Toxicity and larvicidal activity on Aedes aegypti of citronella essential oil submitted to enzymatic esterification

Abstract The essential oil of citronella (Cymbopogon winterianus) has several biological activities, among them the insect repellent action. Some studies showed that cinnamic acid esters can be applied as natural pesticides, insecticides and fungicides. In this context, the objective of the present work was to evaluate the production of esters from citronella essential oil with cinnamic acid via enzymatic esterification. Besides, the essential oil toxicity before and after esterification against Artemia salina and larvicidal action on Aedes aegypti was investigated. Esters were produced using cinnamic acid as the acylating agent and citronella essential oil (3:1) in heptane and 15 wt% NS 88011 enzyme as biocatalysts, at 70 °C and 150 rpm. Conversion rates of citronellyl and geranyl cinnamates were 58.7 and 69.0% for NS 88011, respectively. For the toxicity to Artemia salina LC50 results of 5.29 μg mL-1 were obtained for the essential oil and 4.36 μg mL-1 for the esterified oils obtained with NS 88011. In the insecticidal activity against Aedes aegypti larvae, was obtained LC50 of 111.84 μg mL-1 for the essential oil of citronella and 86.30 μg mL-1 for the esterified oils obtained with the enzyme NS 88011, indicating high toxicity of the esters. The results demonstrated that the evaluated samples present potential of application as bioinsecticide.


Introduction
Citronella, Cymbopogon winterianus Jowitt (Poaceae), is a perennial plant, very resistant to climate change. The essential oil from this plant or their compounds can be applied in the food, pharmaceutical and cosmetic industries, and is a product widely used as a mosquito repellent and plant defense mechanism (Shasany et al., 2000;Cassel and Abstract The essential oil of citronella (Cymbopogon winterianus) has several biological activities, among them the insect repellent action. Some studies showed that cinnamic acid esters can be applied as natural pesticides, insecticides and fungicides. In this context, the objective of the present work was to evaluate the production of esters from citronella essential oil with cinnamic acid via enzymatic esterification. Besides, the essential oil toxicity before and after esterification against Artemia salina and larvicidal action on Aedes aegypti was investigated. Esters were produced using cinnamic acid as the acylating agent and citronella essential oil (3:1) in heptane and 15 wt% NS 88011 enzyme as biocatalysts, at 70 °C and 150 rpm. Conversion rates of citronellyl and geranyl cinnamates were 58.7 and 69.0% for NS 88011, respectively. For the toxicity to Artemia salina LC 50 results of 5.29 μg mL -1 were obtained for the essential oil and 4.36 μg mL -1 for the esterified oils obtained with NS 88011. In the insecticidal activity against Aedes aegypti larvae, was obtained LC 50 of 111.84 μg mL -1 for the essential oil of citronella and 86.30 μg mL -1 for the esterified oils obtained with the enzyme NS 88011, indicating high toxicity of the esters. The results demonstrated that the evaluated samples present potential of application as bioinsecticide.

Kinetic evaluation of citronellyl and geranyl cinnamates production
A preliminary kinetic experiment was performed under the experimental conditions reported by Zanetti et al. (2017) to determine the reaction time for synthesis of citronellyl and geranyl cinnamates. The reaction mixture of cinnamic acid, essential oil (EO), (acid/alcohol molar ratio 3:1), enzyme (concentration of 15 wt%, based on the total amount of substrates) and 40 mL solvent n-heptane was kept under mechanical agitation at 150 rpm and 70 °C during 56 h of reaction. Aliquots of 100 μL were withdrawn from the reaction medium at 0, 0.5, 1, 1.5, 2, 2.5, 3,4,8,12,24,28,32,38,48,52 and 56 h and analyzed by GC.
Reaction conversion was calculated based on the reduction of area of limiting reagent on the basis of reaction stoichiometry (Equation 1).
Where, S b corresponds to the peak area of the limiting reagent before the reaction (geraniol or citronellol) and S corresponds to the peak area of the limiting reagent after the reaction. The esterification activity of the enzyme was quantified through the synthesis reaction of oleic acid and ethanol (molar ratio of 1:3 by volume) described previously (Rigo et al., 2010). The amount of acid consumed was determined by titration with NaOH 0.05 mol/L. One unit of enzymatic activity (U) was defined as the amount of enzyme necessary to consume 1 μmol of fatty acid per minute, under the described assay conditions (Bernardes et al., 2007).

