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

Abstract

Field-induced droplet ionization (FIDI) is a recently developed ionization technique that can transferions 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.

Cholesterol provides nonsacrificial protection of membrane lipids from chemical damage at air–water interface

“…For the reactions initiated by HCl, molecular dynamics simulations show that the fatty acid ester linkages are submerged in the aqueous portion of the air-water interface (47). We propose that HCl dissociates to lower the interfacial pH and effect acidcatalyzed hydrolysis of the ester linkages.…”

Going beyond electrospray: mass spectrometric studies of chemical reactions in and on liquids

“…Subsequent studies examined the interfacial reactivity of surfactant proteins, 96 pulmonary phospholipids, 97 and cholesterol sulfate. 98 In each case, the dynamics of interfacial ozonolysis during droplet exposure were different than in bulk solution, suggesting the importance of a unique environment at the liquid surface.…”

New mass spectrometry techniques for studying physical chemistry of atmospheric heterogeneous processes

“…With extensive ozonolysis, the unsaturated phospholipids formed more hydrophilic products and transitioned into the droplet, and only saturated phospholipids remained on the surface. Recently, Ko et al applied FIDI-MS to studying interfacial reactions of cholesterol sulphate in a POPG layer with ozone [181]. They observed an epoxide of cholesterol sulphate as the major oxidation product.…”