Chemical Composition and Larvicidal Activity of Rollinia leptopetala ( Annonaceae )

No presente trabalho descreve-se a composição química dos óleos essenciais de Rollinia leptopetala R.E. Fries (Annonaceae) e as atividades larvicidas dos óleos essenciais, do extrato metanólico das raízes desta espécie e do alcalóide oxoaporfinico, liriodenina (1), frente às larvas no terceiro estágio do mosquito Aedes aegypti. O extrato metanólico mostrou-se ativo com CL 50 64,6 ± 1,5 ppm e uma forte atividade foi exibida para o composto (1), CL 50 3,6 ± 0,4 ppm. Os óleos essenciais das folhas e galhos também mostraram atividade com CL 50 104,7 ± 0,2 and 34,7 ± 0,3 ppm, respectivamente. Estes dados sugerem que R. leptopetala é fonte potencial de larvicidas naturais. A composição química do óleo essencial e as atividades descritas são comunicadas pela primeira vez.


Introduction
Aedes aegypti is the principal mosquito vector of dengue fever, including the hemorrhagic form, which is endemic to South East Asia, Central and South America, and West Africa. 1 Between 50 and 100 million of cases are registered each year, causing thousands of deaths. 2 The disease has high levels of mortality, and also inflicts great economic losses and social disruption in Brazil. 3lthough some viral diseases, such as yellow fever, have been brought reasonably under control with a vaccine, no vaccine is so far available for dengue.Today, the only way of decreasing the incidence of this disease is by controlling the Aedes aegypti proliferation.This is not an easy task because the mosquitoes have developed resistance to the current synthetic insecticides that, in addition, are toxic to humans and the environment.The mosquito population control in the larval stage is much easier than in the adult stage, and then new strategies are needed for controlling the proliferation of the larvae of A. aegypti.Several studies have focused on natural products as insecticides for controlling A. aegypti larvae.Alkaloids and essential oils from herbal plants have demonstrated larvicidal activity, [4][5][6][7][8] which motivated our group to search for new insecticides from Brazilian plants.
Rollinia leptopetala R. E. Fries, popularly known as pinha brava, is an endemic shrub in Brazil and shows strong toxicity when eaten by animals. 91][12][13] Previous studies on liriodenine showed prominent antibacterial and cytotoxic biological activities. 14,15In this paper, the larvicidal activities of R. leptopetala and of the alkaloid liriodenine (1) against the larvae of Aedes aegypti are reported.

Results and Discussion
The methanol extract from the roots of R. leptopetala showed activity against A.aegypti larvae with LC 50 64.6 ± 1.5 ppm.][18] The alkaloid was evaluated and showed strong larvicidal activity against A. aegypti, with LC 50 3.6 ± 0.4 ppm.][21][22] The chemical composition of the essential oils from the leaves and stems of R. leptopetala showed quantitative and qualitative variation.The leaf oil consisted mainly of oxygenated monoterpenes (47.7%), with linalool and 1,8-cineole as the major compounds.The stem oil was devoid of monoterpenes, whereas sesquiterpenes, mainly oxygenated, were abundant (76.2%).Spathulenol (63.9%) was found in high concentration.The retention indices and relative percentages of the constituents of the oils are shown in Table 1.
The essential oil from the leaves showed moderate activity against the third-instar of Aedes aegypti larvae (LC 50 104.7 ± 0.2 ppm) but higher larvicidal activity (LC 50 34.7 ± 0.3 ppm) was found for essential oil from the stems, which confirmed a higher concentration of oxygenated sesquiterpenes.As no reports on the larvicidal activity were found for spathulenol, and this sesquiterpene was not isolated from the oil to be assayed, we can not suggest the higher activity found in the essential oil from stems is associated to spathulenol.
The quite high larvicidal activity of liriodenine (1) suggests this alkaloid as a model for development of new insecticides for A. aegypti control.The biological activity found for the essential oils, particularly for the essential oil from the stems, corroborates the importance of the investigation of essential oils from plants as potential insecticides and larvicides for controlling Aedes mosquitoes. 5,8,23perimental Plant material R. leptopetala was collected, at the flowering stage, in Guaraciaba do Norte, in the Northeast of Brazil, in February

