(S)-5-(p-nitrobenzyl)-PCTA, a promising bifunctional ligand with advantageous metal ion complexation kinetics

A bifunctional version of PCTA (3,6,9,15-tetraazabicyclo[9.3.1]pentadeca- l(15),11,13-triene-3,6,9,-triacetic acid) that exhibits fast complexation kinetics with the trivalent lanthanide(III) ions was synthesized in reasonable yields starting from N,N′,N″-tristosyl-(S)-2-(p-nitrobenzyl)- diethylenetriamine. pH-potentiometric studies showed that the basicities of p-nitrobenzyl-PCTA and the parent ligand PCTA were similar. The stability of M(N0₂-Bn-PCTA) (M = Mg²⁺, Ca²⁺, Cu ²⁺, Zn²⁺) complexes was similar to that of the corresponding PCTA complexes, while the stability of Ln³⁺ complexes of the bifunctional ligand is somewhat lower than that of PCTA chelates. The rate of complex formation of Ln(N0₂-Bn-PCTA) complexes was found to be quite similar to that of PCTA, a ligand known to exhibit the fastest formation rates among all lanthanide macrocyclic ligand complexes studied to date. The acid-catalyzed decomplexation kinetic studies of the selected Ln(NG=₂-Bn-PCTA) complexes showed that the kinetic inertness of the complexes was comparable to that of Ln(DOTA) chelates making the bifunctional ligand NO₂-Bn-PCTA suitable for labeling biological vectors with radioisotopes for nuclear medicine applications.