Acid—base properties of protoporphyrin IX; its dimethyl ester and heme solubilized on surfactant micelles: spectrophotometric and fluorometric titration

Abstract

Absorption and fluorescence spectra have been obtained and spectrophotometric and fluorometric titration curves recorded for protoporphyrin IX (H 2P(COOH) 2) and its dimethyl ester (H 2P(COOCH 3) 2) in their aqueous solutions, in Triton X-100 and cetyl trimethylammonium bromide (CTAB) micelles. The spectrophotometric titration of heme in Triton X-100 micelles has been performed. The pK of protoporphyrin pyrrole nitrogens were equal to about 6.5 in water solution and to 0.8 and 0.7 in Triton X-100 and CTAB micelles, respectively. The hematin-hemin pK was 3.75 in Triton X-100 micelles. The pK shifts were explained as due to the low effective dielectric constant in Triton X-100 and CTAB micelle nuclei. The pK values of propionic acid residues were determined for H 2P(COOH) 2 and heme in the micelles. A considerable decrease in the critical micelle concentration of surfactants in the presence of protoporphyrins has been observed. It has been suggested that the porphyrin macrocycle is located in the micelle nucleus whereas the propionic acid residues are in the Stern layer.