Affinity-based protein profiling (AfBPP) is a widely applied method for the target identification of bioactive molecules. Probes containing photocrosslinkers, such as benzophenones, diazirines, and aryl azides, irreversibly link the molecule of interest to its target protein upon irradiation with UV light. Despite their prevalent application, little is known about photocrosslinker-specific off-targets, affecting the reliability of results. Herein, we investigated background protein labeling by gel-free quantitative proteomics. Characteristic off-targets were identified for each photoreactive group and compiled in a comprehensive inventory. In a proof-of-principle study, H8, a protein kinase A inhibitor, was equipped with a diazirine moiety. Application of this photoprobe revealed, by alignment with the diazirine background, unprecedented insight into its in situ proteome targets. Taken together, our findings guide the identification of biologically relevant binders in photoprobe experiments.

Falcarinol and stipudiol are natural products with potent anti-cancer activity found in several vegetables. Here, we use a chemical proteomic strategy to identify ALDH2 as a molecular target of falcarinol in cancer cells and confirm enzyme inhibition via covalent alkylation of the active site. Furthermore, the synthesis of stipudiol led to the observation that ALDH2 exhibits preference for alkynol-based binders. Inhibition of ALDH2 impairs detoxification of reactive aldehydes and limits oxidative stress response, two crucial pathways for cellular viability.

Self‐assembled coordination cages are interesting as drug‐delivery systems. Therefore, the synthesis of new M2L4 (M = Pd, Pt) molecular cages derived from highly fluorescent, rigid polyaromatic ligands is reported herein, and the first Pt2L4 cage with a ligand consisting of three pyridine moieties is described. The photophysical properties were examined, and they showed high quantum yields Φ of up to 48 % for the methoxy‐functionalized ligands. Coordination of the ligands to palladium and platinum ions did, however, reduce the fluorescence of the metallocages. The host–guest chemistry of the palladium cages with cisplatin was investigated, which confirmed the encapsulation. The cages encapsulating cisplatin show significantly increased cytotoxicity towards A549 (human lung adenocarcinoma) cells relative to that shown by cisplatin and, thus, appear to be promising delivery vectors for the anticancer drug cisplatin.

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The formation of different conformers of dinuclear silver(i) and gold(i) 1,1'-(2-hydroxyethane-1,1-diyl) bridge-functionalized bis(NHC) complexes with various wing-tip substituents (R = methyl, isopropyl and mesityl) has been investigated using multinuclear NMR spectroscopy and SC-XRD as well as DFT calculations. The ratio of anti/syn isomers strongly depends both on wing-tip substituents and the metal. Moreover, the reaction temperature plays a significant role during the transmetallation process for the ratio of gold(i) conformers, which is further affected by purification procedures. All obtained Au(i)-bis(NHC) complexes have been applied in a standard MTT assay performed for screening the antiproliferative activity against human lung and liver cancer cells. Strong evidence for a significant influence of both wing-tip substituents and conformation on the cytotoxic properties of the applied complexes has been found.

The natural product neocarzilin A ( NCA ) was discovered decades ago, and despite its potent cytotoxic effects no mode of action studies have been performed up to date. Synthesis of neocarzilins A, B, and C and a stereoisomer of NCA provided insights into structural preferences as well as access to probes for functional studies. NCA turned out to be the most active member and was not only effective against cell proliferation but also migration, a novel and so far overlooked activity. To decipher the molecular mode of action, we applied chemical proteomics for target discovery and revealed that NCA targets cancer cell migration via irreversible binding to the largely uncharacterized synaptic vesicle membrane protein VAT-1. A corresponding knockout of the protein confirmed the phenotype, and pull-down studies showed the interaction with an intricate network of key migration mediators such as Talin-1. Overall, we introduce VAT-1 as a promising novel target for the development of selective migration inhibitors with the perspective to limit toxicity in the absence of antiproliferative effects.

Invited for the cover of this issue is the group of Fritz Kühn from Technische Universität München, Germany. The cover image shows a palladium cage encapsulating cisplatin, represented by a bowling ball, knocking down the pins, which symbolize cancer cells.

D iese Autoren haben zu gleichen Teilen zu der Arbeit beigetragen.