Conventional and gemini surfactants embedded within bilayer membranes: Contrasting behavior

Laser microelectrophoresis (coupled with conductance, fluorescence, and dyn amic light scattering) is shown to be a highly instructive tool in comparin g the dynamics of conventional and gemini surfactants embedded within vesic le bilayers. The following can be listed among the more important observati ons and conclusions: a) Cationic conventional surfactant, added to a "solid " (gel) lipid vesicle containing an anionic phospholipid, charge-neutralize s only half the anionic charge. With a "liquid" (liquid crystalline) vesicl e, however. the entire negative charge is neutralized. Thus, the cationic c onventional surfactant can "flip-flop" readily only in the liquid membrane. b) A cationic gemini surfactant charge-neutralizes only the anionic lipid in the outer membrane leaflet of either solid or liquid membranes, thus ind icating an inability to flip-flop regardless of the phase-state of the bila yer. c) Mixed population experiments show that surfactants can hop from one vesicle to another in liquid but not solid membranes. d) In liquid, but no t solid, bilayers, a surface-adsorbed cationic polymer can electrostaticall y "drag" anionic surfactant from the inner leaflet to the outer leaflet whe re the polymer resides. e) Peripheral fluorescence quenching experiments sh ow that a cationic polymer, adhered to anionic vesicles, can be forced to d issociate in the presence of high concentrations of salt or an anionic poly mer. f) Adsorbed polymer, of opposite charge to that imparted to vesicles b y a gemini surfactant. is unable to dislocate surfactant even in a liquid m embrane. g) In our systems, ionic polymers will not bind to neutral vesicle s made solely of zwitterionic phospholipid. On the other hand, ionic polyme rs bind to neutral vesicles if charge neutrality is obtained by virtue of t he membrane containing equimolar amounts of cationic and anionic surfactant . This is attributable to surfactant segregation within the bilayer. h) Exp eriments prove that polymer migration can occur among a population of neutr al ternary vesicles.