A Cyclopropene Electrophile that Targets Glutathione S-Transferase Omega-1 in Cells.

Cyclopropenes are an important new addition to the portfolio of functional groups that can be used for bioorthogonal couplings. The inert nature of these highly strained compounds in complex biological systems is almost counterintuitive given their established electrophilic properties in organic synthesis. Here we provide the first demonstration of a cyclopropene that is capable of direct conjugation to protein targets in cells and show that this compound preferentially alkylates the active site cysteine of glutathione S-transferase omega-1 (GSTO1).

STEFs: Activated Vinylogous Protein-Reactive Electrophiles.

Reported here is the synthesis of a class of semi-oxamide vinylogous thioesters, designated STEFs, and the use of these agents as new electrophilic warheads. This work includes preparation of simple probes that contain this reactive motif as well as its installation on a more complex kinase inhibitor scaffold. A key aspect of STEFs is their reactivity towards both thiol and amine groups. Shown here is that amine conjugations in peptidic and proteinogenic samples can be facilitated by initial, fast conjugation to proximal thiol residues. Evidence that both the selectivity and the reactivity can be tuned by the structure of STEFs is provided, and given the unique ability of this functionality to conjugate by an addition-elimination mechanism, STEFs are electrophilic warheads that could find broad use in chemical biology.

Stendomycin and Pantomycin Are Identical Natural Products: Preparation of a Functionalized Bioactive Analogue.

The natural products pantomycin and stendomycin were both reported as antimicrobial agents. We demonstrate by gene cluster analysis, LC-MS analysis, and isolation that these polypeptides are identical, and we identify previously unknown congeners. We show that stendomycin can be chemically modified at its electrophilic dehydrobutyrine moiety yielding the first bioactive analogue of this natural product which can undergo additional functionalization. This compound may be a valuable starting point for molecular probe development, and we invite its distribution to the scientific community.

Catalytic Asymmetric [4+2]-Cycloadditions Using Tropolones: Developments, Scope, Transformations, and Bioactivity.

An organocatalyzed asymmetric [4+2]-cycloaddition between tropolones and electron-deficient dienophiles is presented. Complex and biologically interesting dihydrohomobarrelenone scaffolds are formed through a Diels-Alder reaction utilizing bifunctional Brønsted-base catalysis, affording the corresponding bridged bicyclic cycloadducts in up to quantitative yields with good enantio- (up to 92 % ee) and diastereoselectivity (up to >20:1 d.r.). The synthetic value of the obtained products is explored by downstream transformations, including photoisomerizations, and their biological relevancy by in vivo testing in MCF-7 cancer cells.

Aptamer-Targeted Drug Delivery for Staphylococcus aureus Biofilm.

Treatment of Staphylococcus aureus biofilm infections using conventional antibiotic therapy is challenging as only doses that are sublethal to the biofilm can be administered safely to patients. A potential solution to this challenge is targeted drug delivery. In this study, we tailored an aptamer-targeted liposomal drug delivery system for accumulation and delivery of antibiotics locally in S. aureus biofilm. In our search for a suitable targeting ligand, we identified six DNA aptamers that bound to S. aureus cells in biofilms, and we demonstrated that one of these aptamers could facilitate accumulation of liposomes around S. aureus cells inside the biofilm. Aptamer-targeted liposomes encapsulating a combination of vancomycin and rifampicin were able to eradicate S. aureus biofilm upon 24 h of treatment in vitro. Our results point to that aptamer-targeted drug delivery of antibiotics is a potential new strategy for treatment of S. aureus biofilm infections.

The covalent reactivity of functionalized 5-hydroxy-butyrolactams is the basis for targeting of fatty acid binding protein 5 (FABP5) by the neurotrophic agent MT-21.

Covalently acting compounds experience a strong interest within chemical biology both as molecular probes in studies of fundamental biological mechanisms and/or as novel drug candidates. In this context, the identification of new classes of reactive groups is particularly important as these can expose novel reactivity modes and, consequently, expand the ligandable proteome. Here, we investigated the electrophilic reactivity of the 3-acyl-5-hydroxy-1,5-dihydro-2H-pyrrole-2-one (AHPO) scaffold, a heterocyclic motif that is e.g. present in various bioactive natural products. Our investigations were focused on the compound MT-21 - a simplified structural analogue of the natural product epolactaene - which is known to have both neurotrophic activity and ability to trigger apoptotic cell death. We found that the central N-acyl hemiaminal group of MT-21 can function as an electrophilic centre enabling divergent reactivity with both amine- and thiol-based nucleophiles, which furthermore translated to reactivity with proteins in both cell lysates and live cells. We found that in live cells MT-21 strongly engaged the lipid transport protein fatty acid-binding protein 5 (FABP5) by direct binding to a cysteine residue in the bottom of the ligand binding pocket. Through preparation of a series of MT-21 derivatives, we probed the specificity of this interaction which was found to be strongly dependent on subtle structural changes. Our study suggests that MT-21 may be employed as a tool compound in future studies of the biology of FABP5, which remains incompletely understood. Furthermore, our study has also made clear that other natural products containing the AHPO-motif may likewise possess covalent reactivity and that this property may underlie their biological activity.

Structure and Function of the Bacterial Protein Toxin Phenomycin.

Phenomycin is a bacterial mini-protein of 89 amino acids discovered more than 50 years ago with toxicity in the nanomolar regime toward mammalian cells. The protein inhibits the function of the eukaryotic ribosome in cell-free systems and appears to target translation initiation. Several fundamental questions concerning the cellular activity of phenomycin, however, have remained unanswered. In this paper, we have used morphological profiling to show that direct inhibition of translation underlies the toxicity of phenomycin in cells. We have performed studies of the cellular uptake mechanism of phenomycin, showing that endosomal escape is the toxicity-limiting step, and we have solved a solution phase high-resolution structure of the protein using NMR spectroscopy. Through bioinformatic as well as functional comparisons between phenomycin and two homologs, we have identified a peptide segment, which constitutes one of two loops in the structure that is critical for the toxicity of phenomycin.