A new approach for the thermal characterization of monocrotaline, a pyrrolizidine alkaloid

Abstract

Monocrotaline (MCT) is a bioactive pyrrolizidine alkaloid (PAs) isolated from Crotalaria retusa seeds. The PAs when taken orally may be toxic to animals and humans. However, studies of pyrrolizidine alkaloids in topic formulations did not show toxic effects, otherwise demonstrated several pharmacological activities. The goal of the present work was to characterize MCT employing thermal analysis (DSC, TG/DTG and DTA) and non-thermal techniques (FTIR and capturing images of the decomposition process). The TG curves presented five mass loss events, at a heating rate of 10 °Cmin−1, being the third event, occurring between 204 °C and 292 °C, the one with the greater decomposition, with a mass loss of 62.2% and with an initial decomposition temperature of about 100 °C. Five events (three endothermic and two exothermic) were observed in the DSC curves at the same heating rate. The first event occurred at temperatures between 133 °C and 149 °C. A sample contraction was observed in the images captured at 150 °C, and it was attributed to a part of the decomposition process. Furthermore, the images of the decomposition process revealed that the sample melts at around 202 °C, which was the temperature of the second event of the DSC curve. The other three events were attributed to the sample decomposition. DTA curves were similar to the DSC curves. Fourier transform infrared spectrometry (FTIR) was employed to compare the spectra of the heated samples at several temperatures (50, 100, 150, 200 and 250 °C), with the spectrum of monocrotaline at room temperature. The spectra were compared by an ad hoc algorithm that performs Pearson correlation analysis. FTIR showed that monocrotaline had a significant change in its structure at 150 °C. The kinetic parameters for degradation process were calculated through the experimental data adjustment in the temperature range of 170–300 °C, using a general kinetic model, which was firstly proposed by Ng and improved by Cai-Liu. The results detected a contribution of kinetic models, mainly geometrical contraction models, with activation energy of 108.39 kJ mol−1 and frequency factor of 4.464 × 1011 min−1. This result corroborates the sample contraction, as seen by the images. With the results of mass loss obtained in the thermogravimetric analysis, the thermal degradation mechanism was proposed in association with the FTIR results.