Diffuse-Reflectance Infrared Fourier Transform and Electron Nuclear Double Resonance Study of the Carotenoid Bixin Attached to Irradiated TiO2

Diffuse-reflectance infrared Fourier transform spectroscopic measurement of the carotenoid cis-bixin on TiO2 shows adsorbance of bixin by the presence of carboxylate, methine, H2O removal or strongly adsorbed OH, and the presence of hydroxyl at the 1200–1800, 2800–3000, 3000–3600, and 3700–3900 cm–1 regions, respectively. The electron paramagnetic resonance/electron nuclear double resonance (ENDOR) spectra of bixin on TiO2 show the formation of bixin radical cation and proton-loss neutral radicals upon light irradiation—similar to the radicals formed and detected by ENDOR for bixin adsorbed on silica alumina. On silica alumina, radicals were generated prior to irradiation, and upon irradiation, the spectral signal was reduced by a factor of 4–5. On the other hand, on TiO2, no signal was detected until after irradiation. The nuclear magnetic resonance of the purified bixin shows the cis-bixin configuration, and the mass spectrometry analysis in both positive and negative modes shows no major peak besides p...