Binding of\na Co(III) Metalloporphyrin to Amines in\nWater: Influence of the p<i>K</i><sub>a</sub> and Aromaticity\nof the Ligand, and pH-Modulated Allosteric Effect

Metalloporphyrins\nhave been widely utilized as building blocks\nfor molecular self-assembly in organic solvents, but their application\nin water is less common due to competition from water molecules for\nthe metal center. However, Co(III) metalloporphyrins are notable for\ntheir strong binding to two aromatic amine ligands in aqueous buffers.\nIn this study, we present a comprehensive investigation of the binding\nbehavior of Co(III) tetraphenyl sulfonic acid porphyrin with selected\naromatic and aliphatic amines in aqueous solution. Our findings reveal\nthat the ligand affinity is influenced by the p<i>K</i><sub>a</sub> values of both the ligand and the porphyrin, as well as the\nhybridization state of the nitrogen atom, with binding to sp<sup>3</sup>-hybridized nitrogen being significantly weaker than to sp<sup>2</sup>-hybridized nitrogen. DFT calculations further suggest that the variations\nin binding affinities are due to differences in the electrostatic\npotential at the nitrogen atoms, with aromatic ligands generally exhibiting\nstronger Co–N coordination due to greater electrostatic attraction.\nMoreover, our study and the binding model we developed demonstrate\nthat changes in pH affect the affinity for each ligand to varying\ndegrees, sometimes resulting in an allosteric cooperative effect.\nThis effect is linked to electronic changes introduced by the binding\nof the first ligand. Our model provides a predictive tool for understanding\nthe assembly behavior of these porphyrins in aqueous buffers, with\npotential applications in developing more efficient catalysts and\nin the creation of smart materials for fields ranging from catalysis\nto nanomedicine and optoelectronics