K. Wagner, D. Harries, S. May, V. Kahl, J.O Rädler and A. Ben-Shaul
Counterion Release upon Cationic Lipid-DNA Complexation,
submitted to Langmuir

The cooperative condensation of DNA and cationic liposomes into an ordered lipid-DNA composite phase in aqueous solution, is associated with the release of partially bound counterions. We directly determine the extent of counterion release by separating the supernatant from the precipitated condensates, measuring the conductivity of the solution before and after the phase transition. The extent of counterions released is calculated for a range of lipid/DNA concentration ratios, based on the nonlinear Poisson-Boltzmann theory. Both experiment and theory show maximal, essentially complete, release of counterions at the isoelectric point, where the positive (lipid)/negative (DNA) charge ratio is 1:1. Furthermore, at this point the entropic contribution to the condensation free energy is maximal and dominant.