MASS SPECTROMETRY FOR CHEMICAL REACTIONS: SYNTHESIS, ANALYSIS, AND APPLICATIONS
Creators: name: Huang, Kai-Hung
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 340199
subject: Analytical chemistry not elsewhere classified
classificationCode: 340599
subject: Organic chemistry not elsewhere classified
classificationCode: 340499
subject: Medicinal and biomolecular chemistry not elsewhere classified

description: Mass spectrometry (MS) has long been recognized as a technology for bioanalysis. However, this thesis focuses on exploiting mass spectrometry for chemical reactions. The work described here covers the (a) investigation of chemistry at interfaces by MS, (b) utilization of MS to accelerate drug discovery processes, and (c) applications of MS techniques for organic synthesis. MS techniques are used to scrutinize the distinctive chemistry and super acidity mechanisms at the gas/liquid interfaces by reacting carbon dioxide (gas phase) with amines (solution, in droplets). The intriguing trace water effect in creating this unique environment at the interfaces is described. A systematic survey of reactions promoted by glass microspheres at liquid/solid interfaces is conducted, revealing that glass surface can act as strong base to speed up reactions. Additionally, the ability of glass surface to degrade biomolecules is revealed, which has implications for bioanalysis. Desorption electrospray ionization (DESI), an ambient ionization method, can be used as a rapid analytical technique for the direct analysis of complex reaction mixtures or bioassays without sample workup. Moreover, DESI can also be used as a small-scale synthetic tool due to accelerated reactions in generated microdroplets. These characteristics make DESI a core technology for high-throughput (HT) experimentation that prioritizes speed to achieve three major roles. <b>(i) HT reaction screening</b> leverages the reaction acceleration phenomenon for rapid chemical space exploration, especially for the late-stage diversification of drug molecules. The entire process, from sampling the reaction mixture by droplets to on-the-fly chemical transformation during millisecond timescales to analysis by MS, achieves an overall throughput of one reaction per second in an integrated fashion. Diverse chemical transformations for various functional groups were achieved, with over 10<sup>4</sup> reactions explored and over 10<sup>3</sup> analogs identified within three hours. <b>(ii) HT synthesis</b> is achieved using an automated homebuilt array-to-array transfer system. The synthetic system uses DESI microdroplets for transferring reaction mixtures from a precursor array to products on a product array. High conversions of diverse reactions with synthetic throughput of 0.2-0.02 Hz and scale of ng-µg (pmole-nmole) in a spatially resolved manner are demonstrated. Hundreds of modified bioactive molecules are generated in an array format, and the spatial distribution of the products is visualized by mass spectrometry imaging. <b>(iii) HT bioassays</b> are demonstrated by combining the label-free nature of MS with the high-speed analysis of DESI. The contactless feature, with high tolerance towards complex mixtures, allows direct bioassays with minimal sample preparation. An opioid receptor binding assay is described with an evaluation of the binding affinity of synthesized opioid analogs. An on-surface enzymatic assay is developed for measuring the bioactivity of deposited molecules <i>in situ</i>. The consolidation of (i) HT reaction screening, (ii) HT synthesis, and (iii) HT bioassays by a single but versatile technique, HT-DESI, can expedite the early drug discovery process. For applications, MS technologies are utilized to probe reactive intermediates and the reaction mechanisms of palladium-catalyzed coupling reactions. MS is also used to explore chemical reactions for natural products, rapidly generating analogs for bioactivity evaluation and benefiting bioanalysis through the discovery of derivatization reactions. HT tandem MS is demonstrated to be powerful for structural elucidation and reaction site identification.