title: DFT and ENDOR Study of Bixin Radical Cations and Neutral Radicals on Silica-Alumina.
abstract: Bixin, a carotenoid found in annatto (Bixa orellana), is unique among natural carotenoids by being water-soluble. We stabilized free radicals from bixin on the surface of silica-alumina (Si-Al) and characterized them by pulsed electron-nuclear double resonance (ENDOR). DFT calculations of unpaired electron spin distribution for various bixin radicals predict the EPR hyperfine couplings. Least-square fitting of experimental ENDOR spectra by spectra calculated from DFT hyperfine couplings characterized the radicals trapped on Si-Al. DFT predicts that the trans bixin radical cation is more stable than the cis bixin radical cation by 1.26 kcal/mol. This small energy difference is consistent with the 26% trans and 23% cis radical cations in the ENDOR spectrum. The remainder of the ENDOR spectrum is due to 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 previously studied. Although carboxyl groups of bixin strongly affect its solubility relative to other natural carotenoids, they do not alter properties of its free radicals based on DFT calculations and EPR measurements which remain similar to typical water-insoluble carotenoids.
authors.affiliation: Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336, United States.
authors.forename: Sefadzi S
authors.initials: SS
authors.lastname: Tay-Agbozo
authors.affiliation: Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336, United States.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Krzyaniak
authors.affiliation: Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336, United States.
authors.forename: Michael K
authors.initials: MK
authors.lastname: Bowman
authors.affiliation: Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336, United States.
authors.forename: Shane
authors.initials: S
authors.lastname: Street
authors.affiliation: Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336, United States.
authors.forename: Lowell D
authors.initials: LD
authors.lastname: Kispert
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry*; D031212:Bixaceae / Q000737:chemistry; D002338:Carotenoids / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D015394:Molecular Structure; D011789:Quantum Theory; D012822:Silicon Dioxide / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D002412:Cations; D005609:Free Radicals; D002338:Carotenoids; D012822:Silicon Dioxide; C004588:bixin; D000537:Aluminum Oxide