ISSN 0006-3509, Biophysics, 2009, Vol. 54, No. 4, pp. 476–480. © Pleiades Publishing, Inc., 2009. Original Russian Text © P.Yu. Novikova, P.M. Krasilnikov, 2009, published in Biofizika, 2009, Vol. 54, No. 4, pp. 675–680. 476 INTRODUCTION Various factors can upset the intracellular balance between oxidative and protective events. Thereby reactive oxygen species cause intense lipid peroxida- tion, which alters the physiological properties of membranes, their structural and barrier functions, impairing the regulation of intracellular processes. Like any deviation from hemeostasis, this is fraught with development of various cell pathologies. There is a large body of evidence that oxidative degradation of lipids underlies the cell alterations associated with aging and age-related diseases (for review see [1]). The organism possesses a complex system to pro- tect itself from peroxidation; it includes both enzymes and small antioxidant molecules. The major antioxi- dants held to operate in biomembranes are vitamin E and carotenoids. The prevalent form of vitamin E in the organism, α-tocopherol, has been shown to inter- rupt the chain reaction of lipid oxidation by interact- ing with peroxyl radicals [2–4]. In the present work we used computer simulation to see how such interaction can take place within the membrane. The chain-breaking reaction involves hydrogen transfer from the hydroxyl of the chromanol ring in tocopherol to the peroxyl radical, for which the reacting sites of the molecules must come into imme- diate proximity; here one can spot a problem. The per- oxyl group in the phospholipid mostly (though not necessarily) occurs about the middle of the fatty acyl length; this is caused by the position of double bonds (which are more susceptible to free radical attack) and has been realized in our model lipid (linoleyl-11-per- oxy-7,9-octadecadienyl phosphatidyl choline). Such a structure apparently implies localization of the per- oxyl deep in the hydrophobic region of the membrane bilayer. On the other hand, the relatively polar chro- manol moiety of α-tocopherol should perhaps reside in the region of phospholipid heads. With such dispo- sition in the membrane (Fig. 1), the hydrogen that must reduce the peroxyl group would be about 10 Å away from it, i.e. much farther than admissible for reaction. However, appearance of polar (oxidized) func- tional groups in the fatty acyl of a membrane lipid may Molecular Modeling of the Reactive Configuration of Peroxidized Lipid and α-Tocopherol in the Membrane P. Yu. Novikova and P. M. Krasilnikov Biological Faculty, Moscow State University, Moscow, 119992 Russia e-mail: polina_novik@mail.ru Received March 12, 2009 Abstract—The mutual arrangement of a phospholipid molecule containing a peroxyl radical and a molecule of membrane-acting antioxidant α-tocopherol (vitamin E) in the lipid bilayer has been studied by molecular dynamics simulation. The geometry of molecules in the membrane is revealed at which the hydrogen atom can be transferred from the exocyclic hydroxyl of α-tocopherol to the peroxyl lipid radical. It is shown that, under equilibrium conditions, the peroxidized fatty acid segment rises nearer to the polar surface of the mem- brane, while α-tocopherol submerges into the hydrophobic part of the lipid bilayer. Key words: molecular modeling, lipid peroxidation, α-tocopherol, peroxyl radical, antioxidant, membrane DOI: 10.1134/S0006350909040137 CELL BIOPHYSICS Abbreviations: MD, molecular dynamics; PLoo, peroxidized phospholipid. H O OO' Fig. 1. Scheme of the initial geometry of a peroxidized phospholipid and α-tocopherol in the membrane. The arrow marks the long distance between the hydrogen of the chromanol hydroxyl and the peroxyl radical in the fatty acyl.