Ginsenoside Rg3, Rk1, and Rg5, rare ginsenosides from Panax ginseng, have many pharmacological effects, which have attracted extensive attention. They can be obtained through the heat treatment of Gynostemma pentaphyllum. In this study, scanning electron microscopy (SEM) and thermal gravity-differential thermal gravity (TG-DTG) were employed to investigate this process and the content change in ginsenosides was analyzed using liquid chromatography-mass spectrometry (LC-MS). SEM and TG-DTG were used to compare the changes in the ginsenosides before and after treatment. In SEM, the presence of hydrogen bond rearrangement was indicated by the observed deformation of vascular bundles and ducts. The before-and-after changes in the peak patterns and peaks values in TG-DTG indicated that the content of different kinds of compounds produced changes, which all revealed that the formation of new saponins before and after the heat treatment was due to the breakage or rearrangement of chemical bonds. Additionally, the deformation of vascular bundles and vessels indicated the presence of hydrogen bond rearrangement. The glycosidic bond at the 20 positions could be cleaved by ginsenoside Rb3 to form ginsenoside Rd, which, in turn, gave rise to ginsenoside Rg3(S) and Rg3(R). They were further dehydrated to form ginsenoside Rk1 and Rg5. This transformation process occurs in a weak acidic environment provided by G. pentaphyllum itself, without the involvement of endogenous enzymes. In addition, the LC-MS analysis results showed that the content of ginsenoside Rb3 decreased from 2.25 mg/g to 1.80 mg/g, while the contents of ginsenoside Rk1 and Rg5 increased from 0.08 and 0.01 mg/g to 3.36 and 3.35 mg/g, respectively. Ginsenoside Rg3(S) and Rg3(R) were almost not detected in G. pentaphyllum, and the contents of them increased to 0.035 and 0.23 mg/g after heat treatment. Therefore, the rare ginsenosides Rg3(S), Rg3(R), Rk1, and Rg5 can be obtained from G. pentaphyllum via heat treatment.
Keywords: Gynostemma pentaphyllum; LC-MS; SEM; TG-DTG; ginsenoside.