Characterization of Mixed Monolayers of Phosphatidylcholine and a Dicationic Gemini Surfactant SS-1 with a Langmuir Balance: Effects Of DNA

Abstract

Monolayers of a cationic gemini surfactant, 2,3-dimethoxy-1,4-bis( N-hexadecyl- N; N-dimethyl-ammonium)butane dibromide (abbreviated as SS-1) and its mixtures with 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC) were studied using a Langmuir balance. More specifically, we measured the force-area ( π- A) curves and determined the elastic area compressibility modulus ( C s −1) as a function of lateral packing pressure and the mole fraction of the cationic lipid ( X SS-1), with and without DNA in the subphase. Both SS-1 and POPC exhibited smooth compression isotherms, indicating their monolayers to be in the liquid expanded state. Even low contents ( X SS-1 < 0.05) of SS-1 in a POPC monolayer condensed the film dramatically, up to 20% at 30 mN/m. This effect is suggested to reflect reorientation of the P −-N + dipole of the POPC headgroup. Accordingly, the magnitude of the condensing effect diminishes with X SS-1 and is not observed for mixed films of dioleoylglycerol and SS-1. Reorientation of the P −-N + dipole is further supported by the pronounced increase in monolayer dipole potential ψ due to SS-1. The presence of DNA in the subphase affected the mixed POPC/SS-1 monolayers differently depending on the constituent lipid stoichiometry as well as on the DNA/SS-1 charge ratio. At a DNA concentration of 0.63 μM (in base pairs) condensation of neat POPC monolayers was evident, and this effect remained up to X SS-1 < 0.5, corresponding to DNA/SS-1 charge ratio of 1.25. An expansion due to DNA, evident as an increase in Δ A/molecule, was observed at X SS-1 > 0.5. At a higher concentration of DNA (1.88 μM base pairs) in the subphase corresponding to DNA/SS-1 charge ratio of 3.75 at X SS-1 = 0.5, condensation was observed at all values of X SS-1.