Sesquiterpenes-Rich Essential Oils of Polyalthia longifolia Thw. (Annonaceae) from Nigeria#

Abstract

The essential oils of the leaf and stem bark of Polyalthia longifolia Thw. (Annonaceae) have been studied for their constituents by means of GC and GC/MS. The leaf oil was almost exclusively composed of sesquiterpene derivatives, being represented by allo-aromadendrene (19.7%), caryophyllene oxide (14.4%), β-caryophyllene (13.0%), β-selinene (7.9%), α-humulene (7.0%) and ar-curcumene (6.8%). However, α-copaene and α-muurolol (ca. 8.7%), β-selinene (8.6%), viridiflorene (8.1%), α-guaiene (7.8%), allo-aromadendrene (7.4%) and δ-cadinene (7.0%) were the major constituents occurring in the oil of the bark sample. All the other sesquiterpenoid compounds were observed in amount greater than 1%. α-Pinene (0.5%) and camphene (tr), which are the two monoterpenoids present in the leaf oil, could not be detected from the bark essential oil.

Key Word Index

Polyalthia longifolia, Annonaceae, essential oil composition, allo-aromadendrene, caryophyllene oxide, β-caryophyllene.

Introduction

As a continuation of our research about die volatile oils of poorly studied species of Nigerian flora (1), we report here die constituents identified from the leaf and bark oí Polyalthia hngifolia Thw (Annonaceae). About 120 species oí Polyalthia are distributed throughout the tropics. Polyalthia longifolia is a tall, handsome, evergreen tree cultivated in gardens in Southwestern Nigeria. In Nigeria and elsewhere, die plant is used for the treatment of skin diseases, fever, diabetes and hypertension. A number of biologically active compounds have been isolated from die non-volatile fractions of die plant. A phytochemical study on the hexane extract of die stem bark of P. longifolia has led to the characterization of clerodane and eni-halimane diterpenes (2,3), two of which have demonstrated significant antibacterial and antifungal activities (4).

Previous studies on die oil of die plant reports on die antimicrobial activity of die essential oils (5-9). Anotiier paper also reports on a preliminary investigation of die essential oil of the leaves, in which azulene derivatives were detected (10).

Experimental

Plant Materiah: The leaves and bark of P. longifolia were collected from plants cultivated in the vicinity of the Forestry Research Institute of Nigeria (FRIN), Ibadan, Nigeria. Mr. T. K. Odewo of the Herbarium Headquarters of die same Institute authenticated the plant materials, where voucher specimens were deposited for future reference.

Extraction of the volatile oils: Essential oils were obtained by separate hydrodistillation (3 h) of the pulverized air-dried plant materials using a Clevenger-type apparatus. Aliquots of each sample (500g) were used for this purpose. The oils (botii light yellow in coloration) which were obtained in 0.3 and 0. 15% (v/w) yields respectively were preserved under refrigeration until use.

Gas Chromatography (GC) analyses: The GC analyses were accomplished with a HP-5890 Series II instrument equipped with a HP-Wax and HP-5 capillary columns (both 30m ? 0.25mm, 0.25 µm film tiiickness), working with die following temperature program: 60°C for 10 min, rising at 5°C/-min to 2200C. The injector and detector temperatures were maintained at 25O0C; carrier gas N2 (2mL/min); detector dual, FID; split ratio 1:30. The volume injected was 0.5 µ?.. The identification of the components was performed by comparison of their retention times with those of pure authentic samples and by means of tìieir linear retention indices (LRI) relative to the series of n- hydrocarbons. The relative proportions of die essential oil constituents were percentages obtained by FID peak-area normalization without die use of response factor.

Gas Chromatography-Mass Spectrometry analyses: GC-EIMS analyses were performed with a Varían CP-3800 gas Chromatograph equipped witii a DB-5 capillary column (30m ? 0.25 mm; coating thickness 0.25 mm) and a Varían Saturn 2000 ion trap mass detector. Analytical conditions: injector and transfer line temperature 220°-240° C respectively; oven temperature programmed from 60°-240°C at 3°C/min.; carrier gas He at lmL/min.; injection of 0.2 mL (10% hexane solution); split ratio 1:30.

Identification of die constituents was based on comparison of the retention times with those of authentic samples, comparing their linear indices relative to the series of n-hydrocarbons, and on computer matching against commercial (NIST 98 and ADAMS) and home-made library mass spectra built up from pure substances and components of known oils and MS literature data (11-16). Moreover, the molecular weights of all the identified substances were confirmed by GC-CIMS, using MeOH as CI ionizing gas

Results and Discussion

The volatile compounds identified from the two oils of P. longifolia are listed in Table I, according to their retention indices on a HP-5 capillary column. Sesquiterpenoid compounds dominated in both oils. None of die monoterpenoid compound was detected in die bark oil, while only small amount of αpinene (0.5%) and camphene (tr) were present in the leaf oil. Sesquiterpenoids were present as hydrocarbons class (69.4% and 70.8%) and oxygen containing class (21.4 and 10.5%) respectively in the leaf and bark oils. The major constituents of the leaf oil were aZZo-aromadendrene (19.7%), caryophyllene oxide (14.4%) and ß-caryophyllene (13.0%). Other significant compounds were ß-selinene (7.9%), a-humulene (7.0%) and «r-curcumene (6.8%). All die constituents of the bark oil were identified in amount > 1%. The main ones being a-copaene and α-muurolol (ca. 8.7%); ß-selinene (8.6%), viridiflorene (8.1%), a-guaiene (7.8%), a?/s-aromadendrene (7.4%) and d-cadinene (7.0%).