Chemical Transformation of Protopanaxatriol-Type Ginsenosides Re and 20(S)-Rf by Heteropoly Acid Based on HPLC-ESI-MSⁿ Analysis

The chemical transformations of protopanaxatriol-type ginsenosides Re and 20(S)-Rf were performed using Keggin-type heteropoly acid catalysis dodeca tungstophosphoric acid. All the transformed products were identified based on a high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MSⁿ) method in association with the comparison of retention time to authentic standard. Each of the generated ginsenosides had isomeric counterparts, all of which were further differentiated through their multiple tandem mass spectra as well as the polarity difference. The molecular mass of ginsenosides could be obtained based on the deprotonated molecular M-H^- ions and formic acid adducted ions M+HCOO^-. Based on the neutral loss information, the kind of the saccharide substitution was recognized rapidly. And the constituents of the saccharide were also identified on the basis of the fragment ions from glycosidic bond cleavage and rearrangement after cross-ring cleavage. Ginsenoside Re was transformed into 8 products, i.e. 20(S)-Rf₂, 20(R)-Rf₂, 20(S)-Rg₂, 20(R)-Rg₂, 25-OH-Rg₆, 25-OH-Rg₄, Rg₆ and Rg₄, while 7 products were derived from ginsenoside 20(S)-Rf, i.e. 20(R)-Rf, 20(S)-Rf₃, 20(R)-Rf₃, 25-OH-Rg₈, 25-OH-Rg₉, Rg₈ and Rg₉. Based on the established HPLC method, all the products were well separated. Moreover, the specific aglycone structure of 3β, 12β, 25-trihydroxy-dammar-20(22)-ene was obtained for the first time through chemical transformation, which has been proved to be safe and effective therapeutic agents. Chemical transformation pathways of ginsenoside Re and 20(S)-Rf were also summarized, which involved deglycosylation, hydration, dehydration, and epimerization reactions. Deglycosylation at C-20 position was thought to occur prior to the other three reactions during the transformation of ginsenoside Re, whereas since there is no saccharide substitutions at C-20 position of 20(S)-Rf, epimerization occurred firstly in the dodeca tungstophosphoric acid dissolved strongly acidic aqueous phase. The double bond between C-24 and C-25 tends to be hydrated, while the tertiary alcohol at C-20 is reactive for dehydration. All the transformation process could be deduced through the analysis of tandem mass spectra. Furthermore, the conversion of ginsenoside Re approximately reached to 100% within 90 min. Especially, the conversion was up to 45% within 15 min. While for 20(S)-Rf, it lasted for 4 h to reach 100% conversion. The conversion of Re was superior to most of that resulting from biotransformation. All the results indicate that HPLC-ESI-MSⁿ is an effective method for the rapid identification of ginsenosides. Heteropoly acid catalysts open up a clean, economical and environmentally benign process in the chemical transformation of saponin-type active components in traditional Chinese medicine.