Chemiluminescent oxidation of luminol in micellar media: development of an anti-radical capacity assay / Oxidação quimiluminescente do luminol em meios micelares: desenvolvimento de uma ensaio para determinação da capacidade anti-radicalar

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

In this work, a methodology to evaluate the anti-radical capacity of hydrophilic and lipophilic compounds based on luminol chemiluminescence in micellar media was developed. The reaction of luminol, hemin and hydrogen peroxide was studied in the presence of charged surfactants (CTAB/CTAC and SDS). The concentration of each reagent was independently varied in order to evaluate its influence on the initial emission intensity (I0), which is proportional to the initial reaction rate. In aqueous SDS solution, I0 showed a linear correlation with the H2O2 concentration between 5,0 10-6 and 6,0 10-5 mol/L, and with the hemin concentration between 8,0 10-9 and 4,0 10-7 mol/L. An increase in the luminol concentration between 5,0 10-7 and 1,0 10-3 mol/L led to an increase in I0 up to 5,0 10-5 mol/L, higher luminol concentrations do not further increased I0. In the presence of CTAB, I0 increased linearly with the H2O2 concentration in the interval studied (2,0 10-5 to 6,7 10-4 mol/L). An increase in I0 was also observed on increasing the hemin concentration from 8,0 10-8 to 8,0 10-6 mol/L. An increase of the luminol concentration from 5,0 10-7 to 5,0 10-5 increased the observed I0, which did not change for higher luminol concentrations. The cmc of CTAB was measured in the reaction conditions (phosphate buffer pH 11,6 and µ= 0,1), and the value determined, 2 10-4 mol/L, was five times lower than the cmc in water. The absorption spectra of hemin and luminol in different CTAB concentrations showed significant variation with the surfactant concentration, indicating an interaction between these reagents and the surfactant. With these studies it was possible to understand well the behavior of the system and to establish experimental conditions which lead to kinetic curves with a slow emission intensity decay and relatively high I0, ideal conditions for the performance of the anti-radical capacity assay. The effect of trolox, the antioxidant used as reference, was evaluated in this conditions in the systems based on the three surfactants. In all the cases a linear correlation between the trolox concentration and the suppressed area in the emission kinetics was observed. This area is proportional to the number of radicals trapped by the antioxidant. Detection limits for trolox were below 1,0 10-7 mol/L, and the linear range was at least one order of magnitude (concentrations higher than 2,0 10-6 mol/L were not evaluated). The antioxidant capacity (n) was determined in the reaction medium containing CTAB for vitamin E (n= 3,5 ± 0,1), rutin (n=4,0 ± 0,2), quercetin (n=3,8 ± 0,4) and uric acid (n=1,3 ± 0,1). The n values determined by this method were very similar to those measured with the DPPH assay. Hence, the assay developed with luminol in micelar media was adequate to evaluate the anti-radical capacity of hidrosoluble as well as liposoluble compounds. The assay conditions established in the presence of CTAB allowed the consecutive injection of antioxidant samples during the same kinetic run. The values determined in this consecutive injection assay proved to be very similar to those obtained in the assay where injections were made individually. This method allows automation, economy of reagents and reduction of assay time

ASSUNTO(S)

antioxidants antiradical assay luminol chemiluminescence quimiluminescência solução aquosas micela bioluminescência química orgânica antioxidantes ensaio anti-radicalar organic chemistry micelle tensoativo surfactant luminol oxidação

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