1.   Introduction

Alternanthera is a diverse genus (of about 80 to 200 species, and the second largest in subfamily Gomphrenoideae of the Amaranthaceae.  The highest diversity of this genus occurs in South America, but many species also occur in the Caribbean, Central America and Mexico. Alternanthera sessilis (L.) R. Br. Ex DC. A plant of the family Amaranthaceae occurs throughout the tropical and subtropical regions of the World [1]. It has been introduced to other parts of the world including Nigeria. The synonyms of A. sessilis include Alternanthera triandra Lam., Alternanthera prostrata Don. And Achyranthus triandra Roxb [2-3]. A. sessilis is a perennial herb with prostrate stems, rarely ascending, often rooting at the nodes. The leaves are obovate to broadly elliptic, occasionally linear-lanceolate, 1-15 cm long and 0.3-3 cm wide. The petioles are glabrous to sparsely villous of about 1-5 mm long. The shiny white and glabrous flowers are spikes, bract and bracteoles 0.7-1.5 mm long while the sepals are 2.5-3 mm long. The plant flowers from December until March. The plant grows wild, but is also cultivated for food, herbal medicines, as an ornamental plant. The leaves and young shoots are consumed as vegetables [4].

There are several reports describing the in-vitro and in-vivo pharmacological studies on A. sessilis. A study reported that both aqueous and ethanolic extracts of aerial parts of A. sessilis possess significant nootropic potential [3,5]. A. sessilis will inhibit the cytotoxic nature of the pathogen causing ocular diseases. Various extracts of A. sessilis were proved to exhibited antimicrobial [6-7], anti-cataract [4,6], wound healing [6, 8], antioxidant [6,9], analgesic [10-11], hepatopro-tective [12-13], anti-cancer [12,14], hematinic [13,15], anti-hyperglycemic [14,16], analgesics [14, 16], anti-diarrhoeal [15,17], anti-inflammatory [18] and anti-diabetic [19]. A. sessilis is a promising tool for enhancing production of potent α-glucosidase inhibitors [20].

The young shoots of A. sessilis contain 3β-Oβ-D-glucopyranosyluronic acid, 2β-Oβ-D-glucopyrano-syloleanolic acid, stigmasterol and β-sitosterol [21-22]. Phytochemical studies yielded β-carotene, ricinoleic acid, myristic, palmitic, stearic, oleic and linoleic acids, α-spiraterol and uronic acid, 2, 4-methylene cycloartanol, cycloeucalenol, choline, oleanolic acid, lupeol, campesterol and 5-α-stigmasta-7-enol [23-24]. The phenolic compounds present in A. sessilis were ferulic acid, catechin, vanillic acid, and epigallo-catechin, gallic acid and chlorogenic acid [9, 25-26]. The polyphenols isolated from A. Sessilis were (+)-catechin, rutin, ellagic acid, and quercetin [10,27]. Moreover, propane-diyl-bis-hexahydro-isochromene, an antifungal compound was also isolated from A. sessilis leaves [28]. The allelopathic potential of A. sessilis was attributed to the content of vanillic acid, gallic acid and chlorogenic acid [25]. Crude extract of A. sessilis analyzed by GC/MS afforded (Z,Z)-9,12,-octadecadienoic acid (25.33%),  vitamin A aldehyde (11.94%) and 12-bromododecanoic acid (11.37%) as major compounds [29]. Analysis of hydro alcoholic and acetone extracts of stems by GC-MS gave the major phytoconstituents as methoxy-bis (cyclopenta-diene), 5,10-dihexyl-5,10-dihydroindolo [3,2-b]indole-2,7-dicarbaldehyde and 1,2-bis[3,4-dimethoxy ben-zyl]-1,2-bis (methoxymethyl) ethane [30]. The fatty acids content of A. sessilis includes ascorbic acid, stearic acid, lignoceric acid, behenic acid, palmitic acid and oleic acid [31]. A mixture of diasteriomers of ionone which showed low antimicrobial activity against Pseudomonas aeruginosa and Trichophyton mentagrophytes were also isolated from the plant [32]. Other compounds such as 11-eicosenoic acid, methyl ester, (E)-9-octadecenoic acid ethyl ester and 20-oxo-heneicosanoic acid methyl ester were also detected from A. sessilis.

The major components of essential oil of leaves of A. sessilis analyzed by the GC-MS were found to be 1,1,1,5,5,5-hexamethyl-3,3-bis[trimethylsilyl)oxy]tris-iloxane (15.43%), S,S-dioxide trans-2-methyl-4-N-pen-tylthiane (11.27%), didodecylphthalate (10.62%) and tetrahydro-2,5-dimethoxy furan (10.01%) [33]. However, 1,1,1,5,5,5-hexamethyl-3,3-bis [trimethyl-silyl) oxy]trisiloxane (17.76%), trans-4-ethyl-5-octyl-2, 2-bis(trifluromethyl)-1,3-dioxolane (11.12%) and tet-rahydro-2,5-dimethoxy furan (9.10%) were the major components of the flower oil [33]. The leaf essential oil of A. sessilis was reported to possess radical scavenging activity [33].

