Test of Electron Delocalization Effects on Water-Proton Spin-Lattice Relaxation by Bromination of [Tetrakis(4-sulfonatopheny)porphine]manganese.

The potential value of electron spin delocalization as a means for substantially increasing the ability of a paramagnetic metal complex to induce nuclear spin relaxation of water protons has been examined by covalent attachment of bromine atoms in the beta-pyrrole positions of the [5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine]manganese complexesMn(III)TPPS(-) and [Mn(II)TPPS].(4)(-) The water-proton spin-lattice relaxivities are reported as a function of magnetic field strength for the brominated and nonbrominated metalloporphyrins over the range of magnetic field strengths corresponding to proton Larmor frequencies between 0.01 and 30 MHz. The brominated metalloporphyrins increase the water-proton relaxativities compared to the nonbrominated metalloporphrins, and, at low magnetic field strengths, the brominated Mn(II)TPPS(-) complex rivals the efficiency of the hexaaquomanganese(II) ion. Attempts to fit the experimental data to theories for paramagnetic relaxation, which are based on the point-dipole approximation, result in distances between the paramagnetic center and the water proton that are unreasonably short based on published structural data. The excess relaxivity implies that the point-dipole approximation may be inappropriate for these porphyrin systems and electron spin delocalization may provide a significant contribution to nuclear spin relaxation that may be fruitfully exploited in construction of contrast agents for magnetic resonance imaging.