Europium(III) DOTA-derivatives having ketone donor pendant arms display dramatically slower water exchange

Abstract

A series of new 1,4,7,10-tetraazacyclododecane-derivatives having a combination of amide and ketone donor groups as side-arms were prepared, and their complexes with europium(III) studied in detail by high resolution NMR spectroscopy. The chemical shift of the Eu(3+)-bound water resonance, the chemical exchange saturation transfer (CEST) characteristics of the complexes, and the bound water residence lifetimes (τ(m)) were found to vary dramatically with the chemical structure of the side-arms. Substitution of ketone oxygen donor atoms for amide oxygen donor atoms resulted in an increase in residence water lifetimes (τ(m)) and a decrease in chemical shift of the Eu(3+)-bound water molecule (Δω). These experimental results along with density functional theory (DFT) calculations demonstrate that introduction of weakly donating oxygen atoms in these complexes results in a much weaker ligand field, more positive charge on the Eu(3+) ion, and an increased water residence lifetime as expected for a dissociative mechanism. These results provide new insights into the design of paramagnetic CEST agents with even slower water exchange kinetics that will make them more efficient for in vivo imaging applications.

Figure 1

Expected resonance contributions in europium (III) chelate bonds in ligands containing a carboxylate oxygen, an amide oxygen, enol oxygen or a ketone oxygen donor and the anticipated trends in the metal-bound water oxygen bond lengths and bound water residence lifetimes. The calculated Mulliken charges on the oxygen donor atom in each type of complex is also shown.

Figure 2

Comparison of chemical shifts of the Eu-OH₂ resonances vs. the H₄ ligand proton resonances in each complex (measured in CD₃CN). The two dashed lines correspond to 66 % and 50 % of the δH₄ / δEu-OH₂ ratios, respectively.

Figure 3

CEST spectra of Eu(4) (red) and Eu(5) (blue) recorded at 9.4 T and 298 K in CH₃CN at B₁ = 10.8 µT (459Hz) and 10.0 µT (426 Hz), respectively. (Inset: B₁= 2.5 µT for both spectra.)

Figure 4

The CEST spectra of Eu(7) (red, 17 mM), Eu(8) (green, 30 mM total, 12 mM SAP), Eu(9) (yellow, 18 mM) and Eu(10) (blue, 3.6 mM) recorded at 400 MHz and 298K in H₂O:CH₃CN (1:1) for Eu(7) and Eu(8) and water for Eu(9) and Eu(10) using applied B₁ field of 10.4 µT.

Scheme 1

Keto-enol tautomerism in ligand 2.

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