Interaction of the tetra(4-sulfonatophenyl)porphyrin with ionic surfactants: Aggregation and location in micelles

Interactions of the water-soluble tetra(4-sulfonatophenyl)porphyrin (TPPS4) with ionic micelles in aqueous solutions have been studied by optical abso rption, fluorescence, resonance light-scattering (RLS), and H-1 NMR spectro scopies. The presence of three different species of TPPS4 in cationic cetyl trimethylammonium chloride (CTAC) solution has been unequivocally demonstra ted: free porphyrin monomers, monomers bound to micelles, and nonmicellar p orphyrin/surfactant aggregates. This result is similar to our previous find ings 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 zwitteri onic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) the aggreg ates were not observed. Binding constants estimated from these data were of the order of 10(4) M-1 for CTAC and HPS. Our data show that solubilization of porphyrins within nonpolar regions of micelles is determined, in genera l, by nonspecific hydrophobic interactions, yet it is significantly modulat ed by electrostatic factors. NMR chemical shift, T-1, and nuclear Overhause r 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.