Essential Oil Variation in Lippia glandulosa Schauer

Journal of Essential Oil Research: JEOR, Nov/Dec 2005 by Maia, José Guilherme S, Silva, Milton Helio L da, Andrade, Eloisa Helena A, Carreira, Léa Maria M

Abstract

The oils of 14 samples of Lippia glandulosa collected in the "Lavrado" area, Roraima state, North Brazil, were obtained by hydrodistillation and analyzed by GC and GC/MS. Two chemical groups were characterized in the oils, one which was rich in thymol (46.3-78.1%), and another containing β-caryophyllene (17.3-59.4%) as the major component. In the first group the compounds p-cymene (5.1-7.2%) and β-caryophyllene (4.1-9.2%) have been associated to thymol in six of the analyzed oil samples. On the other hand, the second group showed variable association with (E)-nerolidol (0.7-23.6%), trans-α-bergamotene (0.2-19.3%), α-alaskene (3.4-16.0%), α-pinene (1.2-7.8%), α-humulene (2.6-6.0%), caryophyllene oxide (1.9-6.0%) and linalool (0.9-5.0%), in the other eight oil samples, justifying the field observation concerning the plant scent variability at the same collection site and the chemical composition differentiation observed to L. glandulosa.

Key Word Index

Lippia glandulosa, Verbenaceae, volatile composition, thymol and β-caryophyllene chemotypes.

Introduction

The genus Lippia (Verbenaceae) comprises nearly 200 species of herbs, shrubs and small trees growing in South and Central America and tropical Africa (1,2). This genus is well known for its aromatic character. More than 50 Lippia essential oils have been previously reported (3-18). The main volatile compounds found previously in the Lippia spp. oils were α-pinene, p-cymene, limonene, 1,8-cineole, linalool, camphor, myrcenone, carvone, α-thujone, tagetenone, piperitenone, peryllaldehyde, neral, geranial, thymol, carvacrol, tliymyl acetate, β-caryophyllene, β-cubebene, β- and γ-elemene and lippifoli-1(6)-ene-5-one.

As part of an ongoing survey of the Amazon aromatic plants, we collected several samples of Lippia glandulosa Schauer in Roraima state, North Brazil. The plant is a perennial shrub reaching 1.2 m in height, occurring in the natural fields and savanna areas of the Amazon, producing new sprouts each year. Locally, the plant is spread out in small populations forming separated spots. Many plant spots are found in an unique area but presenting a variability of aroma in that same area. The volatile composition of these distinguishable plant groups is reported in this paper. No previous references for the oil composition of L. glandulosa were found in the literature.

Experimental

Material and oil isolation: The aerial parts (leaves, flowers and fine stems) of 14 samples of L. grandulosa were collected in different places of the area known as "lavrado," located at the coordinates 2° to 5° North and 59° to 62° West, in Roraima State, Amazon Region, North Brazil, in August 1998. All samples were collected along the road that connect the Boa Vista city to the Pacaraima village, in the north direction to Venezuela border. Voucher specimens have been deposited in the Herbarium of Emilio Goeldi Museum, Belém city, Pará State, Brazil. The samples were air-dried separately for five days and hydrodistilled for 4 h using a Clevenger-type apparatus. The oils were dried over anhydrous sodium sulfate and their percentage content were calculated on basis of the plant dry weight. The plant collection number, site, herbarium number and oil yield of the 14 analyzed samples are showed in Table I.

Analysis of the volatile constituents: The qualitative analysis of the volatile compounds was performed on a Finnigan Mat INCOS XL GC/MS instrument, with the following conditions: a WCOT DB-5ms (30 m x 0.25 mm; 0.25 µm film thickness) fused silica capillary column; temperature programmed: 60°-240°C(3°C/min); injector temperature: 220°C; carrier gas: helium, adjusted to a linear velocity of 32 cm/sec (measured at 100°C); injection type: splitless (1 µL, of a 1:1000 hexane sol.); split flow was adjusted to give a 20:1 ratio; septum sweep was a constant 10 mL/min; EIMS: electron energy, 70 eV; ion source temperature and connection parts: 180°C.

The quantitative data of volatile constituents was obtained by peak area integration using a HP 5890 GC/FID instrument, without the use of correction factors, operated under the same GC/MS conditions, except forthe carrier gas that was hydrogen produced by a Packard hydrogen generator.

Individual components were identified by comparison of both mass spectrum and their GC retention data with those of authentic compounds previously analyzed and stored in the data system. Other identification were made by comparison of mass spectra with those existing in the data system libraries and cited in the literature (19). The retention indices were calculated for all volatiles constituents using a n-alkanes homologous series.

Results and Discussion

In the 14 oil samples of L. glaudulosa 126 volatile compounds were identified, as listed in Table II. The oil composition is quite distinguishable as observed in the chromatographic profiles. The 10 most remarkable compounds found in the oils were: thymol, β-caryophyllene, (E)-nerolidol, trans-α-bergamotene, α-alaskene, α-pinene, p-cymene, α-humulene, caryophyllene oxide and linalool. The 14 samples could be subdiwled in two groups. One of them represented by the 1-6 oil samples, that were dominated by thymol (46.3-78.1%), β-caryophyllene (4.1-9.2%) and p-cymene (5.1-7.2%). The other represented by the 7-14 oil samples, where β-caryophyllene (17.3-59.4%), (E)-nerolidol (0.7-23.6%), trans-α-bergamotene (0.2-19.3%) α-alaskene (3.4-16.0%), α-pinene (1.2-7.8%), α-humulene (2.6-6.0%), caryophyllene oxide (1.9-6.0%) and linalool (0.9-5.0%) were the main compounds. Based on these results, two chemical groups were proposed for the plants of L. plandulosa existing in the Roraima Lavrado area. They are represented by thymol and β-caryophyllene chemotypes, respectively, justifying the field observation concerning the plant aroma variability at the same collection site and the chemical composition differentiation. A transition stage between these two chemical groups can be observed by the analysis of the 6-7 oil samples, where the thymol percentage decreases while the β-caryophyllene content increases.