Adsorption and photodynamic efficiency of meso-tetrakis(p-sulfonatophenyl)porphyrin on the surface of bilayer lipid membranes.

The adsorption and photodynamic efficiency of 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrin (HTPPS) on bilayer lipid membranes (BLM) have been studied. The adsorption of HTPPS on BLM leads to rising of the potential drop on the membrane/water interface which has been detected either by the intramembrane field compensation (IFC) method, or as ζ-potential of liposomes measured by the dynamic light scattering method. The dependence of this potential on the concentration of HTPPS and KCl in the solution can be described in the frame of Gouy-Chapman model of diffuse double layer assuming that the molecules of HTPPS adsorb on the surface of BLM as an anions with four charged groups. The potential disappeared when the pH of solution decreased from 6 to 3 allowing the conclusion that the protonated forms of HTPPS can not adsorb on the BLM probably due to change of conformation or aggregation of the molecules. The photodynamic efficiency of HTPPS was evaluated by measuring the rate of damage of the targets - molecules of styryl dye (di-4-ANEPPS) by singlet oxygen generated under illumination on the surface of BLM. This rate was proportional to the surface density of HTPPS molecules on the membrane which was determined from the change of surface charge of the membrane due to adsorption of the HTPPS. These results indicate that the di-4-ANEPPS molecules are damaged by singlet oxygen generated by monomers of HTPPS molecules adsorbed on the membrane. The rate of oxidation of di-4-ANEPPS molecules adsorbed on the same (cis) side of the membrane together with the HTPPS molecules was either the same or higher than that when di-4-ANEPPS molecules were adsorbed on opposite (trans) side. It indicates that the quenching of singlet oxygen by the di-4-ANEPPS molecules at cis side of the membrane was negligible, in contrast to our earlier study when singlet oxygen was generated by aluminum(III) phthalocyanines with one or two peripheral sulfo groups. The difference between these phthalocyanines and HTPPS was explained by their different adsorption depth in the membrane.