4,​7,​11,​14,​18,​21-​Hexaazatetracosanedi​amide, N1,​N24-​bis(2-​aminoethyl)​-​4,​21-​bis[3-​[(2-​aminoethyl)​amino]​-​3-​oxopropyl]​-​11,​14-​bis[3-​[[2-​[bis[3-​[(2-​aminoethyl)​amino]​-​3-​oxopropyl]​amino]​ethyl]​amino]​-​3-​oxopropyl]​-​8,​17-​dioxo