Highly Efficient Cyclic Dinucleotide Based Artificial Metalloribozymes for Enantioselective Friedel-Crafts Reactions in Water

The diverse secondary structures of nucleic acids are emerging as attractive chiral scaffolds to construct artificial metalloenzymes (ArMs) for enantioselective catalysis. DNA-based ArMs containing duplex and G-quadruplex scaffolds have been widely investigated, yet RNA-based ArMs are scarce. Here we report that a cyclic dinucleotide of c-di-AMP and Cu²⁺ ions assemble into an artificial metalloribozyme (c-di-AMP⋅Cu²⁺ ) that enables catalysis of enantioselective Friedel-Crafts reactions in aqueous media with high reactivity and excellent enantioselectivity of up to 97 % ee. The assembly of c-di-AMP⋅Cu²⁺ gives rise to a 20-fold rate acceleration compared to Cu²⁺ ions. Based on various biophysical techniques and density function theory (DFT) calculations, a fine coordination structure of c-di-AMP⋅Cu²⁺ metalloribozyme is suggested in which two c-di-AMP form a dimer scaffold and the Cu²⁺ ion is located in the center of an adenine-adenine plane through binding to two N7 nitrogen atoms and one phosphate oxygen atom.

Bioorthogonal Metalloporphyrin-Catalyzed Selective Methionine Alkylation in the Lanthipeptide Nisin

Bioorthogonal catalytic modification of ribosomally synthesized and post-translationally modified peptides (RiPPs) is a promising approach to obtaining novel antimicrobial peptides with improved properties and/or activities. Here, we present the serendipitous discovery of a selective and rapid method for the alkylation of methionines in the lanthipeptide nisin. Using carbenes, formed from water-soluble metalloporphyrins and diazoacetates, methionines are alkylated to obtain sulfonium ions. The formed sulfonium ions are stable, but can be further reacted to obtain functionalized methionine analogues, expanding the toolbox of chemical posttranslational modification even further.

Fluorescence Spectroscopic Insight into the Supramolecular Interactions in DNA-Based Enantioselective Sulfoxidation

Interactions of copper(II)-bipyridine cofactors and thioanisole substrate with human telomeric G-quadruplex DNA were studied by UV/Vis absorption, circular dichroism, and fluorescence quenching titration. Three copper(II)-bipyridine complexes are equivalently anchored to the G-quadruplex scaffold at all five fluorescently labeled sites. Thioanisole interacts with the DNA architecture at both the second loop and 3' terminus in the absence or presence of copper(II)-bipyridine complexes. These nonspecificities in the weak interactions of Cu^II complexes and thioanisole with G-quadruplex might explain why DNA only affords a modest enantioselectivity in the oxidation of thioanisole. These findings provide insights toward the construction of highly enantioselective DNA-based catalysts.

Myoglobin-Catalyzed C-H Functionalization of Unprotected Indoles

Functionalized indoles are recurrent motifs in bioactive natural products and pharmaceuticals. While transition metal-catalyzed carbene transfer has provided an attractive route to afford C3-functionalized indoles, these protocols are viable only in the presence of N-protected indoles, owing to competition from the more facile N-H insertion reaction. Herein, a biocatalytic strategy for enabling the direct C-H functionalization of unprotected indoles is reported. Engineered variants of myoglobin provide efficient biocatalysts for this reaction, which has no precedents in the biological world, enabling the transformation of a broad range of indoles in the presence of ethyl α-diazoacetate to give the corresponding C3-functionalized derivatives in high conversion yields and excellent chemoselectivity. This strategy could be exploited to develop a concise chemoenzymatic route to afford the nonsteroidal anti-inflammatory drug indomethacin.

Metal Substitution Modulates the Reactivity and Extends the Reaction Scope of Myoglobin Carbene Transfer Catalysts

Engineered myoglobins have recently emerged as promising scaffolds for catalyzing carbene-mediated transformations. In this work, we investigated the effect of altering the metal center and its first-sphere coordination environment on the carbene transfer reactivity of myoglobin. To this end, we first established an efficient protocol for the recombinant expression of myoglobin variants incorporating metalloporphyrins with non-native metals, including second- and third-row transition metals (ruthenium, rhodium, iridium). Characterization of the cofactor-substituted myoglobin variants across three different carbene transfer reactions (cyclopropanation, N-H insertion, S-H insertion) revealed a major influence of the nature of metal center, its oxidation state and first-sphere coordination environment on the catalytic activity, stereoselectivity, and/or oxygen tolerance of these artificial metalloenzymes. In addition, myoglobin variants incorporating manganese- or cobalt-porphyrins were found capable of catalyzing an intermolecular carbene C-H insertion reaction involving phthalan and ethyl α-diazoacetate, a reaction not supported by iron-based myoglobins and previously accessed only using iridium-based (bio)catalysts. These studies demonstrate how modification of the metalloporphyrin cofactor environment provides a viable and promising strategy to enhance the catalytic properties and extend the reaction scope of myoglobin-based carbene transfer catalysts.

Enantioselective Catalysis by Using Short, Structurally Defined DNA Hairpins as Scaffold for Hybrid Catalysts

A new type of DNA metal complex hybrid catalyst, which is based on single-stranded DNA oligonucleotides, is described. It was shown that oligonucleotides as short as 14 nucleotides that fold into hairpin structures are suitable as nucleic acid components for DNA hybrid catalysts. With these catalysts, excellent enantioinduction in asymmetric Diels-Alder reactions with selectivity values as high as 96 % enantiomeric excess (ee) can be achieved. Molecular dynamics simulations indicate that a rather flexible loop combined with a rigid stem region provides DNA scaffolds with these high selectivity values.