Scheme 1. Oxidation and Decarboxylation of a C-Terminal Cysteine-Bearing Peptide, Catalyzed by a Decarboxylase (LanD)⁶⁸ (Reactive Thioenamide (Red) Then Undergoes Thiol-Michael Cyclization with an Unsaturated Amino Acid (Dha/Dhb) to Provide the Avi(Me)Cys Unit Following Protonation).