Schematic representation of the most relevant chemical modifications introduced on the Arg amino acid. (a) Modifications on the side chain: I coded Arg, II α-dehydro Arg, III β-dehydro Arg, IV γ-dehydro Arg, V Arg in which the side chain has been reduced one methylenic unit (also denoted nor-arginine), and VI Arg in which the side chain has been enlarged one methylenic unit (also denoted homo-Arginine). (b) VII Retro-modification of the main chain to obtain a retroamide bond (retro-Arg).