The genus Croton has about 700 species, many of which are used in folk medicine to treat various diseases. Among them stands out the Croton anisodontus Müll. Arg. belonging to Euphorbiaceae family, which presents an alternative source of 2-hydroxy-3,4,6-trimethoxyacetophenone (C11O5H14, hereafter named HTMCX). This natural product showed promising antimicrobial activity against the multidrug-resistant bacterial strains of Pseudomonas aeruginosa 03 and Staphylococcus aureus 358 when associated with the antibiotic amikacin [1]. Recently, we had reported for the first time the crystalline structure and the assignment of the vibrational modes for this acetophenone crystal [2]. Our crystallographic data showed that at room temperature the HTMCX crystal belongs to the triclinic structure with P1 space group, and two molecules per unit cell. A total of 56 active Raman modes was observed under normal conditions of temperature and pressure. In this work this material was investigated via Raman spectroscopy as function of pressure in the range of 1 atm to 9.7 GPa. The Raman spectra to high hydrostatic pressures were obtained in the laboratory of vibrational spectroscopy and high pressure from Federal University of Pará, Belém, PA, Brazil. The set of experiments was performed in a membrane diamond-anvil cell (MDAC) with the Nujol oil used as a pressure transmitting medium. Some changes were observed in certain spectral regions, including the appearance and/or disappeared of bands, change in the intensity and discontinuities in the curve frequency vs. pressure. These changes evidenced the occurrence of conformational changes in the range of 2.0 to 4.0 GPa and of 5.5 to 7.0 GPa. It should be noted that the Raman spectra obtained after decompression are identical to those obtained at the beginning of the experiment, which indicate that the conformational changes undergone by the crystal are reversible. Therefore, the results of the experiments under high pressures showed that the acetophenone crystal undergoes two conformational changes at high pressure, the first above of 2.0 GPa, and the second one above of 5.0 GPa characterized by the appearance of bands related to lattice modes and little modifications linked to the internal modes.
References:
[1] OLIVEIRA, M.T.A.; TEIXEIRA, A.M.R.; COUTINHO, H.D.M.; MENEZES, I.R.A.; SENA, D.M., JR.; SANTOS, H.S.; DE MESQUITA, B.M.; ALBUQUERQUE, M.R.J.R.; BANDEIRA, P.N.; BRAZ FILHO, R. Identification and Modulatory Activity Assessment of 2-Hydroxy-3,4,6-trimethoxyacetophenone Isolated from Croton anisodontus Mull. Arg.(Euphorbiaceae). Natural Product Communications, v. 9, n. 5, p. 665-668, 2014.
[2] SANTIAGO, R.N.S.; FREIRE, P.T.C.; AYALA, A.P.; TEIXEIRA, A.M.R.; SANTOS, H.S.; BANDEIRA, P.N.; GONÇALVES, F.G.; OLIVEIRA, M.T.A.; CRUZ, B.G.; SENA JR, D.M. Crystal structure, vibrational spectra and quantum chemical parameters of 2-hydroxy-3,4,6-trimethoxyacetophenone isolated from the Croton anisodontus Müll.Arg. (Euphorbiaceae). Journal of Molecular Structure, v. 1171, n. 5, p. 815-826, 2018.