6,6'-bis(α-alanylamino)-2,2'-bipyridine

2-[4-(4-hydroxy-benzyl)-2,5-dioxo-imidazolidin-1-yl]-propionic acid

DL-2-(N'-glycyl-ureido)-propionic acid

N-(3-phenyl-propionyl)-alanine

2-(4-phenyl-butyrylamino)-propionic acid