Complexes of Bifunctional DO3A-N-(α-amino)propinate Ligands with Mg(II), Ca(II), Cu(II), Zn(II), and Lanthanide(III) Ions: Thermodynamic Stability, Formation and Dissociation Kinetics, and Solution Dynamic NMR Studies

Abstract

The thermodynamic, kinetic, and structural properties of Ln3+ complexes with the bifunctional DO3A-ACE4− ligand and its amide derivative DO3A-BACE4− (modelling the case where DO3A-ACE4− ligand binds to vector molecules) have been studied in order to confirm the usefulness of the corresponding Gd3+ complexes as relaxation labels of targeted MRI contrast agents. The stability constants of the Mg2+ and Ca2+ complexes of DO3A-ACE4− and DO3A-BACE4− complexes are lower than for DOTA4− and DO3A3−, while the Zn2+ and Cu2+ complexes have similar and higher stability than for DOTA4− and DO3A3− complexes. The stability constants of the Ln(DO3A-BACE)− complexes increase from Ce3+ to Gd3+ but remain practically constant for the late Ln3+ ions (represented by Yb3+). The stability constants of the Ln(DO3A-ACE)4− and Ln(DO3A-BACE)4− complexes are several orders of magnitude lower than those of the corresponding DOTA4− and DO3A3− complexes. The formation rate of Eu(DO3A-ACE)− is one order of magnitude slower than for Eu(DOTA)−, due to the presence of the protonated amine group, which destabilizes the protonated intermediate complex. This protonated group causes the Ln(DO3A-ACE)− complexes to dissociate several orders of magnitude faster than Ln(DOTA)− and its absence in the Ln(DO3A-BACE)− complexes results in inertness similar to Ln(DOTA)− (as judged by the rate constants of acid assisted dissociation). The 1H NMR spectra of the diamagnetic Y(DO3A-ACE)− and Y(DO3A-BACE)− reflect the slow dynamics at low temperatures of the intramolecular isomerization process between the SA pair of enantiomers, R-Λ(λλλλ) and S-Δ(δδδδ). The conformation of the Cα-substituted pendant arm is different in the two complexes, where the bulky substituent is further away from the macrocyclic ring in Y(DO3A-BACE)− than the amino group in Y(DO3A-ACE)− to minimize steric hindrance. The temperature dependence of the spectra reflects slower ring motions than pendant arms rearrangements in both complexes. Although losing some thermodynamic stability relative to Gd(DOTA)−, Gd(DO3A-BACE)− is still quite inert, indicating the usefulness of the bifunctional DO3A-ACE4− in the design of GBCAs and Ln3+-based tags for protein structural NMR analysis.