The positive influence of fullerene derivatives bonded to manganese(iii) porphyrins on water proton relaxation

Abstract

Manganese-porphyrin compounds as MRI contrast agents have drawn particular attention due to high relaxivities and unique biodistribution. It has been reported that the charge density of the metal center and steric decompression of the substituents, rather than rotational correlation time, were the key factors to determine the relaxivities of manganese(III) porphyrins. In this study, [6,6]-phenyl-C₆₁-butyric acid (PC₆₁BA) was introduced into 5-(4-aminophenyl)-10,15,20-tris (4-sulfonatophenyl) porphyrin (APTSPP) to investigate the influence on water proton relaxation. The obtained PC₆₁BA-APTSPP-Mn possesses a relaxivity of 19.2 mM⁻¹ s⁻¹, which is greater than that of Mn-APTSPP (11.2 mM⁻¹ s⁻¹) and clinically used Gd-DTPA (4.1 mM⁻¹ s⁻¹) at 0.5 T, and even more effective compared with those binding manganese(III) porphyrins to certain macromolecules. It was reasonably speculated that the high relaxivity of PC₆₁BA-APTSPP-Mn should ascribe to the charge density variation of Mn^III and steric decompression induced by PC₆₁BA. Both fluorescence emission spectra and cyclic voltammetry results verified the presence of electronic communication between PC₆₁BA and APTSPP-Mn. In addition, the hydrodynamic diameter of PC₆₁BA-APTSPP-Mn aggregates was much smaller than that of APTSPP-Mn aggregates, which may contribute to the higher relaxivity by inhibiting the formation of dimers of APTSPP-Mn. Therefore, the introduction of fullerene derivatives is suggested to be a good strategy for the improvement of the relaxivities of manganese(III) porphyrins.