High-performance thin-layer chromatography - antibacterial assay first reveals bioactive clerodane diterpenes in giant goldenrod (Solidago gigantea Ait.)

The present work introduces a high-performance thin-layer chromatography (HPTLC)& ndash;direct bioautogra-phy method using the Gram-positive plant pathogenic bacterium, Rhodococcus fascians. The screening and isolation procedure comprised of a non-targeted high-performance thin-layer chromatography-effect -directed analysis (HPTLC & ndash;EDA) against Bacillus subtilis, B. subtilis subsp. spizizenii, R. fascians, and Ali-ivibrio fischeri, a targeted HPTLC & ndash;mass spectrometry (MS), and bioassay-guided column chromatographic (preparative flash and semi-preparative HPLC) fractionation and purification. The developed new separa-tion methods enabled the discovery of four bioactive cis-clerodane diterpenes, solidagoic acid H ( 1 ), sol-idagoic acid E ( 2 ), solidagoic acid I ( 3 ), and solidagoic acid F ( 4 ), in the n-hexane extract of giant golden-rod (Solidago gigantea Ait.) leaf for the first time. These compounds were identified by 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. The initially used HPTLC method (chloroform & ndash; ethyl acetate & ndash; methanol 15:3:2, V/V/V) was changed (to n-hexane & ndash; isopropyl acetate & ndash; methanol & ndash; acetic acid 29:20:1:1, V/V/V/V) to achieve the separation of the closely related isomer pairs ( 1 & ndash;2 and 3 & ndash;4 ). Compounds 1 and 3 exhibited moderate antibacterial activity against the Gram-positive B. subtilis subsp. spizizenii and R. fascians bacterial strains in microdilution assays with half-maximal inhibitory concentration (IC50) values in the range of 32.3 & ndash;64.4 mu g/mL. The mass spectrometric fragmentation of the isolated compounds was interpreted and their previously published NMR assignments lacking certain resonances were completed. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ) Superscript/Subscript Available

Automated summary

This study introduced a high-performance thin-layer chromatography-direct bioautography method to discover four new cis-clerodane diterpenes from giant goldenrod leaves, showing moderate antibacterial activity against certain bacterial strains.