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Thermal decomposition of phospholipid secondary ozonides: Implications for the toxicity of inhaled ozone


While inhalation of ozone is known to cause a variety of health effects, the reactions at a molecular level that lead to these effects are not well understood. One potential path is the reaction of ozone with the unsaturated fatty acid components of pulmonary surfactant at the air-water interface in the lung to form secondary ozonides. These have been proposed to decompose to free radicals, which can then initiate the well-known inflammatory response. We report here the first kinetic studies of the thermal decomposition of the cis and trans secondary ozonides of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a phospholipid found in significant quantities in lung surfactant. The ozonides were synthesized by reaction of O3 with POPC adsorbed on a glass surface, and their thermal decomposition kinetics were followed using high-performance liquid chromatography (HPLC) over the temperature range from 50°C to 106°C in either methanol or 1,1,1,2-tetrachloroethane. The Arrhenius parameters for the thermal decomposition in methanol are A = 10(6.7±0.3) s-1 and E(a) = 19.6 ± 0.6 kcal mol-1 for the cis ozonide, and A = 10(8.7±0.6) s-1 and E(a) = 19.8 ± 1.0 kcal mol-1 for the trans ozonide. In 1,1,1,2-tetrachloroethane, the parameters are A = 10(8.3±2.1) s-1 and E(a) = 18.4 ± 3.4 kcal mol-1 for the cis ozonide, and A = 10(9.3±3.2) s-1 and E(a) = 20.2 ± 5.2 kcal mol-1 for the trans ozonide (all errors cited are ±2σ). Within experimental error, there is no difference in the kinetics of decomposition in the two solvents. However, both the activation energy and the preexponential factor for the decomposition of the phospholipid ozonides are significantly lower than those for decomposition of the long-chain alkene ozonide 1-octene ozonide, determined to be E(a) = 26.7 ± 3.2 kcal mol-1 and A = 10(12.7±1.9) s-1. The latter reaction has been proposed to be initiated by scission of the O-O bond, followed by decomposition of the peroxy biradical to generate free radicals. The kinetics for the decomposition of the POPC ozonides in solution are similar to those of simple alkene ozonides in the gas phase, where a concerted mechanism involving simultaneous intramolecular hydrogen transfer and O-O bond cleavage has been proposed. The only high-molecular-weight major product of the POPC ozonide decomposition identified using liquid secondary ion mass spectrometry (LSIMS) was the lipid acid 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine, which was observed as a product in both solvents. The mechanism and implications for the toxicology of inhaled ozone are discussed.

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