Model studies on the photosensitized isomerization of bixin.
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 N(2)- 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, (3)Bix, 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
MA
Instituto de Ciencias Químicas, Universidad Nacional de Santiago del Estero, Av. Belgrano (S) 1912, 4200 Santiago del Estero, Argentina.
Rios
Alessandro de O
Ade O
Mercadante
Adriana Z
AZ
Nazareno
Mónica A
MA
Borsarelli
Claudio D
CD
D002338:Carotenoids / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D007536:Isomerism; D007700:Kinetics; D008027:Light; D008751:Methylene Blue / Q000737:chemistry; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010782:Photolysis; D017319:Photosensitizing Agents; D012395:Rose Bengal / Q000737:chemistry; D012996:Solutions
D017319:Photosensitizing Agents; D012996:Solutions; D012395:Rose Bengal; D002338:Carotenoids; C004588:bixin; D008751:Methylene Blue