Scm. Gandini et al., Interaction of the tetra(4-sulfonatophenyl)porphyrin with ionic surfactants: Aggregation and location in micelles, LANGMUIR, 15(19), 1999, pp. 6233-6243
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.