Rheum turkestanicum and R. ribes: Characterization of phenolic compounds and a LCESI-QqTOF MS based comparison with the officinal Chinese rhubarb, R. palmatum

The limited supply of Chinese officinal rhubarbs (Rheum palmatum L., R. tanguticum Maxim. ex Balf., and R. officinale Baill.) has resulted in the use of other Rheum species in many places. Therefore, investigating the metabolite profile of unofficinal rhubarbs, and an analytical comparison of their metabolite profile with the officinal ones is crucial. Rheum turkestanicum Janisch. and R. ribes L. are two Rheum species distributed in Iran and are widely being traded in traditional markets as substitutes for R. palmatum. Herein, we isolated different phenolic compounds in the roots and rhizomes of R. turkestanicum and R. ribes by chromatographic techniques and identified them by nuclear magnetic resonance spectroscopy (NMR), electronic circular dichroism (ECD), electrospray ionization-mass spectrometry (ESI-MS), and high-resolution mass spectrometry (HRMS). In addition, the metabolite profile of the two above-mentioned species were compared with that of R. palmatum by liquid chromatography-electrospray ionization-quadrupole time-of-flight mass spectrometry (LC-ESI-QqTOF MS). The obtained MS data were statistically compared by non-metric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM) and a heatmap was plotted for comparing the tested samples. The isolated compounds were catechins, anthraquinones, naphthoquinones, chromones and rhododendrol glycosides, among which (-) 3′,4′′-di-O-methyl-catechin-3-O-gallate was a new compound. The results of LC-MS/MS analysis indicated that while catechins were the most abundant compounds in R. ribes, anthraquinones and naphthalenes were the main constituents of R. palmatum and R. turkestanicum. Interestingly, the substitution in the hexosyl residue of anthraquinone and naphthalene hexosides was a malonyl or an acetyl group in R. palmatum, while this substitution was a 3-hydroxy-3-methylglutaryl group in R. turkestanicum. This may be considered as a potential marker for the differentiation of R. palmatum and R. turkestanicum. The statistical analysis showed that R. palmatum, R. turkestanicum, and R. ribes are distinct species, however, R. turkestanicum had some similarities to R. palmatum. In conclusion, R. turkestanicum can be a subject for future phytochemical, pharmacological, and clinical research regarding its ability to be used as a potential substitute for R. palmatum in pharmaceutical industries.