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

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.

Starch Can Inhibit Nitrite-Dependent Oxidation of Crocin in Gastric Lumen Increasing Bioavailability of Carotenoids

“…A possible explanation for this is the reaction of NO with crocin radicals. This notion is supported by the observation that carotenoids can produce NO/carotenoid complexes as intermediates during degradation of carotenoids in the presence of nitrite (Mendiara et al, 2009). Further studies are required in order to elucidate the oxidation products of crocin and the mechanisms of the reactions between NO and crocin radicals.…”

β‐Carotene and Free Radical Reactions with Nitrogen Oxides

“…Therefore, the solvent dioxane and other later used in our research were adequately tested. It was also verified that they did not react with the nitrogen oxides, NO 2 and NO, even after prolonged times of observation in EPR (unlike what happened with hexane) [9]. NO 2 solutions were very carefully followed.…”

“…In Table 2, each reaction was described and the corresponding kinetic constant assigned. Unfortunately, in our 2009 manuscript, the sixth equation had one mistake, we must write P 1 instead of P 2 [9]. In the 2015 manuscript, the rate constants k i (i = 1 to 7) were not written above the arrows [10].…”

“…The signal is not resolved, due to the movement of the free radical and to the collisions that usually take place at the working temperature. Noticed that our research was always carried out under argon atmosphere [1,9].…”

“…reactive and less efficient in the abstraction of a hydrogen atom than NO 2 [9]. We can compare the following bond dissociation energies (BDE) values at 298 K:…”