title: Counterion-controlled transition of a cationic gemini from submicroscopic to giant vesicles.
abstract: While much is known about the self-assembly of lipids on nanoscale, our understanding of their biologically relevant mesoscale organization remains incomplete. Here, we show for a cationic gemini lipid a sharp and reversible transition from small vesicles with an average diameter of approximately 40 nm to giant vesicles (GVs) with an average diameter of approximately 11 microm. This transition is dependent on proper [NaCl] and specific temperature. Below this transition and in the vicinity of the air/water interface, a series of mesoscale morphological transitions was observed, revealing complex structures resembling biological membranes. On the basis of microscopy experiments, a tentative [NaCl] versus temperature shape/size phase diagram was constructed. To explain this unprecedented transition, we propose a novel mechanism whereby a specific interaction of Cl(-) counterion with the cationic gemini surfactant initiates the formation of a commensurate solute counterion lattice with low spontaneous curvature. In keeping with the high bending rigidity of NaCl crystal, this tightly associated ionic lattice enslaves membrane curvature and the mesoscale 3-D organization of this lipid.
authors.affiliation: Helsinki Biophysics and Biomembrane Group, Institute of Biomedicine/Biochemistry, Biomedicum, University of Helsinki, P.O. Box 63 (Haartmaninkatu 8), FIN-00014, Finland.
authors.forename: Samppa J
authors.initials: SJ
authors.lastname: Ryhänen
authors.forename: V Matti J
authors.initials: VM
authors.lastname: Säily
authors.forename: Mikko J
authors.initials: MJ
authors.lastname: Parry
authors.forename: Paola
authors.initials: P
authors.lastname: Luciani
authors.forename: Giovanna
authors.initials: G
authors.lastname: Mancini
authors.forename: Juha-Matti I
authors.initials: JM
authors.lastname: Alakoskela
authors.forename: Paavo K J
authors.initials: PK
authors.lastname: Kinnunen
mesh_terms: D002412:Cations / Q000737:chemistry; D008055:Lipids / Q000737:chemistry*; D008954:Models, Biological; D008968:Molecular Conformation; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D012965:Sodium Chloride / Q000737:chemistry; D013501:Surface-Active Agents / Q000737:chemistry*; D013696:Temperature
chemical_list: C440188:2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane; D002412:Cations; D008055:Lipids; D000644:Quaternary Ammonium Compounds; D013501:Surface-Active Agents; D012965:Sodium Chloride