Avaliação colorimétrica, eletroanalítica e teórica da atividade antioxidante de óleos essenciais e seus constituintes majoritários

Abstract

Oxidative processes and reactive oxygen species areresponsible for numerous pathologies and economic losses in the food and cosmetic industries. The number of chemical additives, drugs and supplements based on antioxidants, which slow or inhibit such processes, are increasing because they are capable of stabilizing or inhibiting the formation of free radicals. The most widely used antioxidants are the synthetic butyl hydroxyanisole (BHA), 2,6-di-tert-butyl-4-hydroxytoluene (BHT) and propyl gallate (PQ). In addition, the industry also uses ascorbic acid and α-tocopherol. However, the quest for a better quality of life has driven research to replace or associate synthetic products with products of natural origin. There are several compounds of natural origin that have the characteristics required of a good antioxidant, among them phenolic compounds, tannins, alkaloids, anthocyanins and essential oils. Essential oils, secondary metabolites produced by plants, have attracted attention because of their complex chemical compositions, and they can act in the stabilizationor inhibition of free radicals. The objective of the present work was to extract and characterize the essential oils from clove, oregano, spearmint and candeia, as well as to evaluate the antioxidant activities of these oils and their major constituents: eugenol, carvacrol, carvone and α-bisabolol, respectively, using colorimetric, electroanalytical and theoretical methods. The essential oils were extracted by hydrodistillation using a modified Clevenger apparatus for a period of two hours. They were identified and quantified by gas chromatography coupled to a mass spectrometer and a flame ionization detector. The colorimetric evaluation ofthe antioxidant activity was accomplished by means of the stabilization of DPPH radicals, protection of the β-carotene/linoleic acid system, sequestration of hydroxyl radicals and reducing power in relation to Fe +3 ions. The cyclic voltammetry technique was used to evaluate the electrochemical behavior of the samples and their chelating effect on Fe +2 ions. Vitreous carbon was used as the working electrode, platinum wire as the auxiliary electrode and Ag/AgCl as the reference electrode. The evaluations were performed in phosphate buffer (pH 7.4) and sodium sulfate (pH 5.0) as supportive electrolytes, respectively. The enthalpy of dissociation of the OH bond was calculated after the optimization of the structuresby the DFT/ωB97X-D method. Frequency calculations were performed to obtain standard enthalpy values and zero point energy corrections. The enthalpy of solvation was obtained from the thermodynamic cycle. The essential oils of clove and orégano, along with their principal eugenol and carvacrol components, presented antioxidant responses similarto those of the synthetic products, ascorbic acid and BHT. Eugenol and the essential oil from clove yielded better results than those carvacrol and oil of oregano. The other oils and their principal components did not present antioxidante characteristics. Eugenol was shown to be a promising molecule for substitution or association with the antioxidants used in the market.