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 (CholSO₄) in a 1-palmitoyl-2-oleoyl-sn-phosphatidylglycerol (POPG) surfactant layer with ozone (O₃) are investigated using field-induced droplet ionization mass spectrometry (FIDI-MS). Time-resolved studies of interfacial ozonolysis of CholSO₄ reveal that water plays an important role in forming oxygenated products. An epoxide derivative is observed as a major product of CholSO₄ oxidation in the FIDI-MS spectrum after exposure of the droplet to O₃ for 5 s. The abundance of the epoxide product then decreases with continued O₃ exposure as the finite number of water molecules at the air-liquid interface becomes exhausted. Competitive oxidation of CholSO₄ and POPG is observed when they are present together in a lipid surfactant layer at the air-liquid interface. Competitive reactions of CholSO₄ and POPG with O₃ suggest that CholSO₄ is present with POPG as a well-mixed interfacial layer. Compared with CholSO₄ and POPG alone, the overall ozonolysis rates of both CholSO₄ 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 CholSO₄ correlate well with experimental observations and provide a detailed picture of the interactions between CholSO₄, lipids, and water molecules in the interfacial region.