Complexes in the Context of MRI

In the case of Mn11 complexes as potential MRI contrast agents, the presence of a water molecule in the first coordination sphere is crucial to obtain reasonable relaxivity. Given the lower spin of Mn11, the outer sphere contribution to the overall relaxivity is lower than for Gdm complexes. Thus, for complexes that lack inner sphere water the relaxivity is less than 50% as compared to complexes with coordinated water (233-235). 

Aime et al. took advantage of the different redox states of manganese and of the difference in the related relaxation properties to design a p02 responsive contrast agent. The adducts formed between Mn /Mnn tpps complexes and poly-P-cyclodex-trin have considerably different relaxivities depending on the redox state of the metal, itself determined by the partial oxygen pressure of the solution (tpps — 5,10,15,20-tetrakis-(p-sulfonato-phenyl porphinate) (243). 

Thermodynamic Stability (log of Selected Mn11 Complexes and Their Comparison with Other Biogenic Metal Ions at 298 K 

Ligands Mn11 Cu11 Zn11 Ca11 Refs. 

Beside the stable Gdm complexes that we discussed throughout this chapter, other Gdm-based systems have been also investigated as potential contrast agents for MRI. Here, we will shortly review data on gadofullerenes and carbon nanotubes loaded with Gdm. 

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