Alpha radiolysis of DOTA ligand in aqueous solutions with helium ion beams

Abstract

Samples of DOTA (1,4,7,10-tetraazacyclodécanetetraacetic acid) ligand in aqueous solutions have been irradiated with helium ion (4He2+) beams of the ARRONAX (Nantes) and CEMHTI (Orléans) cyclotrons. Formation yields of gaseous radiolysis products (H2, CO2 …) have been measured by micro-gas chromatography. Yield of hydrogen peroxide has in turn been determined by colorimetric titration with titanium (IV). Degraded solutions were also analyzed by electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance spectroscopy (NMR) in order to identify degradation products formed after irradiation and quantify the non-degraded DOTA ligand. Results showed that the presence of DOTA in water leads to an increase of G(H2) and a decrease of G(H2O2). A degradation scheme is proposed: DOTA is degraded through decarboxylation, losing CO2 moieties, condensation of two carboxylic acid moieties with elimination of either a carbonic acid or a glycolic acid group, cleavage of -CH2-COOH, N-CH2-COOH and (CH2-CH2)N-CH2-COOH fragments, or by recombination with a hydroxyl radical (OH). NMR analyses showed that almost a third of DOTA ligand was degraded after 110 kGy alpha irradiation dose.