8-[(2,5-dioxopyrrol-1-yl)methyl]-1,3-dipropyl-7H-purine-2,6-dione 8-[(2,5-dioxo-1-pyrrolyl)methyl]-1,3-dipropyl-7H-purine-2,6-dione 8-(maleimidomethyl)-1,3-dipropyl-7H-purine-2,6-quinone