doi:10.1016/j.cplett.2004.09.093
Copyright © 2004 Elsevier B.V. All rights reserved.
Photo-oxidation of lipids by singlet oxygen: a theoretical study
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Ismael Tejeroa, Angels González-Lafonta, José M. Llucha and Leif A. Erikssonb, c,
, 
aDepartament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
bDepartment of Natural Sciences, Örebro University, Fakultetsgatan 1, 701 82 Örebro, Sweden
cDepartment of Cell and Molecular Biology, Box 596, Uppsala University, 751 24 Uppsala, Sweden
Received 8 September 2004;
revised 8 September 2004.
Available online 7 October 2004.
Abstract
The photo-oxidation reactions between lipid model nona-3,6(c,c)-diene and singlet molecular oxygen are investigated using density functional theory and polarized continuum models. Additions to both the 3- and 4-position of the lipid model (corresponding to the 9(13)- and 10(12)-positions of 9,12 lipid dienes such as linoleic acid) are explored. It is concluded that the modes of attack will lead to adduct intermediates which evolve either to dioxetane formation overcoming a significant energy barrier, or to the final LOOH products (hydro-peroxide bonded to either the 3- or 4-position), for which no transition barriers towards H-abstraction could be located. The computed energy surfaces are in close accord with results for the reactions between singlet oxygen and other unsaturated systems, and explain both the observed difference in product distribution in biological samples and, through the high energy barriers to addition of the initial reactants (15–20 kcal/mol), the low reactivity of singlet oxygen in biological membranes.
Fig. 1. Schematic drawings of linoleic acid and its main lipid peroxidation degradation products MDA and 4HNE, and the lipid model NDE used in the current work.
Fig. 2. Optimized structures of: (a) lipid model NDE; (b) closed shell adduct complex. Distances are given in Ångström.
Fig. 3. Optimized transition state and intermediate for singlet oxygen addition to C3 of NDE (3a and 3b open shell; 3c and 3d closed shell solutions), and (3e) transition state for formation of dioxetane. Distances are given in Ångström.
Fig. 4. Optimized transition state and intermediate for singlet oxygen addition to C4 of NDE (4a and 4b open shell; 4c and 4d closed shell solutions), and (3e) transition state for formation of dioxetane. Distances are given in Ångström.
Fig. 5. Optimized product structures: (a) dioxetane, (b) C3–OOH, (c) C4–OOH. Distances are given in Ångström.
Fig. 6. Energy diagram for singlet oxygen addition to NDE. Solid lines: addition to C3; dashed lines: addition to C4. Top lines at TS1: open shell solutions (3a, 4a); bottom lines at TS1: closed shell solutions (3c, 4c).

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