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-sulfonatophen yl) porphyrins, FeTPPS4 and ZnTPPS4, with ionic and nonionic micelles in aq ueous solutions have been studied by optical absorption, fluorescence, reso nance light-scattering (RLS), and H-1 NMR spectroscopies. The presence of t hree different species of both Fe(III)- and Zn(II)TPPS4 in cationic cetyltr imethylammonium chloride (CTAC) solution has been unequivocally demonstrate d: free metalloporphyrin monomers or dimers (pH 9), metalloporphyrin monome rs or aggregates (possibly mu -oxo dimers) bound to the micelles, and nonmi cellar metalloporphyrin/surfactant aggregates. The surfactant:metalloporphy rin 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 zwit terionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and no n-ionic polyoxyethylene lauryl ether (Brij-35) and t-octylphenoxypolyethoxy etanol (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 a bsorption data and are of the order of 10(4) M-1 for both CTAC and HPS, val ues 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 ZnTP PS4 at pH 4.0 is a factor of 3-5 lower than those for CTAC and HPS, while i n 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 de termined, in general, by nonspecific hydrophobic interactions, yet it is mo dulated 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 t he bound aggregated form (possibly mu -oxo dimers) remains. The metalloporp hyrins were incorporated into the micelles near the terminal part of their hydrocarbon chains, as evidenced by a strong upfield shift of the correspon ding peaks of the surfactants. (C) 2001 Elsevier Science B.V. All rights re served.