Characterization of the carotenoid cis-bixin
Bixin, a carotenoid found in annatto, Bixa orellana, is unique among natural carotenoids by being sparingly water-soluble. Bixin free radicals have been stabilized on the surface of silica alumina and TiO2 and characterized by pulsed electron nuclear double resonance (ENDOR). Least-square fitting of experimental ENDOR spectra calculated from density functional theory (DFT) calculations hyperfine couplings characterized the radicals trapped on silica alumina and TiO2. DFT predicts that the trans bixin radical cation is more stable than the cis bixin radical cation by 1.26 kcal/mol. While this small energy difference is consistent with the 26% trans and 23% cis radical cations in the ENDOR spectrum for silica alumina, the TiO2 spectrum could not be fitted due to poor signal. The ENDOR spectrum for silica alumina shows several neutral radicals formed by loss of a H+ ion from the 9, 9′, 13, or 13′ methyl group, a common occurrence in all water-insoluble carotenoids studied in literature. In addition, the continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy signal of bixin on silica alumina was intense prior to irradiation. Upon irradiation, the intensity is reduced 4-fold. On the other hand, unlike on TiO2 there was no signal prior to irradiation but signal was observed upon irradiation. The extinction coefficient bixin in chloroform is determined to be 1.1 x 105 ± 0.49 and 1.0 x 105 M-1cm-1 ±0.05 at 472 and 503 nm, respectively, while the redox potential in THF is found to be 0.94 ± 0.01V by cyclic voltammogram (CV) measurements. Based on the irreversibility of the CV, the bixin radical cation is estimated to have a short lifetime and decay rapidly at ambient temperature.