Alpha radiolysis of polyamino-carboxylic acids in aqueous solution: application to the DOTA ligand

The radiolytic stability of the DOTA ligand (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), chosen as a reference ligand of the polyamino-carboxylic acid family, has been studied by ex-situ irradiation with helium ion beams or gamma irradiation and by in-situ irradiation by direct introduction of radionuclides in the solution. Formation yields of some species (H₂, CO₂, H₂O₂, HNO₂...) have been determined and the degradation products of DOTA identified by varying the concentrations of DOTA ligand and nitric acid, as well as by introducing metallic cations in solution.It has been shown that the DOTA ligand is mainly degraded in solution by the indirect effect of ionizing radiation, due to the species formed by water radiolysis (e_aq^-, H^. and HO^.). DOTA can react with H^. and HO^. radicals leading to the formation of DOTA radicals after hydrogen abstraction. These radicals can further evolve by C-C or N-C bond cleavage, leading to the loss of carbon dioxide moieties (CO₂) or a carboxylate arm (CH₂COOH) and by recombination with a HO^. radical. In the presence of nitric acid, nitrate ions tend to protect the ligand from degradation by scavenging the reactive species in solution (e_aq-, H^. and HO^.). When the ligand is involved in a metallic complex, the addition of HO^. radicals is promoted whereas the decarboxylation is limited. The introduction of radionuclides in the DOTA solution showed that the ligand undergoes significant direct radiolysis in addition to the indirect effects. (author)