Essential Oils of Nigeria II: Analysis of the leaf Oil of Securidaca longependuculata Fers.

Key Word Index

Securidaca longepedunculata, Polygalaceae, essential oil composition, methyl salicylate.

Introduction

Nigeria is blessed with a diversity of flora, most of which has remained unexploited for their chemical constituents and biological potentials. Securidaca longepedunculata Fers. (Polygalaceae) is commonly used as a medicine in many parts of Africa. In Nigeria, the plant is commonly employed for the treatment of inflammatory conditions and as purgative. An oral administration of a decoction of the root has been shown to produce a sedative effect, mainly attributed to its content of oleanolic acid glycoside (1). Extracts of the plants from Nigeria has been reported to possess both gastrointestinal and trypanocidal effects (2,3). Elsewhere considerable antimalarial (4), insecticidal (5) and insect repellent activities (6) have been confirmed for extracts from various parts of the plant.

On the chemical point of view, ergonine alkaloids (7) and tannins of the presenegamine skeleton (8) have been characterized from the plant. Two minor bitter principles: β-D-(3, 4-dismapoyl)-fructofuranosyl-α-D-(6-sinapoyl)-glucopyrano-side and β-D-(3-disinapoyl)-fructofuranosyl-α-D-(6-sinapoyl)-glucopyranoside have been described from die bark extracts (9). The seed oils have an abundant of hydroxydienoic fatty acids and acetotriacylglycerols (10). Previous research works on the volatde oils have been focused on the root. Costa et al. (11) identified mediyl 4-hydroxybenzoate (mediyl paraben) as die dominant volatile compound of the ether extract of the root oil of the plant. In addition, nine others compounds of the benzoic acid derivatives were also characterized. Lognay et al. (12) reported the occurrence of methyl salicylate and 2-hydroxy-6-methoxybenzoic acid methyl ester in S. longepedunculata root bark from Senegal. Later Jayasekara et al. also identified mediyl salicylate (2-hydroxy benzoic acid methyl ester) as the principal volatile component in the medianol extract of the root bark of the plant from Ghana (13). Methyl salicylate was also reported by Nebe and co- workers (14), as the dominant compound in die root bark oil of the plant from Burkina Faso. Mediyl salicylate was identified as the compound responsible for the fumigant property of the root of the plant (12).

To the best of our knowledge, there is no report in the literature on the constituents of the volatile fraction of the leaf of the plant from Nigeria and elsewhere. This paper reports on the compounds identified from the oil of the leaf of S. longependuculata grown in Nigeria.

Experimental

Plant material and oil isolation: The leaves of S. longependuculata were coUected from trees growing at a location in Bode-Osi, Headquarters of Ola-Oluwa Local Government, Osun State, Nigeria, in October, 2004.

The plant sample was authenticated by F. Usang of the Herbarium Headquarters, Forestry Besearch Institute of Nigeria (FRIN), Ibadan, Nigeria, where a voucher specimen (FHI 106976) has been deposited for future reference. The essential oil was obtained by hydrodistilltion of the air-dried and pulverized plant materials (500g) using a Clevenger-type apparatus with a water-cooled oil receiver to reduce hydrodistillation over-heating artifacts according to the British Pharmacopoeia (15). The oil, dried over anhydrous sodium sulfate, collected in a screw-capped vial and kept under refrigeration untd needed. The yield of the oil was 0.45% v/w.

Analysis of volatile compounds: The composition of the volatde constituents was established by GC and GC/MS analyses. GC analysis was carried out on a Perldn Elmer 8500 gas Chromatograph with FID, fitted with a Supelcowax - 10 fused sdica capdlary column (30 m x 0.32 mm; film thickness: 0.25 pm). The column temperature program was from 75°-250°C at a rate of 2.5°C/min. The injector and detector temperatures were programmed at 230°C and 300°C, respectively. Relative percentage amounts were calculated from GC (FID) analysis. GC/MS analyses were performed on a Hewlett-Packard 5973-6890 system operating in EI mode (70eV) equipped with a split/splitless injector (220°C), a split ratio 1/10, using a fused silica HP-5 MS capdlary column (30m x 0.25mm), film thickness: 0.25µm). The temperature program for HP-5 MS column was from 60°C (5min) to 280°C at a rate of 4°C/min. Helium was used as a carrier gas at a flow rate of 1.0 mL/min. Injection volume of each sample was 2 µL. Retention indices for all compounds were determined according to the Van der Dool approach (16), using n-alkanes as standards. The identification of the components was based on comparison of their mass spectra with those of Wiley and NBS Libraries (17) and those described by Adams (18), as well as by comparison of their retention indices with literature data (18, 19). In some cases, the oil was subjected to co-chromatography with authentic compounds (Fluka, Sigma). Optical rotation was measured on a Perkin-Elmer 341 Polarimeter.

Results and Discussion