Mechanism for the enhanced peroxidation of linoleic acid by a titanium dioxide/hypochlorite system

Nanosized titanium dioxide (TiO2) is a common component of sunscreen preparations and cosmetics as it reflects UV and visible light in accordance to Rayleigh's law. However, in aqueous environments, TiO2 is an efficient photocatalyst, producing superoxide (O-2(-center dot)) and hydroxyl (HO center dot) radicals, which are highly damaging to biomolecules. We investigated the role of TiO2 in promoting the peroxidation of linoleic acid (LA) alone and in the presence of hypochlorous acid (HOCl). TiO2 significantly enhanced peroxidation of LA, which was further enhanced in the presence of HOCl. This latter finding involved the formation of singlet molecular oxygen in a Russell-type mechanism appearing to involve preformed lipid hydroperoxides (LOOH). In addition to lipid peroxidation, HOCl also mediated formation of 18:1 monochlorohydrins, which in the presence of TiO2 appeared to decompose to kinetic products which supplemented peroxidation of linoleic acid. We present a theoretical mechanism which fits the available experimental data and may partially explain the dichotomy associated with HOCls role in lipid modification. (C) 2012 Elsevier Inc. All rights reserved.