title: EPR study of the astaxanthin n-octanoic acid monoester and diester radicals on silica-alumina.
abstract: The radical intermediates of the n-octanoic monoester and n-octanoic diester of astaxanthin were detected by pulsed EPR measurements carried out on the UV-produced radicals on silica-alumina artificial matrix and characterized by density functional theory (DFT) calculations. Previous Mims ENDOR for astaxanthin detected the radical cation and neutral radicals formed by proton loss from the C3 (or C3') position and from the methyl groups. Deprotonation of the astaxanthin neutral radical formed at the C3 (or C3') position resulted in a radical anion. DFT calculations for astaxanthin showed that the lowest energy neutral radical forms by proton loss at the C3 (or C3') position of the terminal ring followed by proton loss at the methyl groups of the polyene chain. Contrary to astaxanthin where proton loss can occur at either end of the symmetrical radical, for the diester of astaxanthin, this loss is prevented at the cyclohexene ends and is favored for its methyl groups. The monoester of astaxanthin, however, allows formation of the neutral radical at C3' and prevents its formation at the opposite end where the ester group is attached. At the terminal ring without the ester group attached, migration of proton from hydroxyl group to carbonyl group facilitates resonance stabilization, similarly to already published results for astaxanthin. However, cw EPR shows no evidence of a monoester radical anion formed. This study suggests the different radicals of astaxanthin and its esters that would form in a preferred environment, either hydrophobic or hydrophilic, depending on their structure.
authors.affiliation: Department of Chemistry, Valdosta State University, Valdosta, Georgia 31689, USA.
authors.forename: A Ligia
authors.initials: AL
authors.lastname: Focsan
authors.forename: Michael K
authors.initials: MK
authors.lastname: Bowman
authors.forename: Julia
authors.initials: J
authors.lastname: Shamshina
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Krzyaniak
authors.forename: Adam
authors.initials: A
authors.lastname: Magyar
authors.forename: Nikolay E
authors.initials: NE
authors.lastname: Polyakov
authors.forename: Lowell D
authors.initials: LD
authors.lastname: Kispert
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry*; D002210:Caprylates / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004952:Esters; D005609:Free Radicals / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D011522:Protons; D012822:Silicon Dioxide / Q000737:chemistry*; D013816:Thermodynamics; D014466:Ultraviolet Rays; D024341:Xanthophylls / Q000737:chemistry
chemical_list: D002210:Caprylates; D004952:Esters; D005609:Free Radicals; D011522:Protons; D024341:Xanthophylls; D012822:Silicon Dioxide; C005948:astaxanthine; D000537:Aluminum Oxide; C031492:octanoic acid