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Influence of -Cyclodextrins upon the Degradation of Carbofuran Derivatives under Alkaline Conditions

Research Article | DOI: https://doi.org/

Influence of -Cyclodextrins upon the Degradation of Carbofuran Derivatives under Alkaline Conditions

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Copyright: © 2018 Jesus Simal-Gandara et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Received: 30 November -0001 | Accepted: 06 September 2018 | Published: 10 September 2018

Keywords: carbofuran, 3-keto-carbofuran, 3-hydroxy-carbofuran, beta-cyclodextrin, inclusion complex, host–guest, degradation.

Abstract

The influence of b-cyclodextrins (b-CDs) on the stability carbufuran-derivatives (3-keto-carbofuran –KCF- and 2-hydroxy carbufuran –HCF-) in basic media has been analysed. An inhibition in the basic hydrolysis has been observed. The observed rate constant decreases because the formation of an unreactive host-guest complex between carbofuran and derivatives and the CDs. The CDs protect the carbamates increasing their half-life time in the presence of basic conditions.

Introduction

            Nowadays, the application of numerous pesticides has become inevitable to protect plantations from diseases and pests. Carbofuran (CF), 3-hydroxy-carbofuran (HCF) and 3-keto-carbofuran (KCF) –see scheme 1- are three of the most toxic carbamate pesticides used to control insects in many crops as potatoes, corn, citric, soybeans or turf grass [1]. They are systemic and contact insecticides and nematicides that act as cholinesterase inhibitors [2]. Further, pesticide pollution has added plenty of compounds to the environment as pollutants increasing concern among the public [3]. In soils, these carbamates are moderately persistent with a half-life ranging 30 and 120 days depending of environmental conditions [4-9]. On the other hand, hydrolysis processes are one of the main pathway for degradation of many xenobiotics in the environment [4-6,10-11]. Finally, CF, HCK and KCF are relatively soluble in water, as compared with other xenobiotics, and highly mobile in soils. [12]

 

              

-dproductsdt=kw3HCFtOHt-=kobs3HCFt=kobs3HCF0-3HCFt    (1)

-dproductsdt=kw3KCFtOHt-=kobs3KCFt=kobs3KCF0-3KCFt    (2)

where [3HCF] and [3KCF] are the carbofuran-derivative concentrations: 3-hydroxy-carboran (equation 1) and 3-keto-carbofuran (equation 2). kw and kobs are the bimolecular rate and the pseudo-first rate constants for the hydrolysis reaction. These equations can be integrated, and expressing the concentration of carbofuran-derivatives in terms of absorbance, yields:

At= A0exp-kobs t+ A (1-exp⁡-kobs t)             (2)

Figure 1-2 shows the influence of [OH-] on kobs in pure water and in CDs, respectively. As can be observed a linear dependence between the pseudo-first order rate constant and [OH-] was obtained for each substrate.

            A large inhibition has been observed for the basic hydrolysis (Figure 3-4). Hence, 3-hydroxy-carbofuran presents a 3.3 times-fold inhibitionand 3-keto-carbofuran yields 3.8 times-fold inhibition. This result is consistent with the corresponding one observed for hydrolysis of carbofuran (4.5 times-fold) [24]. This inhibition probes host-guest complex formation between both carbamates and b-CD.

                         

 Considering, also, that pKa value for β-CD is 12.20 [25] the following kinetic model can be proposed (Scheme 5). According with this the following expression can be obtained for the observed rate constant (kobs) (eq. 4)

            kobs=k0HO-+k1K1HO-1-fCDk2+k3HO-K2f[CD]1+K11-fCD+K2fCD               (4)

where k0, k1 and k3 are the hydrolysis rate constant of free substrate (KCF/HCF), the (KCF-CD/HCF-CD) complex and the (KCF-CD-/HCF-CD-) complex respectively; k2 is the rate constant corresponding to the reaction of the CF with the CD- once complexed. K1and K2 are the inclusion constants. Finally, f is the ionized cyclodextrin fraction and it can be written as

f=[OH-]OH-+Kb                (5)

To facilitate the fit of experimental data to eq. 4 experiments at different [CD] were carried out at [NaOH] concentrations for the limit values of fraction f which is f =0 and f=1. The first conditions imply that the eq. 4 can be simplified by obtaining eq. 6 and the second conditions would allow to rewrite the eq. 4 as eq. 7:

            kobs=koOH-+k1K1OH-CD(1+K1)CD                            (6)

            kobs=koOH-+k2+k3OH-K2CD(1+K2)CD                    (7)

Unfortunately, the errors in the kinetic coefficients and equilibrium constants taken from the fit of equations 6-7 to the experimental data do not allow a detailed analysis of them. Nevertheless, we can indicate that K2, k2, k3 are negligible respect to k1. Finally, we can affirm that the host-guest complex is unreactive, ergo CD hinder the access of hydroxyl ion to the carbonyl group of carbamates, and hence protecting them. This will be very helpful to understand carbofuran behaviour in the analytical, agro-environmental and food areas [26-29].

