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Traditionally, the essential oil of aromatic herbs is obtained using hydrodistillation (HD). Because the emitted volatile fraction plays a fundamental role in a plant's life, various novel techniques have been developed for its extraction from plants. Among these, headspace solid phase microextraction (HS-SPME) can be used to obtain a rapid fingerprint of a plant's headspace. Daucus crinitus Desf. is a wild plant that grows along the west coast of Algeria. Only a single study has dealt with the chemical composition of the aerial part oils of Algerian D. crinitus, in which isochavicol isobutyrate (39.0%), octyl acetate (12.3%), and β-caryophyllene (5.4%) were identified. Using GC-RI and GC-MS analysis, the essential oils and the volatiles extracted from separated organs of D. crinitus Desf. were studied using HS-SPME.
GC-RI and GC-MS analysis identified 72 and 79 components in oils extracted using HD and in the volatile fractions extracted using SPME, respectively. Two types of essential oils were produced by the plant: the root oils had aliphatic compounds as the main component (87.0%-90.1%), and the aerial part oils had phenylpropanoids as the main component (43.1%-88.6%). HS-SPME analysis showed a more precise distribution of compounds in the organs studied: oxygenated aliphatic compounds were well represented in the roots (44.3%-84.0%), hydrocarbon aliphatic compounds were in the leaves and stems (22.2%-87.9%), and phenylpropanoids were in the flowers and umbels (47.9%-64.2%). Moreover, HS-SPME allowed the occurrence of isochavicol (29.6 - 34.7%) as main component in D. crinitus leaves, but it was not detected in the oils, probably because of its solubility in water.
This study demonstrates that HD and HS-SPME modes could be complimentary extraction techniques in order to obtain the complete characterization of plant volatiles.
Daucus is a genus belonging to the Apiaceae family and consists of about 600 species that are widely distributed around the world. D. carota (carrot) is the main species of the Daucus genus, and its cultivated form, Daucus carota ssp. sativa, is one of the most popular root vegetable crops in the world. Carrots have been reported to be endowed with medicinal properties, i.e., hypotensive, diuretic, carminative, stomachic, and antilipemic properties [1-4]. In Algeria, the Daucus genus is represented by species living in dry and uncultivated areas and, among these, D. crinitus Desf. syn. and D. meifolius Brot. are widespread along the Algerian west coast from Tlemcen to Mascara . D. crinitus is characterized by the presence of many subspecies that colonize the sands and cliffs . A survey conducted by herbalists identified that, in folk medicine, a drink made from the roots of D. crinitus is used in decoction to expel the placenta after childbirth, and as a tonic.
Although the phytochemistry of the Daucus genus has been extensively studied (e.g., flavonoids, carotenoids, polyacetylenes, anthocyanins, and volatile constituents), only a single study has dealt with the chemical composition of Algerian D. crinitus oil . The oil obtained from the aerial parts is dominated by phenylpropanoid compounds (45.5%), followed by aliphatic compounds (17.1%), and hydrocarbons sesquiterpenes (16.6%). The main components are isochavicol isobutyrate (39.0%), an uncommon phenylpropanoid associated with octyl acetate (12.3%), and β-caryophyllene (5.4%). Moreover, antibacterial and antifungal activities of separated phenylpropanoid esters of the entire oil have been reported.
The essential oils of aromatic herbs are traditionally obtained using hydrodistillation. Because the emitted volatile fraction plays a fundamental role in a plant's life, various novel techniques have been developed for its extraction from plants. Among these, headspace solid phase microextraction (HS-SPME) allows for the rapid fingerprinting of a plant's headspace [7-10], and HS sampling requires the optimization of the extraction parameters to be carried out. As has been previously reported in the literature [9,10], the most effective fibers from vegetable matrices used are those consisting of three polymers: a liquid (PDMS) for the less polar components, and two solids, DVB and CAR, for the more polar components. Several conditions regarding the time and temperature for equilibrium and extraction have been reported, according the plant material analyzed [7-10].
To obtain a better understanding of the volatiles of D. crinitus, we investigated the chemical composition of Algerian D. crinitus essential oils extracted using hydrodistillation (HD) from separated organs (i.e., the roots, stems, leaves, flowers, and umbels), and the volatile fractions extracted using HS-SPME from the same plant material. In both cases, the analysis was carried out using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS).
