Real-time monitoring of the kinetics and gas-phase products of the reaction of ozone with an unsaturated phospholipid at the air-water interface

While the kinetics and mechanisms of the reaction of O-3 With alkenes in th e gas and condensed phases are reasonably well understood, those with unsat urated organics in the intermediate regime at the air-water interface are n ot. Studies of the reaction of ozone at room temperature with the unsaturat ed phospholipid, 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) at the air-water interface, and, for comparison, the fully saturated dipalmit oyl-L-alpha -phosphatidylcholine (DPPC) were carried out. The phospholipids were exposed at varying surface areas per phosphocholine molecule on a wat er subphase to a flow of O-3 in air (0.25-1 ppm), and atmospheric pressure ionization mass spectrometry (API-MS) was used to monitor the formation of gaseous products in real time. Nonanal was detected as a major gas-phase pr oduct of the reaction of ozone with OPPC; no volatile products were observe d in the case of DPPC. The yield of nonanal, defined as the nonanal produce d per phosphocholine molecule reacted at the air-water interface, was 51 +/ - 13% (20) over this range of ozone concentrations, after correcting for th e solubility of nonanal in the subphase, The nonanal yield was also indepen dent of the available area per molecule over the range from 40 to 158 Angst rom (2) molccule(-1), 56 +/- 11% (2 sigma), at a constant O-3 concentration of 1 ppm. Cyclohexane was used as a scavenger for any OH generated in the reaction, but no evidence for gas-phase OH radical production in the OPPC-O -3 reaction was found using this technique. The time-dependence of the gene ration of nonanal shows that these reactions are enhanced kinetically at th e air-water interface compared to that expected for analogous gas-phase rea ctions. Molecular dynamics simulations of OPPC at the air-water interface s how that the insensitivity of the time dependence and yield of nonanal prod uction to the extent of film compression, as well as the kinetic enhancemen t, can be understood in terms of the structure of the fatty acid chains at the interface. These studies illustrate the utility of real-time monitoring of the gas-phase products of reactions at the air-water interface and the insights into kinetics and mechanisms which can be obtained by combining th ese experimental data with molecular dynamics simulations.