Self-aggregation of free base porphyrins in aqueous solution and in DMPC vesicles

Free base porphyrin (PPhe), derivatized with aminosulfonyl groups linked to the aromatic amino acid phenylalanine at the meso-positions, was mixed with DMPC vesicles. The resulting interaction was studied by absorption, steady-state and transient state fluorescence, at different pHs. At pH=2 to pH=9, the aforementioned porphyrin predominates as an aggregated species, with a co-facial arrangement of the molecules taking into account the blue shift of the Soret band (414 nm for the monomer and 401 nm for the aggregate). Upon interaction with DMPC vesicles, the competing hydrophobic interactions with the bilayer destabilize the aggregated species in favor of monomer incorporation. Fluorescence lifetimes also show that the long component assigned to the monomer contributes only 30% to the overall decay in solution (e.g. pH=7.0) whereas in DMPC vesicles this contribution increases up to 85% independent of the solution pH, which confirms a location of the probe in an environment "protected" from free water. The picture changes completely in the case of TSPP, an anionic porphyrin which does not incorporate in DMPC vesicles. Remarkably, at pH=2.5 all the experimental findings point to the self-assembling of the porphyrin units in J-aggregates induced at the surface of the DMPC vesicle. In fact, upon removal of the aqueous solvent, we could define by fluorescence lifetime imaging microscopy (FLIM) regions where the fluorescence lifetime is that characteristic of the J-aggregate ( 0.11 ns).