Laser-Induced Oxidation of Cholesterol Observed During MALDI-TOF Mass Spectrometry |
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Authors: | Kevin M McAvey Bing Guan Chanel A Fortier Matthew A Tarr Richard B Cole |
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Institution: | Department of Chemistry, University of New Orleans, 2000 Lakeshore Dr., New Orleans, LA 70148, USA. |
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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. |
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