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

Abstract

FTIR spectroscopy is used to study structural aspects of ternary complexes formed by the cationic lipid dimyristoyltrimethylammoniumpropane (DMTAP), the zwitterionic lipid dimyristoylphosphatidylcholine (DMPC), and deoxyribonucleic 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 thermotropic phase behaviour of lipid within the complex. The IR spectra of DNA intercalated between cationic lipid bilayers are determined by subtracting corresponding pure lipid spectra from lipid–DNA complex spectra. These difference spectra indicate deviations of lipid–associated DNA from B-form DNA. Furthermore, two additional water bands arise at positions different from those known for lipid- and DNA-bound water which are indicative of two distinct states of hydration in lipid–DNA complexes. The pure lipid DMTAP exhibits unusual spectroscopic features at the temperature of chain melting, T_m, near 53°C, which are attributed to the existence of a crystalline, headgroup-interdigitated phase existing at temperatures below T_m, in accordance with X-ray diffraction and differential scanning calorimetry (DSC) data.