  We study a simple model of DNA divalent cationic surfactant complexation. We
find that the combination of electrostatic and hydrophobic effects leads to a
cooperative phenomenon in which as the amphiphile is added to the solution
containing DNA, a large fraction of the DNA's charge is neutralized by the
condensed divalent cationic surfactants, forming the surfoplex. This binding
transition occurs for concentrations that are lower for divalent than for
monovalent surfactants. Since the electrostatic strength is larger in the first
case and the amount of surfactant lower, we suggest that multivalent
amphiphilic molecules would be more efficient than monovalent for transfection.
