This project will continue work on the development of catalyst screening methods that employ enzymes as the analytical sensors. This approach has been labeled In Situ Enzymatic Screening (ISES). While chromatographic screening methods are established, these are normally run as serial, time point assays, requiring aliquot removal and work-up, prior to analysis. The ISES method offers the possibility to obtain parallel, real time estimates of relative reaction rates and, in ideal cases, sense and magnitude of enantio-induction, without the need to install a chromophore or mass tag in the substrate. A key goal of the next cycle of this project is to enhance the throughput of the ISES screening, such that larger matrices of potential catalytic combinations may be screened. Also important will be to further develop the most interesting catalysts uncovered in the last cycle, including carbohydrate-based chiral salen ligands, and new combinations of late transition metal halides for formal halocarbocyclization processes. In addition, further expansion of the scope of chemistry that is amenable to screening via ISES will be investigated.
With this award, the Organic and Macromolecular Chemistry Program is supporting the research of Professor David B. Berkowitz in the Department of Chemistry at the University of Nebraska. Professor Berkowitz's research efforts include the development of useful new catalysts for the stereo-controlled opening of small ring compounds, and for carbon-halogen and carbon-carbon bond formation. Successful development of the methodology will have an impact on synthesis in both the academic and industrial communities, the latter particularly in the combinatorial catalysis and pharmaceutical process areas.
Under this NSF Award, the Berkowitz research group at the University of Nebraska has developed and applied new tools to assist synthetic chemists in their efforts to discover new chemistry and to efficiently identify catalysts that support that chemistry. Underlying the Intellectual Merit of this science is the basic premise that organic chemists can harness the power of enzymes as screening tools to facilitate the discovery of new chemistry of interest, and the identification of efficient catalysts to mediate these transformations. Because the "reporting enzymes" are present in the same vessel as the catalysts being screened, this approach is termed In Situ Enzymatic Screening (ISES). The ISES method offers the possibility to obtain real time estimates of catalyst performance without the need to install a chromophore or mass tag in the substrate. Under this award, three major advances in screening were achieved: (1) The throughput was increased by turning to a new set of reporting enzymes; namely an alcohol oxidase/peroxidase couple. This allowed over 1000 potential catalytic combinations to be screened in a parallel readout format (see image, for an example). (2) The first visual colorimetric example of the ISES method was reduced to practice. Namely, by using a peroxidase enzyme, the enzymatic reporting reaction could be linked to the production of green color, upon dye oxidation. This allows the experimentalist to spot successful catalytic combinations with the naked eye (see image). (3) New reporting enzymes were discovered, both from (i) an interesting mesophilic bacterium, Clostridium acetobutylicum, that itself grows in the presence of relatively high concentrations of organic solvents, in its "solventogenic phase" and from (ii) a hyperthermophilic archaeon, Sulfolobus solfataricus, an organism that grows at nearly boiling water termperatures, and whose enzymes have an unusual tolerance for elevated temperatures. Both the Clostridial and the Sulfolobus enzymes were shown to have significant potential, as a chiral catalysts for asymmetric synthesis, and also as useful new screening enzymes for ISES. These new ISES methods were exploited to discover both important new ligands for existing chemistry and fundamentally new reactivity. Thus, on the one hand, promising new types of chiral "salen" ligand were identified that are able to provide for a strong chiral bias in asymmetric synthesis. On the other, a "halometalation-carbocyclization" reaction was uncovered with both rhodium and palladium catalysts that allows for rapid access to natural product core structures of the sesquiterpene lactone variety. These methodology and screening advances are expected to be useful for both the academic and industrial communities, the latter particularly in the combinatorial catalysis and pharmaceutical process areas. This is probably the most important scientific Broader Impact of the project. The project also provided professional training at the interface of chemical synthesis and enzymology for a cadre of postdoctoral fellows, graduate students and undergraduates. Postdoctoral alumni from this project have moved on to both positions in academia and industry, one Ph.D. graduate is now employed in the pharmaceutical industry, and one M.S. student is pursuing a career in chemical education. One undergraduate working on this project earned a prestigious American Chemical Society SURF (Summer Undergraduate Research Fellow) Fellowship, and is currently pursuing his Ph.D. in Chemistry. Another is currently in medical school. Additionally, the Berkowitz group annually has participated in the Chemistry Day outreach activity at the University of Nebraska to showcase opportunities for research/careers in chemistry to high school students and their families from the Great Plains region.
