Effect of electrostatic interactions on binding and retention of DNA oligomers to PNA liposomes assessed by FRET measurements.

A FRET-based method is used to observe the desorption of di-alkyl peptide nucleic acid amphiphiles (PNAA) from liposomes occurring on binding of complementary DNA oligomers. PNA liposomes were prepared containing fluorescein-labeled PNAA and rhodamine-labeled dipalmitoylphosphoethanolamine (DPPE). These liposomes showed efficient energy transfer from the fluorescein to rhodamine, with an average donor-to-acceptor distance of 5.91nm. In low-ionic-strength buffer (50mM Tris-HCl, pH 8.0), the FRET signal was maintained in the presence of a stoichiometric amount of 10- and 20-mers DNA complements, but the signal attenuated for 40-mer complements, indicating that DNA first binds the PNAA before the PNAA/DNA duplex desorbs from the lipid bilayer. The FRET signal was maintained in the presence of 10-, 20-, 40-, and 60-mer DNA in high ionic-strength buffer, showing that the driving force for the desorption is electrostatic repulsion between the bound DNA oligomer and the liposome surface. This conclusion is corroborated by comparison of the PNA/DNA binding energy, the energy of adsorption of the di-alkyl PNAA to the lipid bilayer, and a calculation of the DNA/lipid bilayer electrostatic repulsion using the linearized Poisson-Boltzmann equation.