Lipid Vesicles as Model Membranes in Photocatalytic Disinfection Studies

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hydroperoxide, malondialdehyde, peroxidation, phosphatidylethanolamine, TBARS, titanium dioxide

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The potential use of solar-powered photocatalytic disinfection water systems is an attractive concept and has generated much research over the last two decades. Photocatalytic inactivation of a wide range of water pathogens has shown promise to provide an effective alternative to traditional disinfection methods. However, in order for photocatalysis to be effectively used as a water disinfection process, its inactivation kinetics must be well established. Recent literature points to the peroxidation of phospholipid membranes as the main mechanism for photocatalytic inactivation of bacteria. To test the peroxidation hypothesis, researchers utilized free lipids, particularly lipids with the ethanolamine polar group which is dominant in the cell membrane of Escherichia coli. Although these experiments yielded useful information about byproducts, they did not provide information on the kinetics of lipid peroxidation in cells exposed to photocatalytic treatment.

In this work, lipid vesicles were prepared with a mixture of natural E. coli phospholipids and appropriately sized to be comparable to real cells. The vesicles and E. coli cells were photocatalytically treated in a test tube batch reactor using TiO2 (Degussa P25) and UVA lamps. The rate of phospholipid membrane degradation was determined by measuring the production of malondialdehyde (MDA) and lipid hydroperoxide (LOOH), byproducts of lipid peroxidation. Thiobarbituric Acid Reactive Species (TBARS) and Ferrous Oxidation of Xylenol (FOX) assays were used to assess each byproduct respectively. The fatty acid content of E. coli cells was also modified by adding oleic (C18:1 n-9) and α-linolenic (C18:3 n-3) acids to the growth media. Byproduct formation and oxidation kinetics were compared for all experiments. The results show that the oxidation kinetics of lipid vesicles closely matched the oxidation of E. coli cells in photocatalytic systems proving that the vesicles are useful model systems to study the interaction of cell membranes with TiO2. However, differences in monosaturated fatty acids in E. coli did not appear to affect the overall disinfection kinetics. While these findings further validate membrane peroxidation as an important process in the mechanism of photocatalytic disinfection, they suggest that overall inactivation results from a far more complex collection of processes.

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Citation / Publisher Attribution

Journal of Photochemistry and Photobiology A: Chemistry, v. 221, issue 1, p. 64-70