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.
© 2011 American Society for Mass Spectrometry. Received: 29 August 2011; Revised: 3 October 2011; Accepted: 8 October 2011; Published online: 9 November 2011. The authors acknowledge financial supported for this work by Basic Science Research Program (to H.I.K.; grant no. 2010–0021508) and by WCU Program (to S.M.C. and Y.M. R.; grant no. R32-2008-000-10180-0) through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology. The authors also acknowledge financial support provided by the Beckman Institute Mass Spectrometry Resource Center and National Science Foundation of the United States under grant no. CHE-0416381 (to J.L.B., PI).
Supplemental Material - 13361_2011_275_MOESM1_ESM.doc