DOPC + OTAB + DNA Complexes – Effect of Ionic Strength and Surface Charge Density on DNA Condensation

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 (L_α^c) 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.