FTIR spectroscopic characterization of a cationic lipid-DNA complex and its components

FTIR spectroscopy is used to study structural aspects of ternary complexes formed by the cationic lipid dimyristoyltrimethylammoniumpropane (DMTAP), t he zwitterionic lipid dimyristoylphosphatidylcholine (DMPC), and deoxyribon ucleic acid (DNA). Spectra of the single components are compared with those obtained for both equimolar DMPC-DMTAP mixture and lipid-DNA complex. The IR spectra of mixed lipid-DNA phases are strongly dominated by the lipidic absorption bands. This allows one to easily monitor, in particular, the the rmotropic phase behaviour of lipid within the complex. The IR spectra of DN A intercalated between cationic lipid bilayers are determined by subtractin g corresponding pure lipid spectra from lipid-DNA complex spectra. These di fference spectra indicate deviations of lipid-associated DNA from B-form DN A. Furthermore, two additional water bands arise at positions different fro m those known for lipid- and DNA-bound water which are indicative of two di stinct states of hydration in lipid-DNA complexes. The pure lipid DMTAP exh ibits unusual spectroscopic features at the temperature of chain melting, T -m, near 53 degrees C, which are attributed to the existence of a crystalli ne, headgroup-interdigitated phase existing at temperatures below T-m, in a ccordance with X-ray diffraction and differential scanning calorimetry (DSC ) data.