X-ray diffraction and neutron scattering studies of amphiphile -: Lipid bilayer organization

The lipid bilayer thickness d(L), the transbilayer distance of lipid phosphate groups dpp and the lipid surface area AL of fluid hydrated bilayers of lamellar phases of egg phosphatidylcholine or dipalmitoylphosphatidylcholine containing N-alkyl-N,N-dimethylamine N-oxides (CnNO), 1,4-butanediammonium-N,N'-dialkyl-N,N,N',N'-tetramethyl dibromides (GSn) or monohydrochlorides of [2-(alkyloxy)phenyl]-2-(1-piperidinyl)ethylesters of carbamic acid (CnA) were obtained by X-ray diffraction, and the bilayer thickness in extruded unilamellar dioleoylphosphatidylcholine vesicles containing C12NO was obtained by the neutron scattering. The values of dL, dpp and AL change linearly up to the 1:1 amphiphile:lipid molar ratio. The slopes of these dependencies increase for dL and dpp and decrease for AL with an increasing number of carbons n in the amphiphile long hydrocarbon substituent (18 greater than or equal ton greater than or equal to8 for CnNO, 16 greater than or equal ton greater than or equal to9 for GSn, 12 greater than or equal ton greater than or equal to5 for CnA), while the opposite trends are observed for the short substituent (8 greater than or equal ton greater than or equal to6 for CnNO, 9 greater than or equal ton greater than or equal to7 for GSn, 5 greater than or equal ton greater than or equal to3 for CnA). In case of long substituents, the effects on dL, dpp and AL are caused by the decrease in the difference between the lipid and amphiphile hydrocarbon chain lengths and by the increase in their van der Waals attraction. The short substituent amphiphiles are mobile and exchange between multiple binding sites in the bilayer, minimizing the bilayer surface area.