Search

The coronavirus 2019 (COVID-19) outbreak in March led Davidson College to move from face-to-face classes and laboratories to mostly synchronous Zoom meetings. Prior to COVID-19, the majority of our faculty and students had little experience with remote instruction. With only 5 days to develop a plan, we revisited our individual and department learning goals and worked collectively to help each other redesign and redeploy our courses. In this reflective piece, we provide examples of how each member of our department collaborated with our students to ensure a relatively smooth transition to remote teaching across our entire curriculum while maintaining inclusive excellence. Specific strategies for adapting class meetings, assignments, assessments, additional support, and labs are provided along with select examples. Common themes across the curriculum included increased flexibility, the desire to maintain community, and the need for additional academic, technical, and emotional support. We hope our reflections will be helpful to our chemistry colleagues as we move into the uncertainty of the fall semester.

The synthesis of complex, biologically active molecules by catalyst-controlled, selective functionalization of complex molecules is an emerging capability. We describe the application of Rh-catalyzed conjugate arylation to the modification of thiostrepton, a complex molecule with potent antibacterial properties for which few analogues are known. By this approach, we achieve the site- and stereoselective functionalization of one subterminal dehydroalanine residue (Dha16) present in thiostrepton. The broad scope of this method enabled the preparation and isolation of 24 new analogues of thiostrepton, the biological testing of which revealed that the antimicrobial activity of thiostrepton tolerates the alteration of Dha16 to a range of amino acids. Further analysis of this Rh-catalyzed process revealed that use of sodium or potassium salts was crucial for achieving high stereoselectivity. The catalyst system was studied further by application to the synthesis of amino esters and amides from dehydroalanine monomers, a process which was found to occur with up to 93:7 er under conditions milder than those previously reported for analogous reactions. Furthermore, the addition of the same sodium and potassium salts as applied in the case of thiostrepton leads to a nearly full reversal of the enantioselectivity of the reaction. As such, this study of site-selective catalysis in a complex molecular setting also delivered synergistic insights in the arena of enantioselective catalysis. In addition, these studies greatly expand the number of known thiostrepton analogues obtained by any method and reveal a high level of functional group tolerance for metal-catalyzed, site-selective modifications of highly complex natural products.

Something like the real thing Artificial metalloenzymes ideally combine the favorable properties of natural enzymes with the high efficiency of synthetic catalysts. Inserting new metal groups into existing native proteins, however, often leads to poorer overall catalytic efficiency. To break through this limitation, Dydio et al. replaced the iron in the heme group of cytochrome P450 with iridium and subjected it to directed evolution. The enzyme catalyzed a range of reactions with kinetics similar to those of the native enzyme. It was also able to functionalize fully unactivated C-H bonds, a reaction that previously has only been mediated by synthetic catalysts. Moreover, the artificial enzyme was stable across temperatures and scales that are used industrially. Science , this issue p. 102

Cytochrome P450 enzymes have been engineered to catalyze abiological C-H bond amination reactions, but the yields of these reactions have been limited by low chemoselectivity for the amination of C-H bonds over competing reduction of the azide substrate to a sulfonamide. Here we report that P450s derived from a thermophilic organism and containing an iridium porphyrin cofactor (Ir(Me)-PIX) in place of the heme catalyze enantioselective intramolecular C-H bond amination reactions of sulfonyl azides. These reactions occur with chemoselectivity for insertion of the nitrene units into C-H bonds over reduction of the azides to the sulfonamides that is higher and with substrate scope that is broader than those of enzymes containing iron porphyrins. The products from C-H amination are formed in up to 98% yield and ∼300 TON. In one case, the enantiomeric excess reaches 95:5 er, and the reactions can occur with divergent site selectivity. The chemoselectivity for C-H bond amination is greater than 20:1 in all cases. Variants of the Ir(Me)-PIX CYP119 displaying these properties were identified rapidly by evaluating CYP119 mutants containing Ir(Me)-PIX in cell lysates, rather than as purified enzymes. This study sets the stage to discover suitable enzymes to catalyze challenging C-H amination reactions.

