Analytical applications of desorption chemical ionization tandem mass spectrometry
Abstract
Analytical techniques of desorption chemical ionization tandem mass spectrometry (DCI-MS/MS) are applied to the study of in vitro chemically modified DNA, rapid screening of natural products for the anti-cancer drug taxol, and the identification and characterization of high mass fullerenes. Quantitation of $N\sp3$-methylthymidine, $O\sp4$-methylthymidine, $O\sp6$-methyldeoxy-guanosine and $N\sp7$-methyldeoxyguanosine generated from the reaction of calf thymus DNA with methyl methanesulfonate (MeMS) and 1-methyl-1-nitrosourea (MeNU) by mass spectrometry is reported. Quantitative precision of 10% or better is achieved on samples of 10$\sp{-12}$-10$\sp{-13}$ moles in the presence of a complex biological matrix. The methodology utilizes enzymatic degradation, reverse phase high performance liquid chromatography, and a final stage of separation and quantification by tandem mass spectrometry using desorption chemical ionization. Positive and negative DCI-MS/MS is used to identify and approximately quantitate taxol, an extremely promising anti-cancer drug, in plant extracts so as to direct isolation efforts towards the most appropriate samples. Searches for taxol analogs which contain the biologically active side chain are made using selected parent and neutral loss scans. Using this methodology taxol has been identified in plant cell tissue cultures at the ppm level and taxol and several analogs have been identified in crude extracts from T. baccata and T. brevifolia bark. Positive and negative DCI-MS/MS is used to identify high mass carbon clusters, called fullerenes, from graphitic soot. Cluster up mass 2200 Da., C$\sb{200}$, have been observed. Tandem mass spectrometry is used to characterize the well known C$\sb{60}$ cluster, buckminsterfullerene, using collisionally activated dissociation (CAD) and charge exchange reactions. Mass spectra of positive and negative ions derived from the fullerenes C$\sb{60}$ and C$\sb{70}$ show persistent satellite peaks. They are assigned as adducts of C$\sb{60}$ and C$\sb{70}$ containing CH$\sb2$, CH$\sb3$, O, and OH. CAD of the (M + 17)$\sp{-1}$ and (M + 16)$\sp{-1}$ (M = C$\sb{60}$, C$\sb{70}$) adducts yielded the respective ions of C$\sb{60}$ and C$\sb{70}$. Irradiation, using a mercury arc lamp, of mixtures of C$\sb{60}$ and C$\sb{70}$ increased the yields of the C$\sb{60}$ + O and C$\sb{70}$ + O adducts. In addition, products that corresponded to (C$\sb{60}$O + (CH$\sb2)\sb{\rm n}$) where n = 1,2,3,4,5,6 and (C$\sb{70}$O + (CH$\sb2)\sb{\rm n})$ where n = 1,2 occurred on irradiation of solutions of soot extracts. These compounds were absent upon irradiation of fullerene mixtures that were thoroughly washed with ether. Irradiation of the ether wash containing small amounts of C$\sb{60}$ and C$\sb{70}$ and the monoxides gave increased yields of C$\sb{60}$O and C$\sb{70}$O as well as C$\sb{60}$O$\sb{\rm n}$, where n = 2,3,4,5 and C$\sb{70}$O$\sb{\rm n}$, where n = 2. These series of adducts are interpreted as products resulting from sequential cyclopropanations and epoxidations.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry|Pharmacology
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