HPLC-DAD-ESI/MS and 2D-TLC Analyses of Secondary Metabolites from Selected Poplar Leaves and an Evaluation of Their Antioxidant Potential

Poplar leaves (<i>Populi folium</i>) are a herbal remedy traditionally used for the treatment of rheumatic diseases and prostate inflammation. The aim of our study was a comprehensive identification of secondary metabolites occurring in the leaves of <i>Populus alba</i>, <i>Populus</i> × <i>candicans</i>, and <i>Populus nigra</i>, in order to search for a source of raw plant material rich in active compounds. Total salicylate (TSC), flavonoid (TFC), and phenolic compound (TPC) contents were determined, and the antioxidant potential was assessed using DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2'-azino-bis(3-ethylbenzothiazoline- 6-sulfonic acid) diammonium salt), and FRAP (ferric reducing antioxidant power) assays as well as 2D-TLC (two-dimensional thin layer chromatography) bioautography using DPPH, riboflavin-light-NBT (nitro blue tetrazolium chloride), and xanthine oxidase inhibition tests. Secondary metabolites present in the analyzed poplar leaves were identified under the developed HPLC-DAD-ESI/MS (high performance liquid chromatography with photodiode array detection and electrospray ionization mass spectrometric detection analysis conditions and using the 2D-TLC method. Among the 80 identified compounds, 13 were shown for the first time in the genus <i>Populus</i>. The most diverse and similar set of flavonoids characterized the leaves of <i>P.</i> × <i>candicans</i> and <i>P. nigra</i>, while numerous salicylic compounds were present in the leaves of <i>P. alba</i> and <i>P.</i> × <i>candicans</i>. All analyzed leaves are a rich source of phenolic compounds. The highest flavonoid content was found in the leaves of <i>P. × candicans</i> and <i>P. nigra</i>, while the leaves of <i>P. alba</i> were characterized by the highest content of salicylates. All examined poplar leaves demonstrated antioxidant potential in all the assays used, which decreased in the following order: <i>P. nigra</i>, <i>P. × candicans</i>, <i>P. alba</i>.