4257-92-5 Cyclo(his-pro) Cyclo(histidyl-prolyl) His-pro-dkp Histidyl-proline diketopiperazine Histidyl-proline-diketopiperazine Pyrrolo(1,2-a)pyrazine-1,4-dione, hexahydro-3-(1H-imidazol-4-ylmethyl)-, (3S-trans)-