Phytochemical and biological studies of Tibouchina Semidecandra L

Abstract

Phytochemical studies on Tibouchina semidecandra L., previously known as T. urvilleana have resulted in the isolation of nine pure compounds comprising of one flavonol, three flavonol glycosides, two plant sterols, one fatty acid, an ester and one ellagic acid glycoside. Five compounds have been successfully isolated from the leaves, i.e quercetin, β-sitosterol-O-β-D-glucopyranoside, quercetin 3-O-α-L-(2′′-O-acetyl) arabinofuranoside, avicularin and quercitrin. Three compounds identified as oleic acid, eicosanyl trans-p-coumarate and 23-ethyl-cholest-5-en-3-ol have been isolated from the roots. The stem barks gave one ellagitannin, identified as 3,3′-O-dimethyl ellagic acid 4-O-α-L-rhamnopyranoside. Methylation of quercetin gave quercetin tetramethyl ether, while acetylation of quercetin yielded quercetin tetraacetoxyl acetate. The structures of all compounds were established based on spectral studies using nuclear magnetic resonance, infrared and ultraviolet spectroscopies as well as mass spectrometry. Evaluation of the antioxidative activity on the crude extracts and pure compounds by electron spin resonance (ESR) and ultraviolet-visible (UV-vis) spectrophotometric assays showed that the pure isolated flavonoids and the EtOAc extract of the leaves possessed strong antioxidative capabilities. Quercetin was found to be the most active as radical scavenger in DPPH-UV and ESR methods with SC50 of 0.7 μM ± 1.4 and 0.7 μM ± 0.6, respectively in the antioxidant assay. A combination of quercetin and quercitrin was tested for synergistic anti-oxidative capacity. However, there was no significant improvement observed. The antimicrobial assay on the crude extracts and pure compounds were carried out against the Gram-positive bacteria, Bacillus subtilis and Staphylococcus aureus and the Gram-negative bacteria, Pseudomonas aeruginosa and Escherichia coli. However, no significant activity was observed. Quercetin and quercetin tetraacetoxyl acetate exhibited strong antityrosinase agent with percent inhibition of 95.0% and 93.4% respectively, equivalent to the positive control, kojic acid in the tyrosinase enzyme assay.