Structure elucidation of hydrophillic flavonoids of polygonum Senegalense and trent comparative antifeedant activity with hydrophobic components.

Abstract

Polygonum senegalense is one of the eleven Polygonum species growing in Kenya highlands. P. senegalense finds use in ethno-medical mode of therapy in this country along with other species of the Polygonaceae family. In this work, the aerial parts of the plant were first washed with acetone, then the dried powdered material was sequentially extracted with cold 70 % and 50 % aqueous methanol for two days each and the combined aqueous extracts were partitioned into ethyl acetate. This was followed by chromatragraphic (analytical, preparative t.l.c. and column) analysis of the ethyl acetate fraction using silica gel, impregnated with 3 % oxalic acid solution and Sephadex LH-20. Finally the isolates were characterised using spectroscopic methods (IR, UV, NMR and MS). The secondary metabolites isolated and characterised from the plant material were 2',4' -dihydroxy-6' -methoxychalcone (1), kaempferol (4',5,7- trihydroxyflavonol) (3), quercetin (3' ,4' ,5, 7-tetrahydroxyflavonol) CJ.), quercetin-3-J3-0-galactoside (4), quercetin- 3-J3-0-g1ucoside (5), 2' -J3-0-gluco- 4' -hydroxy-6' -methoxydihydrochalcone'(ji) and quercetin-3-J3-0-glucoside-2"- gallate CD. Antifeedant activity was done on the mid fifth instar nymphs of two species of locust (Locusta migratoria ~d Schistocerca gregaria). Various concentrations of the samples (surface exudate, chloroform partition, ethyl acetate partition, quercetin and quercetin-3-B-O-galactoside) were applied on Whatman No. 1 filter: paper. The surface exudate were found to have high antifeedant activity at all concentrations tested while the inner tissue compounds showed phagostimulatory activity towards the two locust species.

UV analysis of the surface exudates and some of its pure compounds within the uv-visible range indicated the possible use of the surface exudates by the plant as a shield especially for the most dangerous UV-A and UV-B radiations without interfering with the electromagnetic region of photosynthesis.