Observation of a Rectangular Columnar Phase in Condensed Lamellar Cationic Lipid-DNA Complexes

We report on a synchrotron x-ray diffraction study of saturated cationic lipid complexed with DNA. In the lipid gel phase $L_{{\beta }^{\prime }}^c$ the complexes exhibit Bragg reflections due to lamellar lipid bilayer stacking and three nonintegral peaks in agreement with an intercalated centered rectangular columnar superlattice of DNA. The diffuse broadening of the DNA peaks is caused by in-plane translational disorder of the DNA strands and is approximated by Lorentzians with positional correlation lengths of ${\xi }_y\approx 250Å$ out of plane and ${\xi }_x>2000\mathrm{}Å$ in plane. We interpret our results in terms of a system of interacting two-dimensional smectic layers.