Fluorinated Mn(III)/(II)-Porphyrin with Redox-Responsive H and F Relaxation Properties.

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual H relaxivity and F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d /H O evidenced that the H relaxometric and F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (d =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on F T and T relaxation times. Mn-TPP-p-CF has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and F NMR. The broad H and F NMR signals of Mn(III)-TPP-p-CF disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened F NMR signal from Mn(II)-TPP-p-CF . Phantom F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF , retrieved upon complete reoxidation in air within ~24 h. H NMRD curves of the Mn(III)/(II)-TPP-p-CF chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins.