Interaction of Fe(III)- and Zn(II)-tetra(4-sulfonatophenyl) porphyrins with ionic and nonionic surfactants: aggregation and binding

Interactions of the water soluble Fe(III)- and Zn(II)-tetra(4-sulfonatophenyl) porphyrins, FeTPPS4 and ZnTPPS4, with ionic and nonionic micelles in aqueous solutions have been studied by optical absorption, fluorescence, resonance light-scattering (RLS), and H-1 NMR spectroscopies. The presence of three different species of both Fe(III)- and Zn(II)TPPS4 in cationic cetyltrimethylammonium chloride (CTAC) solution has been unequivocally demonstrated: free metalloporphyrin monomers or dimers (pH 9), metalloporphyrin monomers or aggregates (possibly mu -oxo dimers) bound to the micelles, and nonmicellar metalloporphyrin/surfactant aggregates. The surfactant:metalloporphyrin ratio for the maximum nonmicellar aggregate formation is around 5-8 for Fe(III)TPPS4 both at pH 4.0 and 9.0; for Zn(II)TPPS4 this ratio is 8, and the spectral changes are practically independent of pH. In the case of zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and non-ionic polyoxyethylene lauryl ether (Brij-35) and t-octylphenoxypolyethoxyetanol (Triton X-100), the nonmicellar aggregates were not observed in the pH range from 2.0 to 12.0. Binding constants were calculated from optical absorption data and are of the order of 10(4) M-1 for both CTAC and HPS, values which are similar to those previously obtained for the porphyrin in the free base form. For Brij-35 and Triton X-100 the binding constant for ZnTPPS4 at pH 4.0 is a factor of 3-5 lower than those for CTAC and HPS, while in the case of FeTPPS4 they are two orders of magnitude lower. Our data show that solubilization of ZnTPPS4 within nonpolar regions of n-ticelles is determined, in general, by nonspecific hydrophobic interactions, yet it is modulated by electrostatic factors. In the case of FeTPPS4, the electrostatic factor seems to be more relevant. NMR data indicated that Fe(III)TPPS4 is bound to the micelles predominantly as a monomer at pH 4.0, and at pH 9.0 the bound aggregated form (possibly mu -oxo dimers) remains. The metalloporphyrins were incorporated into the micelles near the terminal part of their hydrocarbon chains, as evidenced by a strong upfield shift of the corresponding peaks of the surfactants. (C) 2001 Elsevier Science B.V. All rights reserved.