An analysis of the essential oils from the roots, stems, leaves, flowers, and umbels of D. crinitus harvested in four locations (A-D) identified 72 components, which accounted for 90.9%-98.3% of the total number. Their retention indices and relative percentages are shown in Table Table1.1. Among these, 22 nonterpenic compounds, 12 monoterpenes, nine sesquiterpenes, and four diterpenes were identified. Identification of 33 components was performed by comparing their EI-MS and retention indices with those from the laboratory-produced "Arômes" library; 13 components were identified by comparing their EI-MS and apolar retention indices with those reported in commercial or literature libraries.
However, the main component of the oils in the aerial parts of D. crinitus (53, 84.1%-40.1%) remained unidentified, and its identification was carried out using joint information obtained using data from EI-MS and 13C-NMR spectroscopy. The EI-MS data of 53 were close to those of isochavicol 2-methylbutyrate (4-(prop-(1E)-enyl)phenyl 2-methylbutyrate) 60, except for the molecular ion (m/z = 204 vs. m/z = 218, respectively), and for a signal at m/z = 43, replacing a peak occurring at m/z = 57. These signals suggest the occurrence of an isochavicol-derivative compound that had lost a 14 uma fragment from the acyl part. The 13C-NMR spectra acquired from the stem oil from a sample from Bensekrane (53, 84.1%) exhibited 10 signals, of which, three had double the intensity, assigned from the DEPT spectra of three quaternary carbon atoms (175.60, 149.65, and 135.59 ppm), and two aromatic methine carbon atoms (121.46 and 126.65 ppm), two unsaturated methine carbon atoms (130.15 and 125.78 ppm) and two methyl carbon atoms (18.43 and 18.93 ppm). These signals confirm the presence of phenylpropanoid compounds, and the formula of C13H16O2 was deduced from the DEPT spectra. The identification of isochavicol isobutyrate was unambiguously established from a comparison of its 13C-NMR spectral data with those reported in the literature [6,11] and from the identification of isochavicol as a corresponding alcohol from the LAH reduction of the Bensekrane stem oil sample.
Concerning the plant chemistry, two types of essential oils were produced by D. crinitus. The root oils were mainly composed of aliphatic compounds (87.0%-90.1%), and the aerial part oils (i.e., the leaves, stems, flowers, and umbels) were characterized by the occurrence of phenylpropanoids (43.1%-88.6%). The three main aliphatic compounds in the root oils were: dodecyl acetate 57 (30.3%-48.2%), undecane 17 (14.4%-34.1%), and dodecanal 35 (16.7%-26.3%). It is noticeable that the relative percentage abundance of these compounds differed according to the locality of the sampling; undecane was the major component in the Bensekrane (A) and Sid Abdelli (B) sample oils (34.1% and 30.9%, respectively), whereas dodecyl acetate 57 was identified as the main aliphatic component in the Terni (C) and Beni Snous (D) root oils (48.2% and 42.3%, respectively).
Conversely, in the oils from the aerial parts, the main components were isochavicol esters. These oils were similar qualitatively, but differed in the relative amounts of their major components. We noted that isochavicol isobutyrate 53 was always the major component (84.1%-36.5%), whichever organ was analyzed. The other major components identified were: isochavicol 2-methylbutyrate 60 (1.8%-17.8%), pentadecane 50 (0.4%-15.2%), zizaene 44 (trace-9.2%), and undecane 17 (trace-14.2%). Moreover, except for the stem oil obtained from the Bensekrane (A) sample, it appeared that the sampling locality had a lesser influence on the variability of the component chemicals of the oils. Finally, it should be noted that the relative percentage abundance of the aliphatic and phenylpropanoid compounds was correlated with the position of the organ on the dressed plant. On moving from the bottom to the top of the plant (i.e., the root, leaf, stem, umbel, and then flower), the relative percentage abundance of aliphatic compounds decreased, while the relative percentage abundance of phenylpropanoids increased. In the same way, we noted that the relative percentage abundance of hydrocarbon sesquiterpenes was higher in the umbels and flower oils (10.2%-15.3% and 7.7%-9.7%, respectively) than in the other organs (never more than 5.6%).
