Porphyrin-membrane interactions: binding or partition?

Michal Rotenberg, Rimona Margalit*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


Porphyrins are photodynamic drugs employed in an experimental tumor-treatment modality in which cell membranes are one of the primary drug-action sites. To gain insight into the nature of the interaction of these drugs with those primary sites we have studied the affinity of porphyrins to the lipid moieties of biological membranes, at the molecular level. The association of porphyrins to large unilamellar liposomes, modeling the lipid regions of biological membranes was studied (at equilibrium) for deuteroporphyrin IX and protoporphyrin IX, at neutral pH and 37°C, taking into account porphyrin aggregation. Two thermodynamic approaches were investigated: (i) Simple partition equilibria between the external aqueous phase and the lipid bilayer, for drug monomers and dimers. (ii) Binding equilibria of drug monomers and dimers to the lipid bilayer. Using two types of experimental design and processing the data according to the expectations of both approaches, three different models for the binding (differing in the participation assigned to the dimer) were consdiered. Our major findings are: (a) The data clearly do not fit with the expectations for simple partition equilibria, nor with binding models assuming direct participation of the dimers. (b) The data fit well with a binding process, in which the membrane binds the porphyrin monomers only, with the dimers participating indirectly through the aqueous dimerization equilibrium. (c) At 37°C and neutral pH for liposomes composed of phosphatidylcholine / cholesterol at molar ratios of 3:2, we found for both investigated species a binding constant of 2.3 · 104. (d) For each species the binding constant is independent of the initial and final states of drug aggregation in the aqueous phase.

Original languageEnglish
Pages (from-to)173-180
Number of pages8
JournalBBA - Biomembranes
Issue number1
StatePublished - 27 Nov 1987


FundersFunder number
National Cancer InstituteR01CA033739
U.S. Public Health Service5 R01 CA33739-03a


    • Deuteroporphyrin
    • Liposome
    • Porphyrin-membrane interaction
    • Protoporphyrin


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