RELAXATION ENHANCEMENT OF WATER PROTONS BY MANGANESE(III) PORPHYRINS - INFLUENCE OF PORPHYRIN AGGREGATION

The ability of eight manganese(III) porphyrins to enhance the longitudinal relaxation rate of water protons in aqueous solution has been studied by nuclear magnetic resonance dispersion (NMRD). In NMRD, the relaxation rates of solvent water protons are measured as a function of proton Larmor frequency from 0.01 to 50 MHz (field strengths from 0.23 mT to 1.17 T); a plot of relaxation rate versus frequency is called an NMRD profile. To enable comparison of the data, the observed rates are expressed as relaxivities by normalizing the rates to the concentration of manganese(III) porphyrin. Five porphyrins exhibited similar profiles characterized by high relaxivities, while the profiles for three porphyrins were different and demonstrated much lower relaxivities. Analysis of the NMRD profiles by relaxation theory yields correlation times, the values of which are sensitive to interactions between the hydrated manganese(III) porphyrin and its environment. The values of some correlation times for the three porphyrins with low relaxivities were inconsistent with those showing high relaxivities. Because the values of these correlation times indicate that interactions are occurring between manganese atoms and because differences in structure are apparent between the three porphyrins with lower relaxivities and the five with high relaxivities, it is hypothesized that the three porphyrins exist as aggregates in solution while the others exist as monomers. Addition of perdeuterated acetone to solutions of the porphyrins with low relaxivities caused them to attain relaxivities that are similar in magnitude to the others at all frequencies, while the addition of acetone to solutions of porphyrins with high relaxivities causes no significant change in their relaxivities. Because the addition of acetone is known to break up porphyrin aggregates, the assertion that aggregation is directly responsible for the low relaxivities in aqueous solution is strongly supported.