The volatiles emitted from the D. crinitus roots, leaves, stems, umbels, and flowers harvested in different locations were investigated using HS-SPME under optimized parameters. The optimization of the HS-SPME sampling parameters was carried out using fresh plant material based on the sum of the total peak areas obtained using GC-FID. The maximum sum of the total peak area was acquired for a temperature of 70°C, an equilibrium time of 60 min, and an extraction time of 30 min (Table (Table2).2). The GC-RI and GC-MS analysis identified 84 components: 45 nonterpenic compounds, 17 monoterpenes, 21 sesquiterpenes, and one diterpene (Tables (Tables33 and and4).4). Identification of 45 components was performed by comparing their EI-MS and retention indices with those in the laboratory-produced "Arômes" library, and 18 components were identified by comparing their EI-MS data and their apolar retention indices with those reported in commercial or literature libraries.
Regarding the organ contribution to the aromatic plant fingerprint, it should be noted that the volatile constituents were more abundant in the flowers than in the other parts of the plant. Our analysis showed that, for the same organ, the chemical composition of the HS fractions obtained from different localities was qualitatively similar, but differed by the relative percentage abundance of the main components. However, a correlation between the class of compounds and the organ studied was observed: the oxygenated aliphatic compounds were well represented in the roots, hydrocarbon aliphatic compounds were present in the leaves and stems, and phenylpropanoids were present in the flowers and umbels. In particular, the main volatiles from the roots were aliphatic compounds (88.1%-96.3%) such as dodecanal 35 (20.6%-55.4%), undecane 17 (2.3%-43.5%), dodecyl acetate 57 (14.2%-32.4%), and dodecanol 45 (1.7%-8.8%).
Regarding the aerial organs, both the leaf and stem volatile fractions were dominated by aliphatic hydrocarbon compounds (22.2%-87.9% and 27.6%-37.9%, respectively) and, in particular, alkanes such as pentadecane 50 (11.9%-16.0% and 19.9%-51.6%, respectively) and heptadecane 62 (9.0%-20.7% and 4.1%-20.7%). In addition, hydrocarbon monoterpenes were identified in stems (28.0%-29.4%), e.g., limonene 9 (7.5%-11.7%) and myrcene 5 (7.4%-10.6%), and isochavicol isobutyrate 53 was identified as a volatile emitted from the leaves (0.2%-19.0%). The occurrence of phenylpropanoids was established from both the volatile fractions emitted from the umbels and flowers (43.1%-88.6%), e.g., isochavicol isobutyrate 53 (26.7%-51.8% and 14.3%-19.7%, respectively) and isochavicol 26a (7.1%-15.2% and 29.6%-34.7%, respectively). In addition, hydrocarbon aliphatic compounds, in particular pentadecane 50 and heptadecane 62, were identified and their relative percentage abundance was higher in the volatile fraction extracted from the flowers (the sum was close to 20%) than from the umbels (the sum was close to 10%).
The identification of isochavicol 26a, a compound not present in our MS libraries, was carried out from a comparison of the retention indices and the EI-MS data with those of laboratory-synthesized compounds obtained from the LAH reduction of isochavicol isobutyrate 53, and it was confirmed by a comparison with the 13C-NMR data reported in the literature . Isochavicol 26a has been reported to exhibit interesting antiplasmodial activity , and to the best of our knowledge, this uncommon phenylpropanoid has not been identified in D. crinitus before.
The chemical differences observed between both the essential oils and the volatile fractions extracted using HD and SPME, respectively, can be explained by the fact that the first technique is based on the liquid quasi-total extraction of plant volatiles and the latter technique is controlled by a solid/gas equilibrium step. During hydrodistillation, the most volatile compounds and water-soluble compounds are lost in the gaseous phase and in the hydrolate, respectively, whereas, with HS extraction, it is the fiber affinity of each compound that monitors the sampling of the volatiles. As a consequence, it should be noted that 13 aliphatic compounds (1a-1e, 3a, 5b, 6a, 15a, 17c, 19a, 22c, and 24b) with a low molecular mass and boiling point were identified only in the volatile fractions extracted using HS-SPME. In the same way, isochavicol (26a) was absent in the D. crinitus essential oil, but its occurrence in the leaf HS fractions as a main component (7.1%-34.7%) can be explained by its solubility in water. Twenty-five compounds (identified by a number followed by a letter in Tables Tables33 and and4)4) in total were identified only in the volatile fractions extracted using HS-SPME, and 18 compounds (identified by an asterisk in Table Table1)1) were identified only in the essential oils.
