density Functional Theory calculations (DFT) were conducted to identify the Raman active modes. A high-pressure Raman study was carried out on this material, encompassing a wavenumber range of 50-3450 cm^-1 and a pressure range from 0.0 to 9.0 GPa. Spectral modifications, including wavenumber discontinuities, the emergence and disappearance, broadening and attenuation, as well as the inversion of relative intensities in specific bands associated with both external and internal modes, were observed. These observations indicate a conformational phase transition in LTMEHCl crystal around 1.0 GPa, followed by a second phase transition near 6.0 GPa, which correlates with anincrease in structural disorder. The methylation process likely led to a reduction in hydrogen bond formation ability and an increase in the mobility of the methylated L-tyrosine under high pressure. Consequently, L-tyrosine methyl ester hydrochloride exhibited greater susceptibility to conformational modifications than its non-methylated analogue, L-tyrosine hydrochloride. Furthermore, upon the release of pressure, several bands either reappeared weakly or did not reappear. This behavior suggests a partial amorphization of the material, potentially influenced by the mineral oil medium and its hydrostatic limit.