Studying Interfacial Reactions of Cholesterol Sulfate in an Unsaturated Phosphatidylglycerol Layer with Ozone Using Field Induced Droplet Ionization Mass Spectrometry

Field-induced droplet ionization (FIDI) is a recently developed ionization technique that can transfer ions from the surface of microliter droplets to the gas phase intact. The air-liquid interfacial reactions of cholesterol sulfate (CholSO 4 ) in a 1-palmitoyl-2-oleoyl- sn -phosphatidylglycerol (POPG) surfactant layer with ozone (O 3 ) are investigated using field-induced droplet ionization mass spectrometry (FIDI-MS). Time-resolved studies of interfacial ozonolysis of CholSO 4 reveal that water plays an important role in forming oxygenated products. An epoxide derivative is observed as a major product of CholSO 4 oxidation in the FIDI-MS spectrum after exposure of the droplet to O 3 for 5 s. The abundance of the epoxide product then decreases with continued O 3 exposure as the finite number of water molecules at the air-liquid interface becomes exhausted. Competitive oxidation of CholSO 4 and POPG is observed when they are present together in a lipid surfactant layer at the air-liquid interface. Competitive reactions of CholSO 4 and POPG with O 3 suggest that CholSO 4 is present with POPG as a well-mixed interfacial layer. Compared with CholSO 4 and POPG alone, the overall ozonolysis rates of both CholSO 4 and POPG are reduced in a mixed layer, suggesting the double bonds of both molecules are shielded by additional hydrocarbons from one another. Molecular dynamics simulations of a monolayer comprising POPG and CholSO 4 correlate well with experimental observations and provide a detailed picture of the interactions between CholSO 4 , lipids, and water molecules in the interfacial region.