We report the T1 and T2 NMR (nuclear magnetic resonance) dispersion profiles for a new manganese porphyrin [MnT(2-C)PP] which has an anionic carboxylate group in the ortho position of the phenyl rings on the metalloporphyrin. Previous MEP (molecular electrostatic potential) maps indicated that this judicious derivatization could result in increases in the observed relaxation efficiency. Relaxometric investigations experimentally confirm about a 20 % increase in the relaxation efficiency at clinically relevant field strengths for MnT(2-C)PP compared to the most efficient metalloporphyrin previously reported MnT(4-S)PP. This result supports the hypothesis that electrostatic forces are relevant to the relaxivity of this family of compounds and that the MEP may be used as a tool to design new agents with even larger relaxivities.