Studying interfacial reactions of cholesterol sulfate in an unsaturated phosphatidylglycerol layer with ozone using field induced droplet ionization mass spectrometry

摘要

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.