New evidence for TiO2 photocatalysis during bilayer lipid peroxidation

The photocatalytic peroxidation of the bovine brain L-alpha-phosphatidyl-ethanolamine (PE) vesicle was measured and used as a model for phospholipid peroxidation. Synthetic phosphatidyl-ethanolamine, E. coli phosphatidyl-ethanolamine, lipid polysaccharide (LPS), and cardiopilin were used to compare the TiO2 photocatalysis leading to cell wall membrane degradation. During the photocatalytic degradation, the appearance and growth kinetics were followed for (a) conjugated double bond formation, (b) malondialdehyde (MDA), (c) peroxides, and finally (d) the observed CO2 evolution. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was measured of PE bilayers cast on TiO2 films and to a lesser extent for other phospholipids during TiO2 photocatalysis. The decay kinetics induced by photocatalysis of the isolated cis C=C-H (3008 cm(-1)), -CH2, and -CH3 groups and of the acyl-ester bond of the fatty acid in the glycerol backbone were followed in detail. The photocatalysis was also observed to induce spectral shifts in the CH2 vibrations and changes in the asymmetric phosphate ester (C-(PO4)(-)-C) stretching vibrations. The ATR-FTIR spectral changes suggest structural changes of the lipid bilayer due to peroxidation. The significant part of the photocatalytic peroxidation seems to take place at the TiO2, surface by heterogeneous mediated processes. But concomitantly, a homogeneous chain radical peroxidation of PE occurs in aqueous solution by a radical chain mechanism.