L-Histidine, N α -cbz-L-histidine, L-tryptophan and L-proline react with 1,4-naphthoquinone or 2,3-dichloro-1,4-naphthoquinone to afford modified N-quinonyl amino acids. With free α-amino acids, the quinone moiety is attached to the α-amino group. With blocked α-amino acids the quinone moiety is attached to the heterocyclic nitrogen atom.