Interaction of the Tetra(4-sulfonatophenyl)porphyrin with Ionic Surfactants: Aggregation and Location in Micelles

Abstract

Interactions of the water-soluble tetra(4-sulfonatophenyl)porphyrin (TPPS4) with ionic micelles in aqueous solutions have been studied by optical absorption, fluorescence, resonance light-scattering (RLS), and 1H NMR spectroscopies. The presence of three different species of TPPS4 in cationic cetyltrimethylammonium chloride (CTAC) solution has been unequivocally demonstrated: free porphyrin monomers, monomers bound to micelles, and nonmicellar porphyrin/surfactant aggregates. This result is similar to our previous findings for TPPS4 interactions with biomacromolecules (serum albumin and DNA). The surfactant:porphyrin ratio for maximum aggregate formation is around 4:1−5:1 and 14:1 at pH 3.0 and pH 7.5, respectively. In the case of zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) the aggregates were not observed. Binding constants estimated from these data were of the order of 104 M-1 for CTAC and HPS. Our data show that solubilization of porphyrins within nonpolar regions of micelles is determined, in general, by nonspecific hydrophobic interactions, yet it is significantly modulated by electrostatic factors. NMR chemical shift, T1, and nuclear Overhauser effect data indicate that TPPS4 is located mainly in the hydrophobic core in CTAC and HPS micelles, while in lysophosphatidylcholine its involvement in the polar area is more significant.