- Thermal degradation kinetics of bixin in an aqueous model system
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The kinetics of the thermal degradation of the natural cis carotenoid bixin in a water/ethanol (8:2) solution was studied as a function of temperature (70-125°C), using high-performance liquid chromatography. The curves for the decay of bixin and formation of products (e.g., di-cis and all-trans isomers and a C17 degradation compound) did not adjust well to a first-order rate law, but very good fits were obtained using a biexponential model. This mathematical modeling gave the rate constant values for the formation of the primary products from bixin, and the energy barrier for each step was obtained. The di-cis isomers were formed immediately (15 kcal/mol) together with the decay of bixin, followed by a slow consumption, indicating their role as reaction intermediates. In fact, the di-cis isomers could easily revert to bixin (Ea ≈ 3 kcal/mol) or yield the primary C17 degradation product, with an energy barrier of 6.5 kcal/mol. In turn, 24 kcal/mol was necessary for the Bix - all-trans step, explaining its slower formation.
- Rios, Alessandro De O.,Borsarelli, Claudio D.,Mercadante, Adriana Z.
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p. 2307 - 2311
(2007/10/03)
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- Model Studies on the Photosensitized Isomerization of Bixin
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The photosensitized isomerization reaction of the natural cis carotenoid bixin (methyl hydrogen 9′-cis-6, 6′-diapocarotene-6, 6′-dioate) with rose bengal or methylene blue as the sensitizer in acetonitrile/methanol (1:1) solution was studied using UV-vis spectroscopy, high-performance liquid chromatography (HPLC), and time-resolved spectroscopic techniques, such as laser-flash photolysis and singlet oxygen phosphorescence detection. In both N2- and air-saturated solutions, the main product formed was all-trans-bixin. The observed isomerization rate constants, k obs, decreased in the presence of air or with increase in the bixin concentration, suggesting the participation of the excited triplet state of bixin, 3Bix*, as precursor of the cis→ trans process. On the other hand, bixin solutions in the absence of sensitizer and/or light did not degrade, indicating that the ground state of bixin is stable to thermal isomerization at room temperature. Time-resolved spectroscopic experiments confirmed the formation of the excited triplet state of bixin and its deactivation by ground state bixin and molecular oxygen quenching processes. The primary isomerization products only degraded in the presence of air and under prolonged illumination conditions, probably due to the formation of oxidation products by reaction with singlet molecular oxygen. An energy-transfer mechanism was used to explain the observed results for the bixin transformations, and the consequences for food color are discussed.
- Montenegro, Mariana A.,Rios, Alessandro De O.,Mercadante, Adriana Z.,Nazareno, Monica A.,Borsarelli, Claudio D.
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p. 367 - 373
(2007/10/03)
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- Synthesis of bixin and three minor carotenoids from annatto (Bixa orellana)
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The three apocarotenoids methyl (9Z)-8'-oxo-6,8'-diapocaroten-6-oate (2), methyl (9Z)-10'-oxo-6,10'-diapocaroten-6-oate (4), and methyl (9Z)-14'- oxo-6,14'-diapocaroten-6-oate (5), recently isolated from annatto, were synthesized. The key step of all three syntheses was the Wittig reaction of the (Z)-terminus 6 with the phosphonium salts 15, 18, and 24, carrying the polyene chain. Bixin (1) was synthesized from 2 in a Horner-Emmons reaction.
- Haeberli, Adrian,Pfander, Hanspeter
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p. 696 - 706
(2007/10/03)
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