Conventional and Gemini Surfactants Embedded within Bilayer Membranes: Contrasting Behavior Alexander A. Yaroslavov,* [a] Oleg Yu. Udalykh, [a] Nickolay S. Melik-Nubarov, [a] Viktor A. Kabanov, [a] Yuri A. Ermakov, [b] Vladimir A. Azov, [c] and Fredric M. Menger* [c] Abstract: Laser microelectrophoresis (coupled with conductance, fluores- cence, and dynamic light scattering) is shown to be a highly instructive tool in comparing the dynamics of conventional and gemini surfactants embedded within vesicle bilayers. The following can be listed among the more important obser- vations and conclusions: a) Cationic conventional surfactant, added to a ªsolidº (gel) lipid vesicle containing an anionic phospholipid, charge-neutraliz- es only half the anionic charge. With a ªliquidº (liquid crystalline) vesicle, how- ever, the entire negative charge is neu- tralized. Thus, the cationic conventional surfactant can ªflip-flopº readily only in the liquid membrane. b) A cationic gemini surfactant charge-neutralizes on- ly the anionic lipid in the outer mem- brane leaflet of either solid or liquid membranes, thus indicating an inability to flip-flop regardless of the phase-state of the bilayer. c) Mixed population ex- periments show that surfactants can hop from one vesicle to another in liquid but not solid membranes. d) In liquid, but not solid, bilayers, a surface-adsorbed cationic polymer can electrostatically ªdragº anionic surfactant from the inner leaflet to the outer leaflet where the polymer resides. e) Peripheral fluores- cence quenching experiments show that a cationic polymer, adhered to anionic vesicles, can be forced to dissociate in the presence of high concentrations of salt or an anionic polymer. f) Adsorbed polymer, of opposite charge to that imparted to vesicles by a gemini surfac- tant, is unable to dislocate surfactant even in a liquid membrane. g) In our systems, ionic polymers will not bind to neutral vesicles made solely of zwitter- ionic phospholipid. On the other hand, ionic polymers bind to neutral vesicles if charge neutrality is obtained by virtue of the membrane containing equimolar amounts of cationic and anionic surfac- tant. This is attributable to surfactant segregation within the bilayer. h) Ex- periments prove that polymer migration can occur among a population of neutral ternary vesicles. Keywords: gemini ´ laser microelec- trophoresis ´ membranes ´ polyelec- trolytes ´ vesicles Introduction Bilayer vesicles (hollow spheres composed of native or synthetic lipids) have potential use in drug targeting, [1±4] genetic transformation of cells, [5±9] cancer chemotherapy, [10±15] and in antibacterial and antiviral regimens. [16±20] Efficiency of vesicular preparations depends upon, among other parame- ters, the clearance rate of the vesicles from the bloodstream. The rates, in turn, are governed by the vesicle size and surface charge. Thus, it was shown that large vesicles clear faster than small ones, [10, 21] and that hepatic uptake occurs more readily with negative vesicles than with positive or neutral struc- tures. [22, 23] Having been impressed by the medical relevance of vesicles, and having had past experience not only with vesicles [24] but with surfactants [25] and polyelectrolytes, [26] we decided to investigate the effect of these latter materials on the size, charge, and dynamics of vesicular systems. The hope was to acquire basic information on how lipid, surfactant, and polymer interact with each other at the molecular level and, thereby, to control critical physical/chemical/biological prop- erties of the vesicles. Various preparative methods have been developed that allow vesicle size to be varied from ªsmallº (20 ± 100 nm) to ªlargeº (100 ± 200 nm) to ªgiantº (5 ± 100 mm). [27] For exam- ple, sonication of phospholipid films usually gives small [a] Prof. A. A. Yaroslavov, Dr. O. Yu. Udalykh, Dr. N. S. Melik-Nubarov, Prof. V. A. Kabanov Polymer Department, School of Chemistry Lomonosov Moscow State University Leninskie Gory, 119899 Moscow (Russia) Fax: ( 7) 095-939-0174 E-mail: yaroslav@genebee.msu.su [b] Dr. Yu. A. Ermakov Frumkin Research Institue of Electrochemistry Russian Academy of Science Leninsky Prospekt 32, 117234 Moscow (Russia) [c] Prof. F. M. Menger, V. A. Azov Department of Chemistry, Emory University Atlanta, Georgia 30322 (USA) Fax: ( 1) 404-727-6586 E-mail: menger@emory.edu FULL PAPER Chem. Eur. J. 2001, 7 , No. 22  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0947-6539/01/0722-4835 $ 17.50+.50/0 4835