Bio-orthogonal catalysis

We report the efficient and site selective modification of non-canonical dehydroamino acids in ribosomally synthesized and post-transationally modified peptides (RiPPs) by β-amination. The singly modified thiopeptide Thiostrepton showed an up to 35-fold increase in water solubility, and minimum inhibitory concentration (MIC) assays showed that antimicrobial activity remained good, albeit lower than the unmodified peptide. Also the lanthipeptide nisin could be modified using this method.

We report the fast and selective chemical editing of ribosomally synthesized and post‐translationally modified peptides (RiPPs) by β‐borylation of dehydroalanine residues. The thiopeptide thiostrepton was modified efficiently using Cu(II)‐catalysis under mild conditions and 1D/2D NMR of the purified product showed site‐selective borylation of the terminal Dha residues. Using similar conditions, the thiopeptide nosiheptide, lanthipeptide nisin Z and protein SUMO_G98Dha were also modified efficiently. Borylated thiostrepton showed an up to 84‐fold increase in water solubility, and MIC assays showed that antimicrobial activity was maintained in thiostrepton and nosiheptide. The introduced boronic acid functionalities were shown to be valuable handles for chemical mutagenesis and in a reversible click reaction with triols for the pH‐controlled labeling of RiPPs.

Dehydroalanine (Dha) residues are attractive non-canonical amino acids that occur naturally in ribosomally synthesised and post-translationally modified peptides (RiPPs). Dha residues are attractive targets for selective late-stage modification of these complex biomolecules. In this work, we show the selective photocatalytic modification of dehydroalanine residues in the antimicrobial peptide nisin and in the proteins Small Ubiquitin-like Modifier (SUMO) and superfolder Green Fluorescent Protein (sfGFP). For this purpose, a new water-soluble iridium(III) photoredox catalyst was used. The design and synthesis of this new photocatalyst, [Ir(dF(CF3)ppy)2(dNMe3bpy)]Cl3, is presented. In contrast to commonly used iridium photocatalysts, this complex is highly water-soluble and allows modification of peptides and proteins in water and aqueous solvents under physiologically relevant conditions and with short reaction times and low reagent and catalyst loadings. This work suggests that photoredox catalysis using this newly designed catalyst is a promising strategy to modify dehydroalanine-containing natural products and thus may have great potential for novel bioconjugation strategies.

We report the efficient and selective Cu(II)-catalysed β-silylation of naturally occurring dehydroalanine (Dha) residues in various Ribosomally synthesized and Post-translationally modified Peptides (RiPPs). The method is also applicable to proteins, as was shown by the modification of a Dha residue that was chemically introduced into Small Ubiquitin-like Modifier (SUMO).

We report the late stage chemical modification of ribosomally synthesized and posttranslationally modified peptides (RIPPs) by Diels‐Alder cycloadditions to naturally occurring dehydroalanines. The tail region of the thiopeptide thiostrepton could be modified selectively and efficiently under microwave heating and transition metal free conditions. The Diels‐Alder adducts were isolated and the different site‐ and endo/exo isomers were identified by 1D/2D 1H NMR. Via efficient modification of the thiopeptide nosiheptide and the lanthipeptide nisin Z the generality of the method was established. MIC assays of the purified thiostrepton Diels‐Alder products against thiostrepton‐susceptible strains displayed high activities comparable to that of native thiostrepton. These Diels‐Alder products were also subjected successfully to Inverse‐electron‐demand Diels‐Alder reactions with a variety of functionalized tetrazines, demonstrating the utility of this method for labeling of RiPPs.

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.

Dehydroalanine (Dha) and dehydrobutyrine (Dhb) are remarkably versatile non‐canonical amino acids often found in antimicrobial peptides. Here, we present the selective modification of Dha and Dhb in antimicrobial peptides via photocatalytic activation of organoborates under influence of visible light. Ir(dF(CF3)ppy)2(dtbbpy)PF6 was used as photoredox catalyst in aqueous solutions for the modification of thiostrepton and nisin. The mild conditions and high selectivity for the dehydrated residues, show photoredox catalysis is a promising tool for modification of peptide derived natural products.

Dehydroalanine (Dha) is a remarkably versatile non‐canonical amino acid often found in antimicrobial peptides. Here we present the catalytic modification of Dha via a palladium mediated cross coupling reaction. Using Pd(EDTA)(OAc)2 as water soluble catalyst, a variety of arylboronic acids was coupled to the dehydrated residues in proteins and peptides such as nisin. The cross coupling reaction yields both the Heck product, in which the sp2‐hybridisation of the α‐carbon is retained, as well as the conjugated addition product. The reaction can be performed under mild aqueous conditions, which makes this method an attractive addition to the palette of bio‐orthogonal catalytic methods.