Thermotropic structural changes of saturated-cationic-lipid-DNA complexes

We investigated the thermotropic behavior of Fully hydrated saturated catio nic lipid mixture of DMPC/DMTAP (dimyristoylphosphatidyl-choline/dimyristoy l-trimethyl-ammonium propane) complexed with DNA. Using simultaneous small- and wide-angle X-ray scattering, we found two condensed lamellar phases, a fluid L-alpha(c)- and a gel-state L-beta'(c)-,phase. The chain-melting tra nsition was accompanied by a decrease in the bilayer thickness and an expan sion of the intercalated one-dimensional DNA rod lattice. The interaxial DN A-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.