Cholesterol–diaryl ketone stereoisomeric dyads as models for “clean” type I and type II photooxygenation mechanisms
作者:Inmaculada Andreu、Isabel M. Morera、Francisco Boscá、Laura Sanchez、Pelayo Camps、Miguel A. Miranda
DOI:10.1039/b718068c
日期:——
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 β- and α-Ch and ketoprofen (KP) or tiaprofenic acid (TPA). Upon irradiation under anaerobic conditions, KPâα-Ch dyads were efficiently photolyzed, via intramolecular hydrogen abstraction from C-7. By contrast, KPâβ-Ch, TPAâα-Ch, and TPAâβ-Ch remained unchanged after prolonged irradiation. The transient absorption spectra of KPâα-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 KPâα-Ch dyads, generation of singlet oxygen was expectedly negligible. Conversely, for TPAâα-Ch a ΦΠvalue 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.
胆固醇(Ch)是细胞膜中氧化降解的主要目标,这一过程可以通过两种机制发生:I型(通过自由基)和II型(通过1O2介导)。在本工作中,已从β-和α-Ch以及酮洛芬(KP)或噻洛芬酸(TPA)合成了几种二元组。在厌氧条件下照射下,KP-α-Ch二元组通过C-7的分子内氢提取高效光裂解。相比之下,KP-β-Ch、TPA-α-Ch和TPA-β-Ch在长时间照射后保持不变。KP-α-Ch的瞬态吸收光谱归因于由分子内氢提取产生的短寿命双自由基。有趣的是,从TPA衍生的二元组获得的光谱和寿命与TPA三重激发态非常相似。对于KP-α-Ch二元组,预计单线态氧的生成可以忽略不计。相反,对于TPA-α-Ch,测得的ΦΔ值高达0.5。因此,基于KP的二元组是清洁型I Ch氧化的适当模型,而TPA衍生物是研究纯型II过程的合适系统。