Spectroscopic studies of the interaction of cationic water-soluble iron(III) meso-tetrakis(4-N-methylpyridiniumyl)porphyrin (FeTMPyP) with ionic and nonionic micelles

Abstract

Interactions of cationic FeTMPyP with ionic and nonionic micelles have been studied by optical absorption, resonance light scattering (RLS) and 1 H NMR spectroscopies. The equilibrium behavior of FeTMPyP as a function of pH is described by several species in aqueous solution. The presence of phosphate anions leads to the existence of additional species in the acid p H region, probably due to the coordination of phosphates to the iron. FeTMPyP solution as a function of pH in the presence of anionic SDS showed a simplified equilibrium in acidic pH region, favoring the transition to the dimeric species. Titration of FeTMPyP as a function of SDS surfactant concentration showed the presence of three different porphyrin species: free metalloporphyrin monomers (or dimers depending on pH), metalloporphyrin monomers (or dimers) bound to the micelles, and nonmicellar metalloporphyrin/surfactant aggregates. In the case of zwitterionic LPC and HPS, and nonionic TRITON X-100 the nonmicellar metalloporphyrin/surfactant aggregates were not observed. Binding constants were calculated from optical absorption data and have values of 2 × 103 M −1 for SDS being much smaller for HPS (58 M −1), LPC and TRITON X-100. Comparison with our previous data for anionic FeTPPS 4 shows that both the electrostatic factor and hydrophobic forces are relevant in the porphyrin-surfactant interaction: for FeTPPS 4 binding constants to cationic CTAC and zwitterionic HPS are of the same order of magnitude, 1-3 × 104 M −1; for FeTMPyP the delocalization of the positive charges from the periphery substituents into the macrocycle ring leads to reduction of both electrostatic attraction to the micelle as well as hydrophobic character of the porphyrin ring, leading to a 10-fold reduction of binding to the micelles of opposite charge to the porphyrin. NMR data indicated that FeTMPyP is bound to the micelles as an equilibrium of two forms of monomer at pH 2.0, and at pH 9.0 the bound aggregated form (possibly dimers) is observed predominantly with some amount of a monomeric form.