Deploying Fluorescent Nucleoside Analogues for High-Throughput Inhibitor Screening

Discovery of Fluorescent Naturally-Occurring Inhibitor of SARS-CoV-2 Main Protease by AIE Fluorescent Probe

Target-based high-throughput screening (HTS) is an efficient way to identify potent drugs. However, the accuracy of HTS could be affected by Pan-Assay Interference Compounds (PAINS). One reason for the generation of PAINS is that the inherent photophysical property of screened compounds could interfere with typically used assay signals including absorption and fluorescence. Our previous studies indicate that the fluorescent probe based on the fluorophore with characteristics of aggregation-induced emission (AIE) could provide high accuracy of HTS, especially for the fluorescent natural products. Herein, we report an AIE-based fluorescent probe for the main protease (Mpro) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We designed and synthesized an AIE fluorescent probe ZLHG5, which has a site that can be specifically cleaved by Mpro to produce a light-up fluorescence. Thanks to the large Stokes shift of AIE fluorophore (~200 nm), the probe could be effectively used for HTS of Mpro inhibitors. After screening a library of fluorescent natural products with ZLHG5, we obtained two coumarin-originated natural compounds with potent inhibitory activity towards Mpro protease. This study provides both useful fluorescent HTS probe and potent inhibitors for Mpro protease.

Uridine Bisphosphonates Differentiate Phosphoglycosyl Transferase Superfamilies

Complex bacterial glycoconjugates drive interactions between pathogens, symbionts, and their human hosts. Glycoconjugate biosynthesis is initiated at the membrane interface by phosphoglycosyl transferases (PGTs), which catalyze the transfer of a phosphosugar from a soluble uridine diphosphosugar (UDP-sugar) substrate to a membrane-bound polyprenol-phosphate (Pren-P). The two distinct superfamilies of PGT enzymes (polytopic and monotopic) show striking differences in their structure and mechanism. We designed and synthesized a series of uridine bisphosphonates (UBPs), wherein the diphosphate of the UDP and UDP-sugar is replaced by a substituted methylene bisphosphonate (CXY-BPs; X/Y = F/F, Cl/Cl, (S)-H/F, (R)-H/F, H/H, CH3/CH3). UBPs and UBPs incorporating an N-acetylglucosamine (GlcNAc) substituent at the β-phosphonate were evaluated as inhibitors of a polytopic PGT (WecA from Thermotoga maritima) and a monotopic PGT (PglC from Campylobacter jejuni). Although CHF-BP most closely mimics diphosphate with respect to its acid/base properties, the less basic CF2-BP conjugate more strongly inhibited PglC, whereas the more basic CH2-BP analogue was the strongest inhibitor of WecA. These surprising differences indicate different modes of ligand binding for the different PGT superfamilies, implicating a modified P-O- interaction with the structural Mg2+. For the monoPGT enzyme, the two diastereomeric CHF-BP conjugates, which feature a chiral center at the Pα-CHF-Pβ carbon, also exhibited strikingly different binding affinities and the inclusion of GlcNAc with the native α-anomer configuration significantly improved binding affinity. UBP-sugars are thus revealed as informative new mechanistic probes of PGTs that may aid development of novel antibiotic agents for the exclusively prokaryotic monoPGT superfamily.

Backbone-Anchoring, Solid-Phase Synthesis Strategy To Access a Library of Peptidouridine-Containing Small Molecules

Nucleoside diphosphate sugar (NDP-sugar) substrates provide the inspiration for nucleoside analogue inhibitor scaffolds. By employing solid-phase synthesis, we provide a method to access a library of peptidouridine inhibitors with both minimal compound handling and purification steps. Specifically, this strategy is exemplified by generating uridine diphosphate sugar (UDP-sugar) mimics, which allow for compound elaboration by altering the dipeptide composition, the N-terminal linkage, and a pendant aryl group. To exemplify the versatility, 41 unique nucleoside analogues are presented.

Generation and reaction of alkyl radicals in open reaction vessels

An operationally simple process to transform alkyl iodides into reactive alkyl radicals is described. Aryl diazonium salts react with Hantzsch esters and molecular oxygen to give aryl radicals, which participate in halogen atom transfers to give alkyl radicals. These intermediates react with a variety of acceptors. The reaction cascade occurs at room temperature, in open reaction vessels, with short reaction times.