Boron NMR as a Method to Screen Natural Product Libraries for B-Containing Compounds

Boron catalysis in a designer enzyme

Enzymes play an increasingly important role in improving the benignity and efficiency of chemical production, yet the diversity of their applications lags heavily behind chemical catalysts as a result of the relatively narrow range of reaction mechanisms of enzymes. The creation of enzymes containing non-biological functionalities facilitates reaction mechanisms outside nature's canon and paves the way towards fully programmable biocatalysis1-3. Here we present a completely genetically encoded boronic-acid-containing designer enzyme with organocatalytic reactivity not achievable with natural or engineered biocatalysts4,5. This boron enzyme catalyses the kinetic resolution of hydroxyketones by oxime formation, in which crucial interactions with the protein scaffold assist in the catalysis. A directed evolution campaign led to a variant with natural-enzyme-like enantioselectivities for several different substrates. The unique activation mode of the boron enzyme was confirmed using X-ray crystallography, high-resolution mass spectrometry (HRMS) and 11B NMR spectroscopy. Our study demonstrates that genetic-code expansion can be used to create evolvable enantioselective enzymes that rely on xenobiotic catalytic moieties such as boronic acids and access reaction mechanisms not reachable through catalytic promiscuity of natural or engineered enzymes.

Discovery of Streptomyces species CS-62, a novel producer of the Acinetobacter baumannii selective antibiotic factumycin

Narrow-spectrum antibiotics are of great interest given their ability to spare the microbiome and decrease widespread antibiotic resistance compared to broad-spectrum antibiotics. Herein, we screened an in-house library of Actinobacteria strains for selective activity against Acinetobacter baumannii and successfully identified Streptomyces sp. CS-62 as a producer of a natural product with this valuable activity. Analysis of the cultures via high-resolution mass spectrometry and tandem mass spectrometry, followed by comparison with molecules in the Natural Product Atlas and the Global Natural Products Social Molecular Networking platform, suggested a novel natural product. Genome mining analysis initially supported the production of a novel kirromycin derivative. Isolation and structure elucidation via mass spectrometry and Nuclear Magnetic Resonance (NMR) analyses revealed that the active natural product was the known natural product factumycin, exposing omissions and errors in the consulted databases. While public databases are generally very useful for avoiding rediscovery of known molecules, rediscovery remains a problem due to public databases either being incomplete or having errors that result in failed dereplication. Overall, the work describes the ongoing problem of dereplication and the continued need for public database curation.

11B NMR of the Morphological Evolution of Traditional Chinese Medicine Borax

This article applies nuclear magnetic resonance technology to the study of boron-containing traditional Chinese medicine, in order to explore the morphological evolution of boron elements in traditional Chinese medicine. Borax is a traditional Chinese medicine with anti-corrosion, anti-inflammatory, antibacterial, and anticonvulsant effects. It is made by boiling, removing stones, and drying borax minerals like borate salts. This article introduces an 11B nuclear magnetic resonance method for identifying and characterizing boron-containing compounds in TCM. We applied this technology to borax aqueous solutions in different chemical environments and found that with boron mixed in the form of SP2 hybridization in equilateral triangles and SP3 hybridization in equilateral tetrahedra, the pH changes in alkaline environments significantly affected the ratio of the two. At the same time, it was found that in addition to the raw material peak, boron signals of other boron-containing compounds were also detected in 20 commercially available boron-containing TCM preparations. These new boron-containing compounds may be true pharmaceutical active ingredients, and adding them directly to the formula can improve quality and safety. This article describes the detection of 11B NMR in boron-containing traditional Chinese medicine preparations. It is simple, non-destructive, and can provide chemical fingerprint studies for boron-containing traditional Chinese medicine.

A Drimane Meroterpenoid Borate as a Synchronous Ca+ Oscillation Inhibitor from the Coral-Associated Fungus Alternaria sp. ZH-15

Territrem F (1), a drimane meroterpenoid bearing a unique borate ring system, was isolated together with its diol precursor territrem B (2) from the fungus Alternaria sp. ZH-15 associated with the soft coral Lobophytum crassum collected in the South China Sea. The structure of the new compound was elucidated by spectroscopic analysis and an X-ray single-crystal diffraction study, representing a new type of boron-containing natural product. Both compounds significantly inhibited spontaneous synchronous Ca²⁺ oscillations (SCOs) and epileptic discharges induced by 4-aminopyridine, showing the potential for antiepileptic drug research. The 5,9-boronic ester derivative of 2 did not change its SCO inhibitory activity.

11B and 1H-11B HMBC NMR as a Tool for Identification of a Boron-Containing Nucleoside Dimer

Boron-containing compounds are commonly used in synthetic chemistry and are known to play important roles in biology. Despite the widespread relevance of boronated compounds, there have been limited methods to discover, characterize, and study them. Here, we describe the use of 11B NMR, including 1H-11B HMBC, for the isolation and characterization of the boron-containing natural product diadenosine borate. Utilizing synthetic standards, we optimized coupling parameters for 1H-11B HMBC experiments to allow for the analysis of small quantities (∼1 mg) of boron-containing compounds. This work can facilitate the broader application of 11B NMR to the study of boron in a range of applications, from synthetic chemistry to the role of boron in naturally occurring systems.