Toxicity against Artemia salina
Toxicity test was carried out using the methodology described by Meyer et al. (1982) and adapted by Cansian et al. (2017). The organisms-test were exposed Vargas, 2006;El-Helaly et al., 2021;Espadas-Pinacho et al., 2021). Cansian et al. (2017) observed an increase in toxicity on Artemia salina after enzymatic esterification of clove essential oil (Caryophyllus aromaticus L.), proposing that esterification may be a way to potentiate the insecticidal activity of essential oils.
Some studies are available in the literature on cinnamate production and also on the production of esters from the primary alcohols, geraniol and citronellol, present in the citronella essential oil. Paroul et al. (2012) in a study of bioflavors production by enzymatic esterification of citronella essential oil in a solvent free system, comparing two different acids (oleic and propionic acids), observed that the technique used is promising due to the high ester conversions achieved. In the assay using oleic acid the highest yield was obtained under substrates molar ratio on 1:1, 70 °C and 10 wt% Novozym 435 enzyme concentration, obtaining a conversion of 98.9% citronellyl oleate and 82.5% of geranyl oleate. In the assay using propionic acid the yield was 92.95% citronellyl propionate and 96.51% geranyl propionate under the same conditions described for oleic acid. Zanetti et al. (2017) evaluated the lipase-catalyzed synthesis of geranyl cinnamate and obtained a conversion of 75.85% under 3:1 molar ratio (geraniol:cinnamic acid), 70 °C and 15 wt% of Candida antarctica immobilized lipase (NS 88011). In this study it was observed that geranyl cinnamate presented antimicrobial properties against Staphylococcus aureus and Escherichia coli, but did not present antioxidant activity. Larvicidal activities or geranyl cinnamate toxicity were not evaluated.
There are few studies in the literature evaluating the esterification of citronella essential oil and, to our knowledge, no work about esterification of citronella essential oil with cinnamic acid and evaluation of its toxicity and insecticide properties are available, justifying the accomplishment of the present study that aimed to evaluate the production and the toxicity of the essential oil before and after enzymatic esterification, against A. salina and larvicidal action on A. aegypti, aiming to obtain previous data for future application of the same as bioinsecticide.

Enzyme and substrates
The commercial lipase used in this work was Candida antarctica (NS 88011) immobilized on a hydrophobic polymeric resin, purchased from Novozymes Brazil (Araucária, PR, Brazil). The chemicals used in the study were all AR grade with no further purification: cinnamic acid (97% Sigma-Aldrich); n-heptane (95%, Vetec); commercial citronella essential oil (C. winterianus), obtained by hydrodistillation and purchased from Ferquima (São Paulo, Brazil).

Chemical characterization of citronella essential oil
The chemical composition analysis of citronella essential oil was carried out by gas chromatography coupled to mass spectrometry (Shimadzu, Model QP 5050A). A capillary column (PEG) Rtx-Wax (30 m×0.25 mm×0.25 μm) was used.
to different concentrations (0.5, 1, 5, 10, 15, 20, 25, 50, 75 and 100 μg mL -1 ) of citronella essential oil and citronella essential oil plus cinnamic acid before and after esterification of 48 hours, using test tubes, each one containing at least 10 nauplii of Artemia salina in artificial saline solution (10 g L -1 sea salt, 0.7 g L -1 sodium bicarbonate and 2.0% DMSO). The control was carried using 1, 10, 25, 50, 75, 100, 250, 500, 750 and 1,000 μg mL -1 of cinnamic acid and n-heptane in the same conditions. The LC 50 values were determined in triplicate employing non-linear regression model available in GraphPad Prism 6.0 software and expressed as mean ± standard deviation. The means were compared using the t test, adopting 5% as significance level.

Larvicidal activity against Aedes aegypti
For the larvicidal activity assay, a methodology adapted from WHO (1970) was used, where previously obtained larvae (Soares-Pinheiro et al., 2017) of Aedes aegypti in the third stage of growth were maintained at rest with the samples (essential oil, essential oil plus cinnamic acid before and after esterification) for 24 hours, and then the counting of living and dead organisms was carried out. The samples (essential oil before and after esterification) and control (cinnamic acid and n-heptane) were evaluated, in triplicate, in concentrations between 25 and 500 μg mL -1 .

Chemical characterization of essential oil
The chemical composition of citronella essential oil (Table 1) presented as major components citronellal (38.98%), followed by geraniol (20.56%) and citronellol (17.54%). The results are qualitatively similar to those obtained by Paroul et al. (2012), with quantitative variations. The values found for the primary alcohols, which can be reacted with cinnamic acid, geraniol and citronellol, correspond to 38.1% of the total oil composition.

