Cholesterol-diaryl ketone stereoisomeric dyads as models for "clean" type I and type II photooxygenation mechanisms.
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.
Andreu
Inmaculada
I
Departamento de Química-Instituto de Tecnología Química UPV-CSIC. Universidad Politécnica de Valencia, Camino de Vera s/n, Apdo 22012, 46071, Valencia, Spain.
Morera
Isabel M
IM
Boscá
Francisco
F
Sanchez
Laura
L
Camps
Pelayo
P
Miranda
Miguel A
MA
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
D007659:Ketones; D002784:Cholesterol; D010100:Oxygen