Primary reaction intermediates of Type-I photosensitized lipid oxidation as revealed by time-resolved optical spectroscopies

[Display omitted] •The chlorophyll radical anion yielded via the ultrafast electron transfer reaction from C=C moieties to excited states chlorophyll.•The quenching rates of singlet and triplet chlorophyll excitation by both C=C moieties and oxygen.•The characterized lipoxygenated products including the core aldehyde. Chlorophyll a (Chl a) as a lipophilic photosensitizer can induce biomembrane destruction via Type-II reaction involving singlet oxygen (1O2) as a primary initiator. Type-I oxidation by an excited-state photosensitizer reacting directly with lipid substrate also contributes but the primary intermediate remains to be verified experimentally. We have investigated the reaction dynamics initiated by Chl a-photosensitization involving oxygen and the -C=C- moieties of lipids in the membranes of small unilamellar vesicles (SUVs) prepared from phospholipids of different degree of unsaturation (0, 1 or 2). Under anaerobic condition, femtosecond time resolved absorption (fs-TA) combined with spectroelectrochemical spectroscopies validated the formation of Chl a− in subpicoseconds, and time resolved fluorescence spectroscopy revealed the rapid quenching of 1Chl a* with a rate constant of (28 ns)−1. These ultrafast processes, independent of oxygen, are ascribed to the electron transfer reaction from a -C=C- moiety to 1Chl a* as an initiation step of Type-I reaction. On longer timescales, ns-TA spectroscopy unraveled the drastic quenching of 3Chl a* by either -C=C- or O2, and the quenching by O2 was found to be 20 times more efficient. This together with the 1O2-luminesence analysis prove the involvement of both types of photosensitization. In addition, HPLC-MS spectroscopy confirmed the ketonic, the alcoholic and the core-aldehyde products of lipid oxidation. Moreover, the oxygen dependent partition between Type-I and Type-II reactions is discussed on a detailed kinetics basis, showing that the two mechanisms of lipid oxidation are equally important under an oxygen concentration of 1.3×10−5 M at room temperature.