Characterization of the DNA and Membrane Interactions of a Bioreducible Cell-Penetrating Foldamer in its Monomeric and Dimeric Form

The biophysical properties of a designed bioreducible oligourea foldamer, which shows excellent transfection activities in its dimeric form are presented. Binding isotherms of the monomer as well as of the dimer to both DNA and lipid membranes were determined by indole fluorescence. Comparing the monomer with the dimer allows both a precise biophysical characterization of the role of dimerization and characterization of how the covalent linkage between two monomers affects the transfection activity. The results indicate that dimerization results in pronounced changes in the thermodynamics of different steps of the transfection process, which extend well beyond simple steric effects within the dimer. A model emerges where the imidazole-containing polymers compact DNA at neutral pH, but they liberate the polyurea from the DNA complex at low pH, thus being able to rupture acidified endosomes. Indeed, the dimerization inverts the pH dependence of the binding affinities toward the requirements suggested by this model for efficient transfection.