Thiol modification is an important tool in the chemical, biological, medical, and material sciences. As the only thiol-containing amino acid, cysteine is typically used for protein modification using thiol-based reactions. Despite the ubiquity of cysteine tagging, general chemical approaches do not exist for the site-specific modification of a single cysteine in the presence of other unprotected cysteines within the same peptide or protein chain. A highly efficient method that allows single-site-specific cysteine modification is needed to significantly expand the ability to modify biomolecules. The current technology features several advantages over existing peptide modification methods, including: specificity of thiols over other nucleophiles (e.g., amines, hydroxyls), excellent functional group tolerance, and mild reaction conditions. More importantly, cysteine modifications can be produced on a specific, peptide sequence. This invention has the potential to significantly expand the existing toolkit for modifying biomolecules for research and therapeutics.