ELECTROSTATICS OF DNA CATIONIC LIPID COMPLEXES - ISOELECTRIC INSTABILITY

We propose a Poisson-Boltzmann electrostatic theory for DNA/cationic l ipid complexes modeled as a stack of aligned DNA chains intercalated w ith lipid bilayers, a structure suggested by the recent X-ray synchrot ron studies of Radler et al. Poisson-Boltzmann theory is shown to pred ict that the isoelectric point - where the DNA and cationic lipid char ges are in balance - is unstable against absorption of extra DNA or li pid material. The instability is caused by the entropy gain obtained f ollowing the release of small ions inside the complex and is manifeste d by singular behavior of the rod-rod spacing near the isoelectric poi nt. We apply the theory to a discussion of the results of Radler et al .