Reaction of deuteroferrihaem and m-chloroperoxybenzoic acid in surfactant micelles

Abstract

The effects of non-ionic and anionic surfactant micelles on the UV/VIS spectrum of deuteroferrihaem and the kinetics of its reaction with m-chloroperoxybenzoic acid in pH 7.4 phosphate buffer have been studied. The results were analysed in terms of the micellar association constants of the haem and the transition state. The spectra and association constants are discussed with regard to the effect of surfactant on the ionization and dimerization equilibria of the haem. The visible region of the deuteroferrihaem spectrum changes much more in the presence of Triton X-100 than sodium dodecyl sulfate (sds). The effect of sds concentration on the observed second-order rate constant for the reaction was corrected for the micellar association of the peroxy acid and yielded micellar association constants for the transition state and the haem, K ^TS_mic 150 ± 20 dm³ mol^-1 and K _mic^{haem, pH 7.4} 720 ± 80 dm³ mol^-1, respectively. The latter value is in good agreement with the value of the micellar association constant 870 ± 100 dm³ mol^-1 obtained from the effect of sds concentration on the absorbance in the Soret region of the UV/VIS spectrum of the haem. A similar treatment of the kinetics in Triton X-100 showed that the ratio of K^TS_mic to K _mic^{haem, pH 7.4} increases from 0.06 to 0.14∶1 with increasing surfactant concentration, compared to a constant ratio of 0.21∶1 in sds. The effect of Triton X-100 concentration on the absorbance in the Soret region of the deuteroferrihaem spectrum is qualitatively similar to its effect on the reaction kinetics.

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