Dynamic aggregation of the mid-sized gadolinium complex {Ph4[Gd(DTTA)(H2O)2](-)3}

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Nuclear Magnetic Resonance (NMR) Spectroscopy

Background:

• Diethylenetriaminetetraacetic acid (DTTA) chelates metal ions, forming complexes with potential applications.
• Hydrophobic interactions and π-stacking are key drivers of self-assembly in solution.
• Gadolinium (Gd(3+)) complexes are widely used as MRI contrast agents due to their paramagnetic properties.

Purpose of the Study:

• To synthesize and characterize a novel Gd(3+)-DTTA complex with a hydrophobic core.
• To investigate the self-aggregation behavior of the synthesized compound in aqueous solution.
• To evaluate the impact of self-aggregation on the NMR relaxivity of the Gd(3+) complex.

Main Methods:

• Synthesis of {Ph4[Gd(DTTA)(H2O)2](-)3} and its diamagnetic analogue {Ph4[Y(DTTA)(H2O)2](-)3}.
• Dynamic Light Scattering (DLS) and Nuclear Magnetic Resonance (NMR) spectroscopy (1H, 17O) to study aggregation.
• NMR relaxivity measurements and analysis of NMR dispersion profiles.

Main Results:

• The compound self-aggregates in aqueous solution below 1 mM due to π-stacking of phenyl rings.
• Aggregates consist of 2-3 monomers, confirmed by diffusion and NMR studies.
• Concentration-dependent 1H NMR relaxivities were observed, reaching ~50 mM(-1) s(-1) at >20 mM Gd(3+).
• Increased relaxivity is attributed to slower rotational motion and increased Lipari-Szabo order parameter (S(2)).

Conclusions:

• The synthesized Gd(3+)-DTTA complex exhibits concentration-dependent self-aggregation driven by hydrophobic interactions.
• Self-aggregation significantly enhances the NMR relaxivity of the gadolinium complex.
• This aggregation behavior suggests potential for developing advanced MRI contrast agents with tunable properties.