Reaction of β-carotene with nitrite anion in a homogeneous acid system. An electron paramagnetic resonance and ultraviolet-visible study

Ultraviolet and visible spectroscopy was applied to characterize and to measure the concentration of β-carotene dissolved in a dioxane and water mixture. The reaction of β-carotene in the presence of nitrite anion and acid medium was studied at different temperatures. The reaction systems were homogeneous and were kept anaerobic. Pseudo-first-order rate constants in respect of β-carotene were measured in the range from 293 to 313 K and pH 5.8 ± 0.2. The energy of activation was calculated to be E a = 67.2 ± 3.4 kJ/mol. We interpolate a value that may have biological interest, k β-carotene (310 K) = (9.70±0.78) · 10 −3 s −1 , in the presence of 9.3 · 10 −3 M nitrite anion. Electron paramagnetic resonance spectroscopy was applied to characterize and quantify a persistent intermediate radical generated in the reaction system described. The recorded spectra showed triplet-type signals with a peak-to-peak value of 12.7 G. Nearly the same triplet radical-type intermediates were detected when studying the following reaction systems in pure dioxane: nitrogen dioxide (NO 2 )/β-carotene, nitric oxide (NO)/β-carotene and NO/NO 2 /β-carotene. Therefore, we proposed that the nitrogen oxides have also been intermediates in the reaction system of β-carotene, nitrite anion and acid medium, in the dioxane and water mixture. A mechanism was proposed and checked by employing the chemical kinetics simulation. The explanations developed would lead to a better understanding of the behavior of carotenoids in the presence of nitrite anion and nitrogen oxides.