title: Cholesterol-diaryl ketone stereoisomeric dyads as models for "clean" type I and type II photooxygenation mechanisms.
abstract: Cholesterol (Ch) is a major target for oxidative degradation in cell membranes, a process which can occur by two mechanisms: Type I (via free radicals) and Type II (mediated by 1O2). In the present work, several dyads have been synthesized from beta- and alpha-Ch and ketoprofen (KP) or tiaprofenic acid (TPA). Upon irradiation under anaerobic conditions, KP-alpha-Ch dyads were efficiently photolyzed, via intramolecular hydrogen abstraction from C-7. By contrast, KP-beta-Ch, TPA-alpha-Ch, and TPA-beta-Ch remained unchanged after prolonged irradiation. The transient absorption spectra of KP-alpha-Ch were assigned to the short-lived biradicals resulting from intramolecular hydrogen abstraction. Interestingly, the spectra and lifetimes obtained for the TPA-derived dyads were very similar to those of the TPA triplet excited state. For the KP-alpha-Ch dyads, generation of singlet oxygen was expectedly negligible. Conversely, for TPA-alpha-Ch a Phi delta) value as high as 0.5 was determined. Thus, KP-based dyads are appropriate models for clean type I Ch oxidation, whereas the TPA derivatives are suitable systems for investigation of the purely type II process.
authors.affiliation: Departamento de Química-Instituto de Tecnología Química UPV-CSIC. Universidad Politécnica de Valencia, Camino de Vera s/n, Apdo 22012, 46071, Valencia, Spain.
authors.forename: Inmaculada
authors.initials: I
authors.lastname: Andreu
authors.forename: Isabel M
authors.initials: IM
authors.lastname: Morera
authors.forename: Francisco
authors.initials: F
authors.lastname: Boscá
authors.forename: Laura
authors.initials: L
authors.lastname: Sanchez
authors.forename: Pelayo
authors.initials: P
authors.lastname: Camps
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Miranda
mesh_terms: D002784:Cholesterol / Q000737:chemistry*; D007659:Ketones / Q000138:chemical synthesis / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D010777:Photochemistry; D011789:Quantum Theory; D013237:Stereoisomerism; D013997:Time Factors
chemical_list: D007659:Ketones; D002784:Cholesterol; D010100:Oxygen