Abstract: a-Ketoamide (-NH-CO-CO-) units in intact peptides are generated from Ser/Thr residues via Ru (VII1)-catalyzed C=-C side-chain scission. Facets associated with this novel a-carbon modification have been probed with 75 peptides chosen to represent every possible peptide environment. The reactions were carried out at room temperature with in situ generated Ru (VII1) in biphasic (CH3CN/CC4/pH 3 phosphate buffer, 1: 1: 2 v/v) ...