No abstract

Ultraviolet and visible spectroscopy was applied to characterize and to measure the concentration of b-carotene dissolved in a dioxane and water mixture. The reaction of b-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 bcarotene 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 b-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 )/b-carotene, nitric oxide (NO)/b-carotene and NO/NO 2 /b-carotene. Therefore, we proposed that the nitrogen oxides have also been intermediates in the reaction system of b-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.