Location: Cotton Structure and Quality ResearchTitle: Laser-induced oxidation of cholesterol observed during MALDI-TOF mass spectrometry Author
Submitted to: Journal of American Society for Mass Spectrometry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2011
Publication Date: N/A
Citation: N/A Interpretive Summary: Cholesterol, found in biological membranes, have been widely studied due to the deleterious effects of their oxidation compounds. Cholesterol oxidation products (COPs) have been implicated in the formation in the onset of cancer. Matrix-Assisted Laser Desorption Ionization Time-of-Flight (MALDI-TOF) spectroscopy was successfully used to prove the mechanistic role of hydroxyl radicals in the oxidation of cholesterol. Support for this mechanism was determined through the use of free radical scavengers and optimized conditions necessary for the formation of three-odd electron cholesterol oxidation products. It was determined that the 2,5-dihyroxybenzoic acid (DHB), the matrix compound, was instrumental in the decomposition of cholesterol after being the target of laser irradiation. A dithranol matrix was also shown to be instrumental in the degradation of cholesterol but yielded lower intensities compared to DHB.
Technical Abstract: Conditions for the detection of three odd-electron cholesterol oxidation peaks were determined and these peaks were shown to be artifacts of the matrix-assisted laser desorption time of flight (MALDI-TOF) process. Matrix choice, solvent, laser intensity and cholesterol concentration were systematically varied to characterize the conditions leading to the highest signals of the radical cation peaks, and it was found that initial cholesterol solution concentration and resultant density of solid cholesterol on the MALDI target were important parameters in determining signal intensities. It is proposed that hydroxyl radicals, generated as a result of laser irradiation of the employed 2, 5-dihydroxybenzoic acid (DHB) matrix, initiate cholesterol oxidation on the MALDI target. An attempt to induce the odd-electron oxidation peaks by means of adding an oxidizing agent succeeded using an acetonitrile solution of DHB, cholesterol, and cumene hydroperoxide.Moreover, addition of free radical scavengers reduced the abundances of some oxidation products under certain conditions. These results are consistent with the mechanism of oxidation proposed herein involving laser-induced hydroxyl radical production followed by attack on neutral cholesterol. Hydroxyl radical production upon irradiation of dithranol matrix may also be responsible for generation of the same radical peaks observed from cholesterol in dithranol by an analogous mechanism.