LongLarf

Science, Volume 381, Issue 6665, Page 1474-1479, September 2023.

Identification of combinatorial cyclic peptides: Complex dynamic combinatorial libraries of cyclic peptides can be generated by thiol-to-thioester exchanges. Structural tools were developed onto the peptidic scaffold to facilitate the identification of a hit by chromatography and mass spectrometry. A chemically inert UV-Vis probe was introduced as well as an ornithine residue for site-selective ring-opening in MS/MS.

Abstract

Target-directed dynamic combinatorial chemistry is a very attractive strategy for the discovery of bioactive peptides. However, its application has not yet been demonstrated, presumably due to analytical challenges that arise from the diversity of a peptide library with combinatorial side-chains. We previously reported an efficient method to generate, under biocompatible conditions, large dynamic libraries of cyclic peptides grafted with amino acid's side-chains, by thiol-to-thioester exchanges. In this work, we present analytical tools to easily characterize such libraries by HPLC and mass spectrometry, and in particular to simplify the isomers’ distinction requiring sequencing by MS/MS fragmentations. After structural optimization, the cyclic scaffold exhibits a UV-tag, absorbing at 415 nm, and an ornithine residue which favors the regioselective ring-opening and simultaneous MS/MS fragmentation, in the gas-phase.

Abstract

Vanillin is a popular and versatile flavor compound, almost entirely produced from petroleum-derived phenol by a multi-step chemical synthesis. The process is hazardous to the environment and unsustainable for its fossil oil usage. Therefore, developing environmentally friendly, efficient, and sustainable routes to biobased vanillin is essential. Here, we report on vanillin production from 4-n-propylguaiacol (4PG), one of the main components in lignin oil obtained through reductive catalytic fractionation (RCF) of soft wood, by employing recombinant Escherichia coli cells. Conversion is based on the expression of two engineered oxidative enzymes: a 4-n-propylguaiacol oxidase and an isoeugenol dioxygenase. A high yield of vanillin, 66% from 4PG in RCF lignin oil was achieved through rounds of optimisation of the whole-cell conversion process. This high-performance strategy was readily scaled up to produce vanillin at an unprecedented 18% and 3% yield based on lignin oil and spruce wood respectively. The whole-cell bioconversion process shows good tolerance even at high loadings of starting material, showcasing the robustness and applicability of the employed biocatalysts. This work paves the way for further development towards the efficient production of high-titer biobased vanillin using depolymerised lignin as the feedstock.

The first platform for oxidative alkyl halide-olefin metathesis is described. The procedure employs diazenes as catalysts, which effect cyclization of alkenyl alkyl halides to generate cyclic olefins. Mechanistically, the diazene catalyst is converted to a hydrazonium intermediate by alkylation with the alkyl halide component followed by tautomerization. The resulting hydrazonium ion reacts with the olefin component through [3+2] cycloaddition and cycloreversion reactions, leading to new olefin and hydrazonium moieties. Hydrolysis of the newly formed hydrazonium was found to outcompete retro-tautomerization / dealkylation, thus furnishing a carbonyl product instead of an alkyl halide. For this reason, the process is termed oxidative alkyl-halide olefin metathesis. Adventitious O2 and H2O were sufficient for catalyst turnover. The synthesis of phenanthrene, coumarin, and quinolone derivatives is demonstrated, as well as the potential to apply this strategy to other electrophiles.

Most flavin-dependent enzymes contain a dissociable flavin cofactor. We present a new approach for installing a covalent bond between a flavin cofactor and its hosting protein. By using a flavin transferase and carving a flavinylation motif in target proteins, we demonstrate that ‘dissociable’ flavoproteins can be turned into covalent flavoproteins. Specifically, three different FMN-containing proteins were engineered to undergo covalent flavinylation: a light-oxygen-voltage (LOV) domain protein, a mini singlet-oxygen-generator (miniSOG), and a nitroreductase (BtNR). Optimizing the flavinylation motif and expression conditions led to covalent flavinylation of all three flavoproteins. The engineered covalent flavoproteins retained function and often exhibited improved performance such as higher thermostability or catalytic performance. Crystal structures of all three covalent flavoproteins confirmed the designed threonyl-phosphate linkage. The targeted flavoproteins differ in fold and function, indicating that this method of introducing a covalent flavin-protein bond is a powerful new method to create flavoproteins which cannot lose their cofactor, boosting their performance.

