Direct electrospray tandem mass spectrometry of the unstable hydroperoxy bishemiacetal product derived from cholesterol ozonolysis

Cholesterol is the most abundant neutral lipid in the epithelial lining fluid of the lower airways of the lung also known as pulmonary surfactant and a potential target for reaction with ambient ozone when inspired into the human lung. The isolated double bond of cholesterol has been shown to be susceptible to attack by ozone, but the major reaction product from cholesterol ozonolysis had been remarkably difficult to structurally characterize. Recently, NMR and X-ray crystallography have been used to suggest the formation of a hydroperoxy, hydroxy hemiacetal product, using various derivatives and models of cholesterol to stabilize this chemically reactive product. Electrospray ionization mass spectrometry was used to study the somewhat unstable ozonolysis product of cholesterol which was found to display unique ionization and fragmentation properties when collisionally activated. The electron-deficient carbon atoms of this highly oxygenated product permitted covalent attachment of an acetate anion during negative ion electrospray ionization, leading to the formation of abundant adduct ions at m/z 511. Surprisingly, positive ions were not readily formed. Collision induced dissociation of the adduct anion yielded a major ion at m/z 477, corresponding to the loss of hydrogen peroxide. The most abundant fragment ion following collisional activation was observed at m/z 93, resulting from a complex rearrangement subsequent to the attack of the hydroperoxide anion on the carbon center of the acetate adduct. Based on the interpretation of the tandem mass spectral data, the major cholesterol ozonization product was characterized as a hydroperoxy, hydroxy hemiacetal derivative, which was consistent with the NMR and X-ray crystallographic studies which were carried out on the more stable methyl ether derivative.