The effects of porphyrin structure and aggregation state on photosensitized processes in aqueous and micellar media.

Abstract

AbstractThe efficiency of several porphyrins at 10 μM and 83 μM as sensitizers of the photooxidation of 0.1 mM tryptophan and histidine via a singlet oxygen‐mechanism was studied in pH 7.4‐buffered aqueous solutions and in aqueous dispersions of Triton X‐100 micelles. The porphyrins were either solubilized in the bulk aqueous medium or associated with the micellar phase, whereas the amino acids were always located in the aqueous phase. With those porphyrins, such as uroporphyrin I, meso‐tetra (4‐sulfonatophenyl)porphine, meso‐tetra(4‐carboxyphenyl)porphine and meso‐tetra)N,N,N‐trimethylanilinium)porphine, which are > 98% monomeric in both media, the efficiency of histidine photooxidation was independent of the site of O2(1Δg) generation, as shown by the closely similar values for the photooxidation rate constant and oxygen‐consumption quantum yield in the presence and absence of Triton micelles; the same indications were provided by photokinetic experiments with tryptophan. Actually, laser flash photolysis studies showed that the micelle‐incorporation of the above mentioned porphyrins brought about only minor changes in their photophysical properties, including the relative yield of O2(1Δg) generation. On the other hand, hematoporphyrin IX, its Zn2+‐complex, and coproporphyrin III are largely aggregated in homogeneous aqueous solution; their incorporation into Triton micelles caused an increase of the triplet quantum yield and an enhancement of the oxygen‐consumption quantum yield and photooxidation rate constant for both histidine and tryptophan. The lower photosensitizing efficiency of aggregated porphyrin species in comparison with the corresponding monomeric porphyrin was confirmed by measuring the initial rate and quantum yield of oxygen consumption upon irradiation of 1 mM histidine and tryptophan in the presence of different hematoporphyrin concentrations within the 0.3‐100μM range.