Replacing the iron atom in Fe-porphyrin IX proteins with a noble-metal atom enables the creation of enzymes that catalyse reactions not catalysed by native Fe-enzymes or other metalloenzymes; this approach could be used to generate other artificial enzymes that could catalyse a wide range of abiological transformations. Naturally occurring metalloenzymes are promising alternatives to transition-metal catalysts and non-metal enzymes for the synthesis of chemicals and biologically active compounds, but they catalyse only a narrow range of reactions. One way of broadening that range is to replace the native catalytic metal with an abiological metal. John Hartwig and colleagues report the effect of substituting the iron atom in Fe-porphyrin IX (Fe-PIX) proteins. Myoglobin variants containing an Ir(Me) site catalyse the functionalization of C–H bonds to form C–C bonds and add carbenes to β-substituted vinylarenes and unactivated aliphatic α-olefins. Directed evolution of the Ir(Me)-myoglobin generates mutants that form either enantiomer of the products of C–H insertion and catalyse the enantio- and diastereoselective cyclopropanation of unactivated olefins. The rich chemistry of free metalloporphyrins and the ease of preparation and evolution of substituted haem proteins using the methods adopted here open the way to the creation of many artificial metalloenzymes. Enzymes that contain metal ions—that is, metalloenzymes—possess the reactivity of a transition metal centre and the potential of molecular evolution to modulate the reactivity and substrate-selectivity of the system1. By exploiting substrate promiscuity and protein engineering, the scope of reactions catalysed by native metalloenzymes has been expanded recently to include abiological transformations2,3. However, this strategy is limited by the inherent reactivity of metal centres in native metalloenzymes. To overcome this limitation, artificial metalloproteins have been created by incorporating complete, noble-metal complexes within proteins lacking native metal sites1,4,5. The interactions of the substrate with the protein in these systems are, however, distinct from those with the native protein because the metal complex occupies the substrate binding site. At the intersection of these approaches lies a third strategy, in which the native metal of a metalloenzyme is replaced with an abiological metal with reactivity different from that of the metal in a native protein6,7,8. This strategy could create artificial enzymes for abiological catalysis within the natural substrate binding site of an enzyme that can be subjected to directed evolution. Here we report the formal replacement of iron in Fe-porphyrin IX (Fe-PIX) proteins with abiological, noble metals to create enzymes that catalyse reactions not catalysed by native Fe-enzymes or other metalloenzymes9,10. In particular, we prepared modified myoglobins containing an Ir(Me) site that catalyse the functionalization of C–H bonds to form C–C bonds by carbene insertion and add carbenes to both β-substituted vinylarenes and unactivated aliphatic α-olefins. We conducted directed evolution of the Ir(Me)-myoglobin and generated mutants that form either enantiomer of the products of C–H insertion and catalyse the enantio- and diastereoselective cyclopropanation of unactivated olefins. The presented method of preparing artificial haem proteins containing abiological metal porphyrins sets the stage for the generation of artificial enzymes from innumerable combinations of PIX-protein scaffolds and unnatural metal cofactors to catalyse a wide range of abiological transformations.

Organotransition metal complexes catalyze important synthetic transformations, and the development of these systems has rested on the detailed understanding of the structures and elementary reactions of discrete organometallic complexes bound to organic ligands. One strategy for the creation of new organometallic systems is to exploit the intricate and highly structured ligands found in natural metalloproteins. We report the preparation and characterization of discrete rhodium and iridium fragments bound site-specifically in a κ(2)-fashion to the protein carbonic anhydrase as a ligand. The reactions of apo human carbonic anhydrase with [Rh(nbd)2]BF4 or [M(CO)2(acac)] (M=Rh, Ir) form proteins containing Rh or Ir with organometallic ligands. A colorimetric assay was developed to quantify rapidly the metal occupancy at the native metal-binding site, and (15)N-(1)H NMR spectroscopy was used to establish the amino acids to which the metal is bound. IR spectroscopy and EXAFS revealed the presence and number of carbonyl ligands and the number total ligands, while UV-vis spectroscopy provided a signature to readily identify species that had been fully characterized. Exploiting these methods, we observed fundamental stoichiometric reactions of the artificial organometallic site of this protein, including reactions that simultaneously form and cleave metal-carbon bonds. The preparation and reactivity of these artificial organometallic proteins demonstrate the potential to study a new genre of organometallic complexes for which the rates and outcomes of organometallic reactions can be controlled by genetic manipulation of the protein scaffold.