Gas-phase studies on the reactivity of charged, aromatic polyradicals by using distonic ion approach and Fourier tansform ion cyclotron resonance (FT-ICR) mass spectrometry, and development of LIAD/APCI and HPLC/APCI for ionization of hydrocarbons

Jinshan Gao, Purdue University

Abstract

Radicals play significant roles in biochemistry, plasma chemistry, polymerization, combustion, atmospheric chemistry, development of new organic materials, and many other chemical processes. Aromatic σ,σ-biradicals of the para-benzyne type formed from enediyne prodrugs, the most potent antitumor agents discovered thus far, are thought to abstract two hydrogen atoms from each strand of double stranded DNA, resulting in irreversible damage to tumor DNA. Unfortunately, the high toxicity of these agents hinders their clinical use. Hence, a better understanding of the chemical properties of the biradical intermediates is highly desirable. meta-Benzynes have substantially stronger coupling between the readical sites than para-benzynes, which reduces their radical reactivity. Tuning of meta-benzynes' reactivity may lead to better synthetic drugs with higher biological potency and less cytotoxicity than the enediynes. Hence, an investigation was carried out on the reactivities of nineteen meta-benzyne analogs to obtain information on the factors that control meta-benzynes' reactivity. The important factors were found to include the energy needed for the biradicals to reach the transition state geometry and the biradicals' electron affinity at the radical sites. It was discovered that the reactivity of meta-benzynes can be "tuned" from electrophilic to radical-like by changing the type and position of substituents, which is beneficial for the rational design of synthetic antitumor drugs. Almost all studies carried on tridehydroarenes and tetradehydroarenes are theoretical due to the difficulty in studying such highly reactive species experimentally. Hence, the chemical reactivities of substituted tridehydropyridinium and tetradehydropyridinium cations were examined experimentally. This study demonstrates that substitution strongly affects the reactivity of these organic tri- and tetraradicals. Their reactivities are vastly different from those of their unsubstituted analogs. Laser-induced acoustic desorption (LIAD), which enables the evaporation of nonvolatile and thermally labile compounds as intact neutral molecules into the gas phase, was successfully coupled with an APCI source in a commercial linear quadrupole ion trap mass spectrometer (LQIT). This makes it possible to decouple desorption and ionization processes, thus enabling the simultaneous characterization of both polar and nonpolar organic compounds at atmospheric pressure. Finally, hydrocarbons were employed as the reagents in an APCI source of a commercial LQIT, thus greatly expanding its analytical utility. Nonpolar, nonfunctionalized, saturated and unsaturated, linear, branched and cyclic hydrocarbons, as well as polyaromatic and heteroaromatic hydrocarbons, were successfully ionized. This method was coupled with HPLC by using hexane as the mobile phase.

Degree

Ph.D.

Advisors

Kenttamaa, Purdue University.

Subject Area

Analytical chemistry|Organic chemistry

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