The Evolution, Application, and Discovery of Photoenzymatic Reactions

Abstract

Society needs more efficient and sustainable processes for manufacturing. This means chemists must develop methods which utilize renewable resources, eliminate waste, and avoid the use of toxic reagents. Biocatalysts are uniquely attractive for chemistry because they are derived from renewable biomass and are inherently biodegradable. Reactions involving enzymes, are operationally mild, highly selective, and can be incredibly efficient. Because enzymes are evolved to perform a particular reaction in nature, biocatalysts are thought to only catalyze their natural reactivity. As such, developing strategies to expand the reactivity patterns available to enzymes is an enduring challenge in catalysis. This work is focused on developing novel biocatalysts to address long-standing challenges in organic synthesis. Specifically, we have used a combination of enzymes and visible light excitation to generate, stabilize, and enantioselectively terminate highly-reactive radical intermediates. To develop these systems, we exploit the enzymes’ natural catalytic machinery as a scaffold for non-natural photoactivation modes. This approach is illustrated through our work on ‘ene’-reductase (ERED) based photoenzymes. EREDs are a class of oxidoreductase that naturally perform 1,4 reductions of α,β-unsaturated carbonyl compounds using a flavin cofactor. Our group has discovered that the flavin cofactor, is not limited to ionic chemistries as it can stabilize a radical semiquinone intermediate. This work has leveraged this latent radical intermediate as a scaffold for open-shell catalysis enabled by visible light excitation. Here we describe efforts in evolving, modeling, and applying these photoenzymatic systems. Further, we have looked to develop synergistic approaches using ruthenium-based photocatalysts to address the limitations of EREDs in oxidative chemistry.