Scheme 1

The fact is that many pesticides can form inclusion with CD –see scheme 2 and 3-, often resulting in improvements in the properties of the complexed substances. With some pesticide-CD combination, it was found that as [CD] increases, the CD-pesticide inclusion complex precipitated out of solution, thus reducing the solubility of the pesticide. Also, in other cases, the observed behaviour is the opposite, CDs may form CD-pesticide inclusion complexes where it is more soluble than the free solute in a polar solvent. For this reason, CDs were successfully used to extract numerous commonly used pesticides from contaminated soil via cyclodextrin complexation [13-16]. In fact, that is a more environmentally friendly method compared to surfactants and organic solvents. CDs might become a great alternative as agents for improve remediation of contaminated soil and groundwater [17-22]. The must kept in mind that the cost of cyclodextrins is declining actually and, hence, the potential use of β-cyclodextrin could be of great interest in the pesticide solutions formulations and enhancement of their biological activity [23].

Materals and Methods

            b-CD,3-hydroxy-2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate and 3-keto-2,3- dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate, better known as 3-hydroxy-carbofuran and 3-keto-carboruan respectively, were supplied by Sigma-Aldrich (Steinheim, Germany). Sodium hydroxide and acetonitrile were Panreac reagent (Barcelona, Spain). All aqueous solutions were prepared by weight using double-distilled degasified water. All reagents used in the present investigation were of the maximum commercially available purity and were used without further purification. Experimental data of carbofuran was taken from the literature [24]

            The kinetic tests were conducted under pseudo first-order conditions ([PESTICIDE] << [OH]). Reactions were monitored through the first-order basic hydrolysis of carbofuran-derivatives using a Varian Cary 50 Bio spectrophotometer with an observation cell thermostated at 25.0 ± 0.1 °C using a Polyscience thermostat-cryostat temperature controller. When it was necessary, a rapid mixing stopped-flow unit supplied by Applied Photophysics, thermostated at 25.0 ± 0.1 °C (Polyscience thermostat-cryostat) was used. Detailed experimental procedure has been described elsewhere [24]

            Nonlinear regressions were carried out in a commercial software (pro Fit 6.2) supplied by QuantumSoft (Uetikon am See, Switzerland).

Results

            To guarantee that the presence of CDs would not alter the product of the reaction studied, a reaction spectra were carried out between 200 and 800 nm, and because CDs absorb in the UV-vis region, the spectrum of CDs in absence of reaction was used as blank. In each instance, it was observed that the final spectrum of the products of the reaction coincided with one obtained in pure water (data not shown).

The influence of CDs upon the basic hydrolysis of carbofuran-derivatives has been analysed in the present manuscript. Pseudo-first order conditions were kept in all experiments. Carbofuran-derivatives concentration was kept in all of experiments and equal to 8.33 ´ 10-5 M. This concentration was chosen to optimize the change in absorbance with time during the kinetic process. Sodium hydroxide concentrations were chosen to obtain a suitable half-life time to monitor the reaction. Finally, CDs concentration was varied between 0 and 0.017 M.

The disappearance of the absorbance at its maximum wavelength, (l= 290 nm and l= 280 nm KCF and HCF respectively) was use to follow spectrophometrically the reaction advance. Rate equation is the following:

Discussion

            As quote above, the effects of b-CD on the basic hydrolysis of carbofuran-derivatives (KCF and HCK) with [NaOH] constant was carried out (figures 3-4). A clear non-linear decrease in the hydrolysis rate was found and this decrease reach saturation in the reactivity of carbofuran which is related to host-guest complex formation between both carbamates and b-CD. Also, we must remake that at [CD] constant and in the present of different values [HO-] a linear relationship was found, and the absence of intercept implies that the basic hydrolysis goes though the nucleophilic attack of OH- on the carbonyl group of carbamate exclusively and water is not involved in the hydrolysis process as scheme 4 shown.

Conclusions

In summary, the inhibition of the hydrolysis process of the carbamates due to the encapsulation of them in the CD cavity implies a longer half-life of these xenobiotics. In this sense, the use of formulations consisting of the encapsulation of carbamates by CDs would allow, given their greater persistence, to reduce the concentration applied to the crops or the number of treatments needed during the harvest.          

Acknowledgements

            The authors thank Xunta de Galicia and European Union for financial support. J. Morales thanks the University of Vigo for a research-training grant. G. Astray would like to give his warm thanks to Xunta de Galicia, Consellería de Cultura and Educación e Ordenación for the postdoctoral grant B, POS-B/2016/001 which he received from them. A. Cid acknowledges the post-doctoral grants SFRH/BD/78849/2011 granted to Requimte and UID/MULTI/04378/2013 granted to Unidade de Cien̂cias Biomoleculares Aplicadas (UCIBIO), both from the Portuguese Foundation for Science and Technology.

References

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