Oxidation of carotenoids in dichloromethane and mesoporous molecular sieves: HPLC and spectroscopic studies.

摘要

In the oxidation of the carotenoids, ethyl all-trans-8 ′-apo-β-caroten-8′-oate (I) and all-trans-β-carotene (II), with ferric chloride, several equilibria occur between Fe3+, Fe 2+, Cl−, the neutral carotenoid, and its radical an electron from Fe2+. Isomerization of carotenoids occurs during the oxidation. In the presence of air a stable product is formed in high yield during the oxidation. 1H NMR, LC-MS and optical studies show that this product is the 5,8-peroxide of the starting material. A mechanism for the formation of this compound is proposed.; Carotenoid radical cations (Car·+) produced by oxidation with FeCl₃ in CH₂Cl₂ solution were found to coexist as two species which were spectroscopically identified: the free radical cation (FCR) and an ion pair. The ion pair is formed by interaction between Car·+ and Cl−. Optical and EPR studies show that the ion pair is solvent-separated (SSIP). The interaction causes a shift in the maximum absorption wavelength of Car·+ . The blue shift is for symmetrical Car·+ and the red shift is for asymmetrical Car·+. Semiempirical calculations predict these results and a theoretical explanation is given for these phenomena.; Electron transfer (ET) and isomerization of the carotenoids (Car) imbedded in MCM-41, Ti-MCM-41, Cu-MCM-41 and Fe-MCM-41 were studied. The ET efficiency in MCM-41 was highest for the carotenoid with the lowest oxidation potential. The presence of transition metal ions in the framework of MCM-41 increased the ET efficiency of all carotenoids, but the enhancement did not depend on oxidation potential, but rather on whether complexes are formed between carotenoids and metal ions. The presence of transition metal ions results in a large blue shift of the maximum absorption wavelength due to changes in the carotenoid conformation. A very high trans to cis isomerization efficiency of Car was found in these solid hosts. Isomer types of Car can be controlled by selecting different hosts.