Antibacterial potential of the phenolics extracted from the Paulownia tomentosa L. leaves as studied with use of high-performance thin-layer chromatography combined with direct bioautography

Abstract

Paulownia tomentosa (Thunb.) Steud. is a medicinal plant with a very long established and considerable position in traditional Chinese medicine (TCM) and in those of the other Far East Asian countries (mainly in Vietnam, Laos, Japan, and Korea). In Europe, Americas and Australia, P. tomentosa is known as princess tree, empress tree, or foxglove tree, and it is cultivated as a beautifully blossoming ornamental tree. In TCM, all parts of P. tomentosa (i.e., timber, bark, leaves, flowers and fruits) are used for curative purposes. From a literature survey, a conclusion can be drawn that in phytochemical and ethnopharmaceutical studies relatively less attention has been paid to investigating the chemical composition and biological activity of the leaves than that of the other parts of this medicinal plant and for this reason, our research was focused on antibacterial properties of the phenolics extracted from the P. tomentosa leaves. There are two important novelties of our approach. One novelty is that the phenolics extract from the P. tomentosa leaves was fractionated into six fractions (I–VI, expectedly containing flavonoid aglycons, free phenolic acids, non-polar flavonoid glycosides, polar flavonoid glycosides and phenolic acids liberated by the acidic and basic hydrolysis, respectively). The other novelty is that antibacterial assay was performed for each isolated fraction with the use of high-performance thin-layer chromatography combined with direct bioautography (HPTLC-DB). The best separations were obtained for fractions I and II, somewhat worse separations were obtained for fractions III–V, and the lack of characteristic bands was observed with fraction VI. In the antibacterial assay performed upon the HPTLC fingerprints, the Gram positive Bacillus subtilis and the Gram negative Aliivibrio fischeri bacteria were used. Well pronounced antibacterial activity was established with the selected fingerprint zones derived from fractions I–IV, whereas a lack of antibacterial activity was observed with fractions V and VI. Identification of apigenin and p-coumaric acid in the chromatographic zones from fractions I and II was performed with use of HPTLC and HPLC-DAD-MS, aided by external standards. Fractions III and IV were found to have similar chromatographic patterns, which suggests the presence of semi-polar flavonoid sugar conjugates in the P. tomentosa leaves.