Unprecedented Relaxivity Gap in pH-Responsive Fe^III -Based MRI Probes

Area of Science:

• Inorganic Chemistry
• Materials Science
• Biomedical Imaging

Background:

• Molecular Magnetic Resonance Imaging (MRI) faces sensitivity limitations.
• Developing responsive contrast agents is crucial for advanced diagnostics.
• Current contrast agents often lack significant signal amplification.

Purpose of the Study:

• To synthesize and characterize novel mononuclear ferric complexes.
• To investigate the pH-responsive relaxation enhancement capabilities of these complexes.
• To explore the structural basis for their performance in MRI.

Main Methods:

• Synthesis of two hexa-dentate macrocyclic ferric complexes.
• Relaxivity measurements at varying pH.
• Structural analysis of the ligand and coordination environment.

Main Results:

• Complexes exhibit 27- and 71-fold increases in water relaxation rate acceleration upon pH change.
• The phenomenon is fully reversible and fatigue-less.
• Perturbed pKa values of 3.5 were observed, linked to pendant lactam/carbamate arms.

Conclusions:

• The novel ferric complexes show exceptional pH-responsive relaxivity.
• Macrocyclic structure and pendant arms are key to performance.
• These findings advance the development of highly sensitive molecular MRI agents.