Description Application Methyl (4-bromobenzoyl)-L-cysteinate is a chemoselective thiol probe for detection of electrophilic metabolites. As it is often difficult to isolate low-abundance natural products from extracts, metabolities with bioactivity or therapeutic potential may be missed. Castro-Falcón, et al developed a reactivity-guided approach using thiolate probes to discover electrophilic natural products - a cysteine-based probe and thiophenol-based probe. Both consist of • A chemoselective reagent for covalent reactivity with a pharmacophore, • A conspicuous UV/VIS or mass spectrometric (MS) tag, • A linker between the tag and reagent. Methyl (4-bromobenzoyl)-L-cysteinate contains a bromine MS tag and phenyl bromide UV tag that was demonstrated to chemoselectively label enone-, β-lactam-, and β-lactone-based natural products. Complementarily, a thiophenol probe (cat# 125237) with a chlorine MS tag and phenyl chloride UV tag labeled epoxide-based natural products. These probes can be used in diverse extracts (bacterial, fungal, plant) to chemically faciliate access to high-value metabolites. Other Notes Thiol Probes To Detect Electrophilic Natural Products Based on Their Mechanism of Action Thiol-Based Probe for Electrophilic Natural Products Reveals That Most of the Ammosamides Are Artifacts Neolymphostin A Is a Covalent Phosphoinositide 3-Kinase (PI3K)/ Mammalian Target of Rapamycin (mTOR) Dual Inhibitor That Employs an Unusual Electrophilic Vinylogous Ester