Journal of Photochemistry and Photobiology A: Chemistry 221 (2011) 64–70 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem Lipid vesicles as model membranes in photocatalytic disinfection studies O.K. Dalrymple, W. Isaacs, E. Stefanakos, M.A. Trotz, D.Y. Goswami ∗ Clean Energy Research Center, University of South Florida, Tampa, FL 33620, USA article info Article history: Received 2 November 2010 Received in revised form 12 April 2011 Accepted 24 April 2011 Available online 30 April 2011 Keywords: Hydroperoxide Malondialdehyde Peroxidation Phosphatidylethanolamine TBARS Titanium dioxide abstract 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 disin- fection 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 appro- priately sized to be comparable to real cells. The vesicles and E. coli cells were photocatalytically treated in a test tube batch reactor using TiO 2 (Degussa P25) and UVA lamps. The rate of phospholipid mem- brane 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 -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 TiO 2 . 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Over the past decade the mechanism of photocatalytic disin- fection has been heavily debated among researchers. Increasing evidence suggests that the oxidation of cell membrane lipids plays an important role in the photocatalytic inactivation of bacterial pathogens in water [1–5]. The general hypothesis is that the unsatu- rated fatty acids, mainly polyunsaturated fatty acids, present in the phospholipid membranes are very sensitive to oxidation by radical species, particularly the hydroxyl radical. The repeating arrange- ment of lipids in the membrane allows cell injury to occur at sites relatively distant from the initiation source due to radical-induced chain reactions (Fig. 1). ∗ Corresponding author. Tel.: +1 8139740956; fax: +1 8139746438. E-mail addresses: odalrymp@mail.usf.edu (O.K. Dalrymple), goswami@usf.edu (D.Y. Goswami). During photocatalysis hydroxyl radicals are generated on the surface of a solid semiconductor catalyst, such as titanium diox- ide (TiO 2 ), when exposed to light of the appropriate wavelength [6]. The hydroxyl radical is known to oxidize all macromolecules found in cells including proteins [7,8], polysaccharides [9], lipids [10–12], and nucleic acids [13,14]. However, the disinfection pro- cess is mainly characterized by an interaction between the cell membrane and the photocatalyst [15]. The phospholipid compo- nents of the cell are localized in the cell membrane. TiO 2 is capable of initiating an irreversible oxidation of the fatty acids present in the membrane of the pathogen when hydroxyl radicals extract H- atoms from unsaturated lipids. The initiation process is followed by a propagation cycle in which the newly formed lipid radical reacts with oxygen to produce a lipid peroxyl radical. The prop- agation cycle continues as the lipid peroxyl radical reacts with a nearby unsaturated lipid producing a new lipid radical and a lipid hydroperoxide (LOOH). The process is terminated when two radicals react forming a non-radical species. The lipid hydroper- 1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2011.04.025