Spectroscopic studies of water-soluble porphyrins with protein encapsulated in bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles: Aggregation versus complexation

We have investigated the interaction of two water-soluble freebase porphyrins (negatively charged meso-tetrakis(p-sulfonatophenyl)porphyrin sodium salt (TSPP) and positively charged meso-tetrakis (N-methyl pyridinium-4-yl)porphyrin (TMpyP)) with two drug-carrier proteins (human serum albumin (HSA) and beta-lactoglobulin (PLG)) in bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane/water reverse micelles (RM) by using steady-state and transient-state fluorescence spectroscopy. TSPP exhibited a complex pattern of aggregation on varying the RM size and pH in the absence of the protein: at low omega(0) (the ratio of water concentration to AOT concentration, the emission of H-aggregates prevails under acidic or neutral "pH(ext)" conditions. Upon formation of the water-pool, J-aggregates and monomeric diacid species dominate at low "pH(ext)" but only monomer is detected at neutral "pH(ext)" The aggregation number increases with omega(0) and the presence of the protein does not seem to contribute to further growth of the aggregate. The presence of protein leads to H-deaggregation but promotes J-aggregation up to a certain protein/porphyrin ratio above which, complexation with the monomer bound to a hydrophobic site of the protein prevails. The effective complex binding constants are smaller than in free aqueous solution; this indicates a weaker binding in these RM probably due to some conformational changes imposed by encapsulation. Only a weak quenching of TMpyP fluorescence is detected due to the presence of protein in contrast to the negative porphyrin.