Self-assembled DNA–cationic-lipid complexes: Two-dimensional smectic ordering, correlations, and interactions

We report a synchrotron small-angle x-ray scattering (SAXS) study of the mutilayered, self-assembled structure (complex) that is formed by mixing DNA with cationic liposomes. In these complexes the DNA is confined between charged lipid bilayers and orders as a two-dimensional (2D) smectic liquid crystal. The power-law bilayer-bilayer correlations of the 3D multilayer smectic liquid crystal, which are coupled to the 2D lattice of DNA chains, are found to deviate significantly from those described by the standard Caillé model of smectic-$A$ phases. To model the DNA ordering, the 2D smectic correlation function and the corresponding structure factor are derived from the smectic Hamiltonian in harmonic approximation. The resulting line shape is then fitted to the DNA correlation peak. It is found that for samples of higher $d,$ short-range correlations between the DNA in adjacent sheets have to be assumed to explain the data. From the least-square fitting, the 2D DNA interchain compressibility modulus $B$ is extracted as a function of $d$ and discussed in view of different possible microscopic interactions responsible for the ordering.