| Abstract
| - 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 andsinglet oxygen phosphorescence detection. In both N2- and air-saturated solutions, the main productformed was all-trans-bixin. The observed isomerization rate constants, kobs, decreased in the presenceof air or with increase in the bixin concentration, suggesting the participation of the excited tripletstate of bixin, 3Bix*, as precursor of the cis→ trans process. On the other hand, bixin solutions in theabsence of sensitizer and/or light did not degrade, indicating that the ground state of bixin is stableto thermal isomerization at room temperature. Time-resolved spectroscopic experiments confirmedthe formation of the excited triplet state of bixin and its deactivation by ground state bixin and molecularoxygen quenching processes. The primary isomerization products only degraded in the presence ofair and under prolonged illumination conditions, probably due to the formation of oxidation productsby reaction with singlet molecular oxygen. An energy-transfer mechanism was used to explain theobserved results for the bixin transformations, and the consequences for food color are discussed. Keywords: Bixin; photoisomerization; photosensitization; singlet oxygen and triplet states
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