Disclosed are methods of single-site-specific cysteine modification on peptide/protein molecules under physiologically relevant conditions. This process features several significant advantages over existing methods of peptide modification, such as specificity towards thiols over other nucleophiles (e.g., amines, hydroxyls), excellent functional group tolerance, and mild reaction conditions. Especially important is the specificity observed for thiols appearing in an X-Cys-Pro-X sequence over other thiols or disulfides, where X is Phe, Trp, or Tyr; under the inventive conditions, other cysteines or reactive functional groups on the same peptide/protein chain are not functionalized.