Because the experiments were optimized for the SPME extraction parameters, the extraction temperature was the most important parameter in our plant headspace study. The distribution constants of each component were temperature dependent: the extraction of hydrocarbon monoterpenes and oxygenated sesquiterpenes was improved at a medium temperature (50°C) and at a high temperature (90°C), respectively (Table (Table2).2). The optimal temperature (70°C) used for the HS extraction was an analytical compromise based on the maximum amount of volatiles extracted. Regarding the comparison of both techniques in terms of the isolation time, HS-SPME was clearly faster (70 min), whereas 300 min was required for hydrodistillation. In the same way, the amount of plant material used for the headspace analysis was less (1 g), whereas the production of D. crinitus oil using hydrodistillation required 200-300 g of plant material. This may be a major reason for the difference in the chemical HS and HD data.
Several conclusions can be drawn concerning the chemistry of D. crinitus from this study.
(i) Two types of essential oil were produced: the root oils, which were mainly composed of aliphatic compounds (87.0%-90.1%); and the aerial part oils, which were mainly composed of phenylpropanoids (43.1%-88.6%).
(ii) HS-SPME analysis showed a more precise distribution of volatiles in the organs studied: oxygenated aliphatic compounds were well represented in the roots (44.3%-84.0%), hydrocarbon aliphatic compounds in the leaves and stems (31.3%-88.7%), and phenylpropanoids in the flowers and umbels (47.9%-64.2%).
(iii) Except for two samples, the location of the samples had a minor influence on the plant volatile production.
Finally, this study has demonstrated that HS-SPME extraction can be considered as an alternative technique for isolating volatiles from aromatic plants.
Samples of each organ (roots, stems, leaves, flowers and umbels) from D. crinitus Desf., were collected on November 2008, in Bensekrane (A) [260 m, 35°04'N 1°13'O], Sid Abdelli (B) [258 m, 35°05'N 1°12'O], Terni (C) [1199 m, 34°47'N 1°21'O] and Beni Snous (D) [854 m, 34°37'N 1°34'O] forests near Tlemcen, Algeria. Voucher specimens were deposited in the herbarium of the Tlemcen University Botanical Laboratory (Voucher number: UBL 05.09). The oils were isolated by hydrodistillation (200 - 300 g of plant per sample) for 6 h using a Clevenger-type apparatus  according to the European Pharmacopoeia and yielded 0.02 for roots and 0.03-0.04% for aerial parts.
The single organs of D. crinitus were cut roughly with scissors (1 - 2 cm long) before subjection to HS-SPME. The SPME device (Supelco) coated with divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS, 30 μm) was used for extraction of the plant volatiles. Optimization of conditions was carried out using fresh aerial parts of the plant (1 g in a 20 mL vial) and based on the number and the sum of total peak areas measured on GC-FID. Temperature, equilibration time and extraction time were selected after nine experiments combining four temperatures (30, 50, 70 and 90°C), four equilibration times (20, 40, 60 and 80 min) and three extraction times (15, 30 and 45 min). After sampling, SPME fibre was inserted into the GC and GC-MS injection ports for desorption of volatile components (5 min), both using the splitless injection mode. Before sampling, each fibre was reconditioned for 5 min in the GC injection port at 260°C. HS-SPME and subsequent analyses were performed in triplicate. The coefficient of variation (1.9% < CV < 19.8%) calculated on the basis of total area obtained from the FID-signal for the samples indicated that the HS-SPME method produced reliable results.
