Abstract
We investigated the thermotropic behavior of fully hydrated saturated cationic lipid mixture of DMPC/DMTAP (dimyristoylphosphatidyl-choline/dimyristoyl-trimethyl-am monium propane) complexed with DNA. Using simultaneous small- and wide-angle X-ray scattering, we found two condensed lamellar phases, a fluid Lcα- and a gel-state Lcβ'-phase. The chain-melting transition was accompanied by a decrease in the bilayer thickness and an expansion of the intercalated one-dimensional DNA rod lattice. The interaxial DNA-DNA spacings are quantitatively described by conservation of local volume fractions for both phases in excess water. The line broadening of the wide-angle scattering exhibited a systematic variation in the hydrocarbon chain tilt as a function of the cationic lipid molar fraction.