A novel insect-derived flavoenzyme from the honeybee Apis mellifera (beeHMFO) can selectively oxidize 5-hydroxymethylfurfural (HMF) to the corresponding dialdehyde 2,5-diformylfuran (DFF), which is an interesting bio-based polymer precursor. Moreover, activity toward a number of other aromatic alcohols is observed. The predicted structure of beeHMFO shows high similarity to other flavoprotein oxidases capable of oxidizing HMF.

Abstract

The chemical 5-hydroxymethylfurfural (HMF) can be derived from lignocellulose and is an interesting bio-based platform chemical as it has the potential to be transformed into numerous valuable building blocks such as the polymer-precursor 2,5-diformylfuran (DFF). To date, only a few oxidases acting on HMF are known and by sampling atypical species, we discovered a novel flavin-dependent oxidoreductase from the honeybee Apis mellifera (beeHMFO). The enzyme can perform the chemoselective oxidation of HMF to DFF but can also readily accept other aromatic alcohols as substrates. The function of the enzyme may well be the antimicrobial generation of hydrogen peroxide using HMF, which is very abundant in honey. The discovery of this insect-derived flavoprotein oxidase holds promising potential in the synthesis of renewable products and demonstrates that insects can be an interesting source of novel biocatalysts.

Nature Synthesis, Published online: 05 October 2023; doi:10.1038/s44160-023-00411-6

The selective synthesis of non-symmetrical diamides and amido-esters is a challenge. Now a Pd-catalysed dicarbonylation method is reported that generates non-symmetrical diamides and amido-esters through diamino- and amino-alkoxy carbonylations of propargylic acetates using two different nucleophiles. Mechanistic studies reveal that the process occurs through a sequential carbonylation process.

Nature Synthesis, Published online: 02 October 2023; doi:10.1038/s44160-023-00398-0

Performing enantioselective photocatalytic reactions with visible light is a challenging task that has seen substantial advances with the development of new catalysts. Although many approaches utilize dual-catalytic systems in which the photocatalyst is separate from the chirality-inducing moiety, in this Review bifunctional photocatalysts that perform both roles are discussed.

A wide range of 2,5-diketopiperazines underwent stereochemical editing at position C6 leading to the formation of products with a defined configuration which was retained in consecutive reactions. Spectroscopic evidence was collected corroborating a hydrogen atom transfer to the photocatalyst as the pivotal step of the process.

Abstract

2,5-Diketopiperazines are cyclic dipeptides displaying a wide range of applications. Their enantioselective preparation has now been found possible from the respective racemates by a photochemical deracemization (53 examples, 74 % to quantitative yield, 71–99 % ee). A chiral benzophenone catalyst in concert with irradiation at λ=366 nm enables to establish the configuration at the stereogenic carbon atom C6 at will. If other stereogenic centers are present in the diketopiperazines they remain unaffected and a stereochemical editing is possible at a single position. Consecutive reactions, including the conversion into N-aryl or N-alkyl amino acids or the reduction to piperazines, occur without compromising the newly created stereogenic center. Transient absorption spectroscopy revealed that the benzophenone catalyst processes one enantiomer of the 2,5-diketopiperazines preferentially and enables a reversible hydrogen atom transfer that is responsible for the deracemization process. The remarkably long lifetime of the protonated ketyl radical implies a yet unprecedented mode of action.

Nature Chemical Biology, Published online: 28 September 2023; doi:10.1038/s41589-023-01426-y

The flavoenzyme nicotine oxidoreductase degrades nicotine in the bloodstream. Now, genetic selection in bacteria has been used to improve the catalytic performance of nicotine oxidoreductase, isolating variants with increased O2 reactivity that were more effective at degrading nicotine in the blood of rats.

Simple heating of muconic acid in water at high temperature in an autoclave under autogenic pressure provides levulinic acid in high yield, as described by Bert U. W. Maes et al. in their Communication (e202309597). The method can also be applied to make substituted levulinic acids from the corresponding muconic acids. Lignin oil rich in propylguaiacol obtained from pine sawdust has been successfully transformed into 3-propyllevulinic acid.