Tissue-Specific Accumulation and Isomerization of Valuable Phenylethanoid Glycosides from Plantago and Forsythia Plants.

A comparative phytochemical study on the phenylethanoid glycoside (PhEG) composition of the underground organs of three Plantago species (P. lanceolata, P. major, and P. media) and that of the fruit wall and seed parts of Forsythia suspensa and F. europaea fruits was performed. The leaves of these Forsythia species and six cultivars of the hybrid F. × intermedia were also analyzed, demonstrating the tissue-specific accumulation and decomposition of PhEGs. Our analyses confirmed the significance of selected tissues as new and abundant sources of these valuable natural compounds. The optimized heat treatment of tissues containing high amounts of the PhEG plantamajoside (PM) or forsythoside A (FA), which was performed in distilled water, resulted in their characteristic isomerizations. In addition to PM and FA, high amounts of the isomerization products could also be isolated after heat treatment. The isomerization mechanisms were elucidated by molecular modeling, and the structures of PhEGs were identified by nuclear magnetic resonance spectroscopy (NMR) and high-resolution mass spectrometry (HR-MS) techniques, also confirming the possibility of discriminating regioisomeric PhEGs by tandem MS. The PhEGs showed no cytostatic activity in non-human primate Vero E6 cells, supporting their safe use as natural medicines and allowing their antiviral potency to be tested.

Role of different tautomers in the base-pairing abilities of some of the vital antiviral drugs used against COVID-19.

Repurposed drugs are now considered as attractive therapeutics against COVID-19. It is shown that Remdesivir, a nucleoside drug that was originally invented for the Ebola virus, is effective in suppressing the replication of SARS-CoV-2 that causes COVID-19. Similarly, Galidesivir, Favipiravir, Ribavirin, N4-hydroxycytidine (EIDD-1931), and EIDD-2801 (a prodrug of EIDD-1931) were also found to be effective against COVID-19. However, the mechanisms of action of these drugs are not yet fully understood. For example, in some experimental studies, these drugs were proposed to act as a RNA-chain terminator, while in other studies, these were proposed to induce base-pair mutations above the error catastrophe limit to stall the replication of the viral RNA. To understand the mutagenic effects of these drugs, the role of different tautomers in their base-pairing abilities is studied here in detail by employing a reliable dispersion-corrected density functional theoretic method. It is found that Remdesivir and Galidesivir can adopt both amino and imino tautomeric conformations to base-pair with RNA bases. While the insertions of G and U are preferred against the amino tautomers of these drugs, the insertion of C is mainly possible against the imino tautomers. However, although Favipiravir and Ribavirin can make stable base pair interactions by using their keto and enol tautomers, the formation of the latter pairs would be less probable due to the endothermic nature of the products. Interestingly, the insertions of all of the RNA bases are found to be possible against the keto tautomer of Favipiravir, while the keto tautomer of Ribavirin has a clear preference for G. Remarkably, due to the negligible difference in the stability of EIDD-2801 and EIDD-1931, these tautomers would coexist in the biological environment. The insertion of G is found to be preferred against EIDD-1931 and the incorporations of U, A, and G are preferred opposite EIDD-2801. These findings suggest that base-pair mutations are the main causes of the antiviral properties of these drugs.