Visible Light‐Mediated Functionalization of Selenocystine‐Containing Peptides

Unlocking New Prenylation Modes: Azaindoles as a New Substrate Class for Indole Prenyltransferases

Aza-substitution, the replacement of aromatic CH groups with nitrogen atoms, is an established medicinal chemistry strategy for increasing solubility, but current methods of accessing functionalized azaindoles are limited. In this work, indole-alkylating aromatic prenyltransferases (PTs) were explored as a strategy to directly functionalize azaindole-substituted analogs of natural products. For this, a series of aza-l-tryptophans (Aza-Trp) featuring N-substitution of every aromatic CH position of the indole ring and their corresponding cyclic Aza-l-Trp-l-proline dipeptides (Aza-CyWP), were synthesized as substrate mimetics for the indole-alkylating PTs FgaPT2, CdpNPT, and FtmPT1. We then demonstrated most of these substrate analogs were accepted by a PT, and the regioselectivity of each prenylation was heavily influenced by the position of the N-substitution. Remarkably, FgaPT2 was found to produce cationic N-prenylpyridinium products, representing not only a new substrate class for indole PTs but also a previously unobserved prenylation mode. The discovery that nitrogenous indole bioisosteres can be accepted by PTs thus provides access to previously unavailable chemical space in the search for bioactive indolediketopiperazine analogs.

Selenium in Peptide Chemistry

In recent years, researchers have been exploring the potential of incorporating selenium into peptides, as this element possesses unique properties that can enhance the reactivity of these compounds. Selenium is a non-metallic element that has a similar electronic configuration to sulfur. However, due to its larger atomic size and lower electronegativity, it is more nucleophilic than sulfur. This property makes selenium more reactive toward electrophiles. One of the most significant differences between selenium and sulfur is the dissociation of the Se-H bond. The Se-H bond is more easily dissociated than the S-H bond, leading to higher acidity of selenocysteine (Sec) compared to cysteine (Cys). This difference in acidity can be exploited to selectively modify the reactivity of peptides containing Sec. Furthermore, Se-H bonds in selenium-containing peptides are more susceptible to oxidation than their sulfur analogs. This property can be used to selectively modify the peptides by introducing new functional groups, such as disulfide bonds, which are important for protein folding and stability. These unique properties of selenium-containing peptides have found numerous applications in the field of chemical biology. For instance, selenium-containing peptides have been used in native chemical ligation (NCL). In addition, the reactivity of Sec can be harnessed to create cyclic and stapled peptides. Other chemical modifications, such as oxidation, reduction, and photochemical reactions, have also been applied to selenium-containing peptides to create novel molecules with unique biological properties.

Nonyl Acridine Orange as a Prospective Photocatalyst in Chalcogenylation of Coumarins and Quinolinones

A mild and efficient method for preparation of 3-sulfenyl and 3-selenyl coumarins and quinolinones mediated by artificial light or sunlight is presented. The elaborated protocol highlights the use of nonyl acridine orange as a photocatalyst to generate a sulfenyl radical from thiols that is further trapped by a heterocycle. The utility of the protocol is justified by a diverse scope of thiols, including short cysteine-containing peptides. The same reaction conditions can be applied for preparation of 3-selenyl coumarins and quinolinones. Various protected and unprotected selenocysteine-containing peptides were successfully utilized demonstrating high tolerance for amino acids with sensitive groups (Arg, Lys, Trp, His, and Tyr).

Selenocysteinyl electrophiles efficiently promote the formation of coumarin and quinolinone cores by 6-endo-dig cyclization

Efficient methods have been disclosed for the construction of nitrogen and oxygen containing heterocyclic systems attached to selenocysteine or selenoglutathione.