GC analyses were carried out using a Perkin Elmer Autosystem GC apparatus (Walhton, MA, USA) equipped with a single injector and two flame ionization detectors (FID). The apparatus was used for simultaneous sampling to two fused-silica capillary columns (60 m × 0.22 mm, film thickness 0.25 μm) with different stationary phases: Rtx-1 (polydimethylsiloxane) and Rtx-Wax (polyethylene glycol). Temperature program: 60 to 230°C at 2°C min-1 and then held isothermal 230°C (30 min). Carrier gas: helium (1 mL.min-1). Injector and detector temperatures were held at 280°C. Split injection was conducted with a ratio split of 1:80. Injected volume: 0.1 μL. For HS-SPME-GC analysis, only Rtx-1 (polydimethylsiloxane) column was used and volatile components were desorbed in a GC injector with a SPME inlet liner (0.75 mm. I.D., Supelco).
The oils obtained were investigated using a Perkin Elmer TurboMass Quadrupole Detector, directly coupled to a Perkin Elmer Autosystem XL equipped with two fused-silica capillary columns (60 m × 0.22 mm, film thickness 0.25 μm), Rtx-1 (polydimethylsiloxane) and Rtx-Wax (polyethylene glycol). Other GC conditions were the same as described above. Ion source temperature: 150°C; energy ionization: 70 eV; electron ionization mass spectra were acquired with a mass range of 35-350 Da. Oil injected volume: 0.1 μL. The volatile fractions sampling by HS-SPME were analyzed only on a Rtx-1 capillary column and volatile components were desorbed in a GC injector with a SPME inlet liner (0.75 mm. I.D., Supelco).
Identification of the components was based (i) on the comparison of their GC retention indices (RI) on non polar and polar columns, determined relative to the retention time of a series of n-alkanes with linear interpolation, with those of authentic compounds or literature data [13-18]; and (ii) on computer matching with commercial mass spectral libraries [15-19] and comparison of spectra with those of our personal library. Relative amounts of individual components were calculated on the basis of their GC peak areas on the two capillary Rtx-1 and Rtx-Wax columns, without FID response factor correction.
13C-NMR spectra of the stem oil from Bensekrane station (isochavicol isobutyrate 53: 84.1 %) were acquired in deuterated chloroform using a Bruker Avance 400 Fourier Transform spectrometer (Wissembourg, France) operating at 100.13 MHz for 13C-NMR and equipped with a 5 mm probe. All shifts were referred to the internal standard tetramethylsilane (TMS). 13C-NMR spectra of the chromatographic fractions were recorded with the following parameters: pulse width, 4 μs (flip angle, 45°); acquisition time, 2.7 s for 128 K Data table with a spectral width of 25,000 Hz (250 ppm); CPD mode decoupling; digital resolution, 0.183 Hz/pt. The number of accumulated scans was 5000 for a sample (around 40 mg of the oil in 0.5 mL of deuterochloroform) depending of the amount of product. Exponential line broadening multiplication (1 Hz) of the tree induction decay was applied before Fourier Transformation.
The isochavicol isobutyrate-rich stem oil (200 mg) from Bensekrane, was dissolved in dry Et2O (4 mL) and was carefully added to a suspension of lithium aluminum hydride (LAH) (100 mg) in dry Et2O (6 mL) at 0°C. The mixture was stirred at room temperature and then refluxed for 3 h. The reaction mixture was hydrolysed by addition of a solution of NaOH 15 % (2 mL) and cold water. The organic layer was separated, washed with water to neutrality, dried over Na2SO4 and concentrated under vacuum. The mixture (80 mg) exhibited isochavicol (22.3 %) as main components.
The authors declare that they have no competing interests.
MAD collected the plant material. MAD, ND, and JMD performed the HD and HS-SPME extractions, obtained the essential oils and the volatile fractions, and participated in the data analysis. MAD, HA, BT, AM, and JC conceived the study and helped draft the manuscript. HA, BT, AM, and JC performed the coordination of the study, and worked on the data analysis and interpretation. All the authors read and approved the final manuscript.
The authors are grateful to Prof. M. Bouazza (Botanical Laboratory, Biology Department, Aboubekr Belkaïd University) for the identification of the vegetable matter and Dr. P. Bradesi (University of Corsica, UMR-CNRS 6134, Equipe Chimie et Biomasse) for NMR data acquisition. They are indebted to the Agence Universitaire de la Francophonie (AUF) for providing a research grant to N.D.