Abstract
The effect of ionic strength and surface charge density of the lipoplexes formed from the cationic surfactant octadecyltrimethylammonium bromide (OTAB) and the neutral phospholipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) on DNA condensation was studied using fluorescence spectroscopy. The structure of lipoplexes was examined by small- and wide-angle X-ray scattering (SAXS/WAXS). The efficiency of DNA condensation was monitored by relative changes in the intensity of the emission of the fluorescence probe ethidium bromide (EtBr). A high OTAB/DNA charge ratio of 10:1 is required for efficient condensation at all ionic strengths. The increasing mole ratio of OTAB/DOPC supports DNA condensation, while the process is significantly hindered by a high ionic strength. Only ~55% of DNA was found to be condensed by OTAB/DOPC = 1 mol mol−1 complexes in 150 mmol l−1 of NaCl. A condensed fluid lamellar phase (Lαc) was detected in the DOPC + OTAB + DNA complexes. The SAXS patterns show regular DNA–DNA packing only in complexes prepared in a medium of low ionic strength (<50 mmol l−1). Interestingly, free DNA in the supernatant of DOPC + OTAB + DNA complexes was not detected by UV/Vis spectrometry, indicating its trapping in the lipoplexes.