Investigating Cu(II) Complexes for MRI: A Comprehensive Approach Using EPR, Relaxometry, and Computational Modeling

Area of Science:

• Inorganic Chemistry
• Materials Science
• Biomedical Imaging

Background:

• Gadolinium-based contrast agents (GBCAs) are standard for MRI but carry risks.
• Developing safer, Gadolinium-free (Gd-free) contrast agents is a critical research area.
• Understanding paramagnetic relaxation in transition-metal complexes is key to designing novel agents.

Purpose of the Study:

• To investigate the structure-relaxivity relationships in two copper(II) complexes, [Cu(TACN)]2+ and [Cu(TREN)]2+.
• To determine how coordination environment, geometry, and hydration affect paramagnetic relaxation pathways.
• To establish a framework for designing Cu(II)-based MRI contrast agents.

Main Methods:

• Electron Paramagnetic Resonance (EPR) spectroscopy
• Q-band Electron Nuclear Double Resonance (ENDOR)
• Variable-temperature 17O Nuclear Magnetic Resonance (NMR)
• Field-dependent 1H relaxometry
• Density Functional Theory (DFT) calculations

Main Results:

• EPR and ENDOR provided accurate rotational correlation times and metal-proton hyperfine couplings.
• 1H relaxometry showed distinct water exchange dynamics for [Cu(TACN)]2+ (fast) and [Cu(TREN)]2+ (slow).
• [Cu(TREN)]2+ exhibited significant scalar relaxation under basic conditions due to hydroxide (OH-) substitution.

Conclusions:

• Cu(II) relaxivity is highly sensitive to subtle structural variations.
• Geometric and hydration control can effectively modulate inner-sphere and prototropic exchange pathways.
• The integrated experimental-computational approach enables rational design of Cu(II)-based MRI contrast agents.