Production of citronellyl and geranyl cinnamates
The preliminary kinetic results of citronellyl and geranyl cinnamates are presented in Figure 1, and indicates that the optimum reaction time at the evaluated experimental conditions was 52 hours.
The results obtained for the peak areas of the components of interest before and after the esterification reaction are described in Table 2. The reduction of the areas of limiting substrates (citronellol and geraniol) showed conversions of 58.7% for citronellol and 69% for geraniol, confirmed by the increase of ester areas. These results are similar to those obtained by Paroul et al. (2012) in a study of esterification of citronella essential oil with propionic acid, where it was obtained a conversion of 55.4% for citronellol and 77.9% for geraniol into its respective esters, under reaction conditions of 60 °C, 1:1 molar ratio (oil:acid) and 1 wt% of Novozym 435.
After enzyme recovery from reaction medium by filtration, its activity was determined. A reduction of 39% was observed after 52 hours of reaction (initial activity of 1,559.4 U g -1 and final activity of 949.0 U g -1 ). This reduction is similar to that obtained by Padilha and Augusto-Ruíz (2007) who observed a 44.4% reduction in porcine pancreatic lipase enzyme solution activity under much milder conditions (60 min reaction at 38 °C), which can possibility the reuse of the biocatalyst or evaluate the fed batch mode of operation.
The reaction mixture, without prior purification, was used for the biological activities' tests.

Toxicity against Artemia salina
The toxicity results of citronella essential oil and citronellyl and geranyl cinnamates against nauplii of A. salina are shown in Figure 2. The mean lethal concentrations (LC 50 ) of the essential oil and esterified citronella oil obtained from the correlation equations between sample concentration and A. salina mortality are shown in Table 3.
It can be observed that the esters showed similar toxicity to citronella essential oil (5.29 and 4.36 μg mL -1 , respectively). The control cinnamic acid and the reaction components before esterification presented low toxicity (439.59 and 329.74 μg mL -1 , respectively) and n-heptane did not cause toxicity at the evaluated concentrations. The LC 50 results of the evaluated samples show that they present high toxicity, following the definition of Meyer et al. (1982) who considered that a product is toxic when it has a LC 50 value <1000 μg mL -1 and Amarante et al. (2011) who report that a sample is very toxic when its LC 50 is less than 100 μg mL -1 .
The same toxicity behavior for essential and esterified oils was obtained by Cansian et al. (2017) which obtained LC 50 of 0.59 μg mL -1 for clove essential oil (Caryophyllus aromaticus) and 0.12 μg mL -1 for eugenyl acetate produced by the esterification of clove essential oil, although with 10 to 40 times higher toxicity to these last.

Larvicidal activity in Aedes aegypti
The determination of the larvicidal activity was made from the percentage of dead Aedes aegypti larvae as a function of the concentration of the evaluated samples. Results are presented in Table 4.
Similar result for citronella oil on Aedes aegypti mosquito larvae was reported by Silva et al. (2017) who obtained a LC 50 value of 120 μg mL -1 for the citronella essential oil of the species Cymbopogon winterianus. Cheng et al. (2003) considered good larvicidal agent substances with an LC 50 value of less than 100 μg mL -1 . It can be observed that the esterification of the geraniol and citronellol monoterpenes promoted an increase in the larvicidal activity in the test with A. aegypti larvae (111.84 to 86.30 μg mL -1 ). However, the control (cinnamic acid) also showed larvicidal activity (98.85 μg mL -1 ) and its presence in the esterified oil contributed to the increased activity in the product.   The reaction components before esterification showed intermediate larvicidal activity between essential oil and cinnamic acid due to the dilution of the latter, as expected. The n-heptane did not cause toxicity at the evaluated concentrations. As esters are fairly stable in relation to essential oil (Surburg and Panten, 2016), this esterification product has the potential to use as larvicide to control diseases transmitted by A. aegypti.

Conclusions
New data on the production of cinnamic acid esters by enzymatic esterification of geraniol and citronellol from citronella essential oil were presented in this paper, showing a promising methodology for the production of these compounds. Conversion of citronellol and geraniol to corresponding esters reached 58.7% and 69%, respectively. The esterified oil presented high toxicity to Artemia salina (4.36 μg mL -1 ), being effective against Aedes aegypti mosquito larvae (86.30 μg mL -1 ), showing that these substances can be used as larvicide to prevent the spread of vector of various diseases.