riDOM, a Cell-Penetrating Peptide. Interaction with DNA and Heparan Sulfate

DNA condensation in the presence of polycationic molecules is a well-known phenomenon exploited in gene delivery. riDOM (retro-inverso dioleoylmelittin) is a cell-penetrating peptide with excellent transporter properties for DNA. It is a chimeric molecule where ri-melittin is fused to dioleoylphosphoethanolamine. The physical-chemical properties of riDOM in solution and in the presence of DNA and heparan sulfate were investigated with spectroscopic and thermodynamic methods. Dynamic light scattering shows that riDOM in solution aggregates to well-defined nanoparticles with a diameter of similar to 13 nm and a xi-potential of 22 mV, composed of about 220-270 molecules. Binding of riDOM to DNA was studied with dynamic light scattering, xi-potential measurements, and isothermal titration calorimetry and was compared with microscopic binding constant of S X 10(7) M-1 and a stoichiometry of 12 riDOM per 10 DNA base pairs. In the complex the DNA double strand is completely shielded by the more hydrophobic riDOM molecules. Authentic melittin binds to DNA with a much lower binding constant of 5 X 10(6) M-1 and lower stoichiometry of 5 melittin per 10 DNA base pairs. The binding enthalpies for riDOM and melittin are small and the binding reactions are entropy-driven. Sulfated glycosaminoglycans such as heparan sulfate are also linear molecules with a negative charge. riDOM binding to heparan sulfate on cell surfaces can therefore interfere with DNA-riDOM binding. riDOM-heparan sulfate complex formation was characterized by isothermal titration calorimetry and spectroscopic methods. The binding constant of riDOM for heparan sulfate is K approximate to 2 X 10(6) M-1. Authentic melittin has a similar binding constant but riDOM shows a 3-fold higher packing density on heparan sulfate than the distinctly smaller melittin.