Extraction and isolation of liriodenine (1)
The air-dried and pulverized roots (500 g) of R. leptopetala were washed with 20% (v/v) aqueous NH 4 OH, and extracted with CH 2 Cl 2 in a Soxhlet apparatus.The organic solution was distilled under reduced pressure giving a brown viscous syrup (50 g).This material was chromatographed on a silica gel column (63-200 µm, Vetec), previously washed with NaHCO 3 , and eluted with a gradient system (0→30%) of CH 2 Cl 2 /MeOH.Fractions were pooled according to TLC analysis.From the combined fractions eluted with an 80:20 mixture of CH 2 Cl 2 :MeOH, a residue was obtained which was submitted to exclusion chromatography on Sephadex TM LH-20 (Amersham Biosciences, Sweden) by elution with a mixture of MeOH/CH 2 Cl 2 to yield liriodenine (1, 78.0 mg, 0.16% yield), 278.0-280.4°C.Liriodenine (1) was also isolated from the methanol extract (18.5 g) of the roots (500 g), which was obtained by extraction of roots with cold MeOH for 72 h at room temperature.This material was chromatographed on a silica gel column (63-200 µm, Vetec) using hexane, CHCl 3 , EtOAc and MeOH as eluents.The combined fractions eluted with CHCl 3 (2.23 g) were chromatographed using a gradient system (0→30%) of CH 2 Cl 2 /MeOH.The fractions pooled with CH 2 Cl 2 /MeOH (90:10 v/v), provided 1 (12.7 mg, 0.07% yield).Recrystallization of 1 was from MeOH and structural determination was performed by spectral analysis, including comparison with literature data. 16 1H and 13 C NMR spectra were recorded on a Bruker Advance DRX 500 spectrometer and the mass spectrum was obtained from a VG Auto M Spec Fisions Instruments spectrometer.

Essential oil isolation and chemical analysis
The oils from the fresh leaves (500 g) and stems (500 g) of R. leptopetala were extracted by hydrodistillation for 4 h, using a modified Clevenger-type apparatus with a water-cooled oil receiver to reduce distillation overheating artifacts.Pale yellowish oils were obtained in 0.03 and 0.05% yield from the leaves and stems, respectively, which were dried over anhydrous sodium sulfate and kept at 4 °C until GC-MS and GC analysis.
The essential oil constituents were quantified by Analytical GC-FID, that was carried out on a Shimadzu GC-17A gas chromatograph using dimethylpolysiloxane DB-5 fused silica capillary column (30 m × 0.25 mm × 0.25 µm film thickness).Hydrogen was used as carrier gas at a flow rate of 1 mL min -1 and 30 psi inlet pressure; split, 1:30; temperature programmed, 35-180 °C at 4 °C min -1 , then heated at a rate of 17 °C min -1 to 280 °C and held isothermal for 10 min; injector temperature, 250 °C; detector used FID, detector temperature, 250 °C.The analysis of the oils was performed by GC-MS analysis on a Hewlett-Packard 5971 GC-MS instrument employing the following conditions: column: dimethylpolysiloxane DB-5 fused silica capillary column (30 m × 0.25 mm × 0.1 µm film thickness); carrier gas: helium (1mL min -1 ); injector temperature: 250 °C; detector temperature: 200 °C; column temperature: 35°-180 °C at 4 °C min -1 then 180-250 °C at 10 °C min -1 ; mass spectra: electron impact 70 eV.Individual components were identified using the Wiley L-built library and two other computer libraries 24,25 MS searches using retention indices as a pre-selection routine, as well as by visual comparison of the fragmentation patterns with those reported in the literature. 26,27No linear retention index was calculated and no standard was used.The chemical components identified in the essential oils are presented in Table 1.

Screening for larvicidal activity
Liriodenine (1), essential oils, and the methanol extract were placed in beakers (50 mL) and dissolved in H 2 O/DMSO 1.5% (v/v) at concentrations of 1-500 ppm, followed by the addition of 50 larvae at the third-instar.For each experiment, both positive (Temephos at 3.22 ppm) and negative (distilled water containing 1.5% DMSO) control assays were carried out.Mortality was recorded after 24 h of exposure, during which no nutritional supplement was added.The experiments were carried out at 28 ± 2 °C.Each test was performed in triplicate.Data were evaluated through regression analysis.From regression line, the LC 50 values were read representing the lethal concentration for 50% larval mortality of A. aegypti.The bioassays were performed at the Laboratório de Entomologia, Núcleo de Endemias, Secretaria de Saúde do Estado do Ceará, Brazil.

Table 1 .
Chemical composition of essential oils from Rollinia leptopetala .The plant was identified by Dr. F. S. Cavalcanti and Prof. E. P. Nunes from the Herbário Prisco Bezerra (EAC), Universidade Federal do Ceará, Brazil where a voucher specimen, # 33496, is deposited.
a Constituents listed in order of elution of DB-5 column.2004