Characterization of mixed monolayers of phosphatidylcholine and a dicationic gemini surfactant SS-1 with a Langmuir balance: Effects of DNA

Monolayers of a cationic gemini surfactant, 2,3-dimethoxy-1,4-bis(N-hexadec yl-N;N-dimethyl-ammonium)butane dibromide (abbreviated as SS-1) and its mix tures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were stu died using a Langmuir balance. More specifically, we measured the force-are a (pi -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 (XSS-1), with and without DNA in the subphase. Both SS- 1 and POPC exhibited smooth compression isotherms, indicating their monolay ers to be in the liquid expanded state. Even low contents (XSS-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 XSS-1 and is not observed for mixed films of dioleoylglyce rol and SS-1. Reorientation of the P--N+ dipole is further supported by the pronounced increase in monolayer dipole potential psi due to SS-1. The pre sence of DNA in the subphase affected the mixed POPC/SS-1 monolayers differ ently depending on the constituent lipid stoichiometry as well as on the DN A/SS-1 charge ratio. At a DNA concentration of 0.63 muM (in base pairs) con densation of neat POPC monolayers was evident, and this effect remained up to XSS-1 < 0.5, corresponding to DNA/SS-1 charge ratio of 1.25. An expansio n due to DNA, evident as an increase in DeltaA/molecule, was observed at XS S-1 > 0.5. At a higher concentration of DNA (1.88 muM base pairs) in the su bphase corresponding to DNA/SS-1 charge ratio of 3.75 at XSS-1 = 0.5, conde nsation was observed at all values of XSS-1.