In continuation of our on-going study aimed at the characterization of the chemical constituents and biological activities of essential oils from Nigerian plants [34] we report herein the volatile compounds identified in the leaves of A. sessilis.

2. Materials and methods

2.1 Collection of A. sessilis leaves

The leaves of A. sessilis (220 g) were collected from Lagos State University, Nigeria. The collection of the plant samples was done during the month of April 2018. The plant was identified by Mr Adeniji, K.A. of the Forestry Research Institute, Ibadan with voucher number FHI 112139; and a voucher specimen was deposited at the herbarium.  Prior to hydrodistillation process the samples were air-dried under laboratory shade for two weeks (27ºC) to reduce the moisture contents. In addition, sediments and other unwanted materials were separated from the samples.

2.2 Hydrodistillation of the essential oil fromA. sessilis

Dried samples were pulverized into coarse powder in locally made grinder prior to hydrodistillation. In this experiment, 220 g of pulverized samples were used. The pulverized leaves of A.sessilis were loaded into a 5 L flask. Distilled water was added until the sample was covered completely. The sample was then subjected to hydrodistillation for 3 h in Clevenger-type apparatus according to an established protocol [35] to obtained essential oil which was stored under refrigeration (4oC) in weighed sample bottle as described in previous studies [34-35].

2.3 Chemical analysis of the oil

In the analysis of the chemical constituents of the essential oil, gas chromatography-flame ionization detector (GC-FID) and gas chromatography coupled with mass spectrometry (GC/MS) were used. For the GC-FID, an HP-5890 Series II gas chromatograph that has two capillary columns (HP-Wax and HP-5), both of dimension 30 m x 0.25 mm and film thickness of 0.25 μm was used for the analysis. The GC was temperature program at 60ºC and isothermally held for 10 min, rising at 5ºC/min to 220ºC. The injector and detector temperatures were both maintained at a temperature of 250ºC. Nitrogen was used as a carrier gas at a flow rate of 2 mL/min. The GC was equipped with a dual detector (FID). The method of splitting at ratio of 1:30 was employed to inject the essential oil samples (0.5 mL) into GC. Quantification was done by external standard method using calibration curves generated by running GC analysis of representative compounds.

During the analysis of the essential oil by gas chromatography-mass spectrometry (GC/EIMS), a Varian CP-3800 gas-chromatograph equipped with a HP-5 capillary column of dimension 30 m x 0.25 mm and a film thickness 0.25 μm was used. The gas chromatograph was coupled to a Varian Saturn 2000 ion trap mass detector. The analytical conditions employed include injector and transfer line temperatures of 220ºC and 240ºC, respectively, while the oven temperature was programmed from 60ºC to 240ºC at 3ºC/min. In this analysis, the carrier gas was helium at a flow rate of 1mL/min. The volume of essential oil injected into the GC was 0.2 μL (10% n-hexane solution) at a split ratio of 1:30. In addition, the mass spectra were recorded at 70 eV while the acquisition mass range was recorded at m/z 30-300 with a scan rate of 1 scan/sec.

The identification of the constituents was based on comparison of their retention times with those of authentic samples, comparing their linear indices relative to a series of n-alkanes (C6-C36). Further identifications were also made possible by the use of a homemade library of mass spectra built up from pure substances and components of known oils  [36].

and MS literature data as described in our previous report [34]. Moreover, the molecular weights of all the identified substances were confirmed by GC-CIMS, using MeOH as CI ionizing gas.

3.     Results and discussion

The average yield of essential oils was 0.21% ± 0.01 (v/w), calculated on a dry weight basis. The oil sample was colourless. The identities, retention indices and percent compositions of the oils are shown in Table 1 and Fig. 1.

Table 1. Constituent of essential oil of Alternanthera sessilis

aElution order on HP-5; b LRI, Linear retention indices on HP-5column; cLiterature retention indices; dStandard deviation are insignificant and excluded from the Table to avoid congestion; Sr. No serial number.

Figure. 1. GC chromatogram and representative compounds of Alternanthera sessilison HP-5 column

4. Conclusions

The chemical constituents of essential oils from the leaf of A. sessilis from Nigeria were being reported for the first time. The major constituents of the essential oil were identified as hexahydrofarnesyl acetone, β-caryophyllene and n-heptadecane. None of the monoterpene compounds was present in the oil sample. The data presented herein were found to differ completely from a previous study on the oil sample. 

Authors’ contributions

Conceptualization, O.N.A.; Methodology, I.A.O., O.N.A; Software, R.A; Validation, I.A.O; Formal analysis, F.M.M., R.A.; Investigation, A.T.A., O.N.A.; Resources, P.S.; Data curation, I.A.O; Writing–original draft preparation, A.T.A.; Writing – review & editing, R.A., O.N.A.; Project administration, O.N.A.

Acknowledgements

The authors would like to express their appreciation to the Department of Biochemistry, Lagos State University, for their technical assistance during the animal study.

Funding

No funding was received for this research.

Conflicts of interest

The authors declare that they have no conflict of interests.