1986 Volume 59 Issue 2 Pages 515-523
The morphological behavior of various combinations of the following synthetic lipids was investigated in the aqueous dispersion state by means of electron microscopy, differential scanning calorimetry, and fluorescence polarization spectroscopy as well as by turbidity and X-ray diffraction measurements: N+C5Ala2Cn and N+(H)C5Ala2Cn as cationic, (SO3−)C5Ala2Cn as anionic, and QC5Ala2Cn as nonionic (n=14, 16). Aqueous dispersions of various combinations of cationic and anionic lipids at the equimolar ratio afforded nonlamellar aggregates, having highly ordered and three-dimensional network structure with repeating distances in the 70–130 Å range, above their phase transition temperatures (Tm). However, any slight loss of the fractional balance retained at the equimolar ratio led to the formation of bilayer aggregates. Under such conditions that allow the formation of nonlamellar aggregates, the high microscopic homogeneity of cationic and anionic lipid molecules was attained, originating from the intramembrane electrostatic interaction between cationic and anionic head moieties to form highly developed ion-pair clusters on the aggregate surface. Meanwhile, the nonlamellar phase appeared above Tm was spontaneously transformed into the normal bilayer phase below Tm as confirmed by electron microscopy and turbidity measurements. The nonlamellar phase was also characterized by the low-angle X-ray diffraction method and was consistent with the highly developed domain with the face-centered cubic lattice.
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