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The thermodynamic, kinetic, and structural properties of Ln 3+ complexes with the bifunctional DO3A-ACE 4- ligand and its amide derivative DO3A-BACE 4- (modelling the case where DO3A-ACE 4- ligand binds to vector molecules) have been studied in order to confirm the usefulness of the corresponding Gd 3+ complexes as relaxation labels of targeted MRI contrast agents. The stability constants of the Mg 2+ and Ca 2+ complexes of DO3A-ACE 4- and DO3A-BACE 4- complexes are lower than for DOTA 4- and DO3A 3- , while the Zn 2+ and Cu 2+ complexes have similar and higher stability than for DOTA 4- and DO3A 3- complexes. The stability constants of the Ln(DO3A-BACE) - complexes increase from Ce 3+ to Gd 3+ but remain practically constant for the late Ln 3+ ions (represented by Yb 3+ ). 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 DOTA 4- and DO3A 3- 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 1 H 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 C α -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-ACE 4- in the design of GBCAs and Ln 3+ -based tags for protein structural NMR analysis.

molecules/molecules online/molecules annual

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