29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

THE VERTICAL DEPTH OF PORPHYRIN IN A LIPID BILAYER AND THE MEMBRANE PROPERTIES AFFECT THE SENSITIZING EFFICIENCY

Abstract-
Hydrophobic porphyrin sensitizers partition mainly into lipid environments. Singlet oxygen that is generated by such membrane-bound sensitizer diffuses very rapidly, within <1 sec, out of the lipid domain into the water phase, where it decays very rapidly. Membrane cellular damage can therefore be caused only while singlet oxygen is diffusing in the membrane. Deeper insertion of the sensitizer in the membrane should increase the dwell time of singlet oxygen in the membrane and enhance the sensitizing efficiency. We synthesized and employed a series of protoporphyrin derivatives with alkyl carboxylate groups of varying lengths. They partitioned in the membrane with the charged carboxylate end of the molecule anchored at the lipid/water interface and with the tetrapyrrole ring residing at varying depths. The relative penetration depths were determined by quenching of their fluorescence with iodide ions or with spin probes. A strong depth-dependent effect on the sensitization efficiency of a membrane-bound target was obtained between the shortest and longest PP derivatives, where the depth changed by only a few Angstroms. When DMPC was added to the membrane, the porphyrins were pushed deeper, towards the inter layer area. This relocation decreased the extent of damage caused to a bilayer-residing target. We observed a strong change in the sensitizing efficiency at the membrane's gel-to-liquid phase transition. This arises from a temperature effect on the diffusion of singlet oxygen below and above the phase transition temperature.

Keywords: photosensitization, lipid bilayer, singlet-oxygen diffusion