Mechanisms of Radical-Initiated Alkylation of Alkenes, Peptides and Proteins - A Green Chemistry Route

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In this project, funded by the Chemical Structure, Dynamic & Mechanism B Program of the Chemistry Division, Professor Lisa Kelly of the Department of Chemistry at the University of Maryland, Baltimore County (UMBC) is investigating the light-induced production and fate of reactive free radicals using new precursors which absorb light at long wavelength. Radicals are a versatile class of reactive chemical species that are useful to carry out carbon-carbon bond formation without the requirement of toxic reagents or harsh reaction conditions. In polymer chemistry, radicals initiate formation of the polymer. In biochemistry, radicals can be used to label proteins in order to identify the sites of weak intermolecular interactions. Materials science makes use of radicals to attach specific molecules onto surfaces. This research uses a class of compounds called naphthalene diimides that undergo the light-induced release of carbon dioxide (CO2) to produce reactive radicals. The chemistry is carried out with light in aqueous solution and does not require other harmful additives. Because this chemistry produces only CO2 as a byproduct, it represents a green route to carry out otherwise difficult chemical transformations. Integrated with her research activities, Professor Kelly develops curriculum innovations to teach entrepreneurial chemists and chemical engineers how to make field-portable computerized sensor platforms that monitor photochemical reactions in the environment. Through these advanced laboratory projects, chemistry students build electronic circuits and develop basic computer programming skills that might serve as a technology basis for future start-up ventures. Professor Kelly is an active participant in UMBC's diversity-advancement initiatives. These activities include mentorship for chemistry and biochemistry students who are members of underrepresented minorities by sharing her experiences and facilitating networking opportunities with graduate alumni in government, industry and academia. Photochemical transformations are attractive because they do not require heat and can be initiated in specific locations "on demand." Using pulsed laser spectroscopies, this research identifies and maps the mechanism of photoinduced electron transfer (PET)-induced decarboxylation. The chromophores, naphthalene diimides, absorb very strongly in the long-wavelength UV region of the spectrum, requiring significantly lower concentrations than other radical-producing compounds. A novel set of the naphthalene diimides are synthesized and the rate constants for radical production and decay are measured. Specific aims of this work are to first understand the kinetics and quantum efficiency of radical production, then investigate viable reactivity targets that are relevant to protein labeling and polymer formation. Applications in photoaffinity labeling and deactivation of specific enzymes are also being tested. While the products from photodecarboxylation have been characterized in a number of peptide and alkyl carboxylates, there has been little direct evidence for the involvement of carbon-centered radicals. Photoinduced decarboxylation of naphthalene diimides is expected to have broad utility in environmentally friendly synthetic organic chemistry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.