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The elasticity of uniform unilamellar vesicles of acidic phospholipids during osmotic swelling is dominated by the ionic strength of the media


Biochemistry 26(17): 5439-5447
The elasticity of uniform unilamellar vesicles of acidic phospholipids during osmotic swelling is dominated by the ionic strength of the media
Uniform, unilamelar vesicles have been prepared by the pH-modification technique. The initial sizes of the vesicles were from 200 to 700 nm and were measured to within 1-3% by photo correlation spectroscopy. Vesicles were made of the dioleoyl esters of phosphatidic acid, phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, the diphytanyl ethers of phosphatidylglycerol, Escherichia coli lipids, and lac permease reconstituted into E. coli lipids. The vesicle suspensions were prepared and then diluted with electrolyte (KCl) and/or nonelectrolyte (sucrose, trehalose, pentaerythritol) impermeants. The amplitude of the swelling is linearly proportional to the osmotic pressure difference across the bilayer. We have determined the elastic modulus, the elastic limit (percent surface expansion at bursting), and the transbilayer pressure difference at bursting for each of these vesicles at constant osmolarity but at different ionic strengths. We find that the elastic properties of the bilayer vary by a factor of 10 in electrolyte media as compared to isosmolal nonelectrolyte media and that this variation appears to be related to both the charge density at the surface and the ionic strength of the media. Anionic lipid vesicles in 150 mM KCl have a significantly higher modulus (50 .times. 107 dyn/cm2) and transbilayer pressure difference (40 mosM) at bursting with a small capacity to stretch (3-4% surface expansion) compared to the same vesicles suspended in nonelectrolyte impermeants. The latter vesicles undergo a significant surface expansion (8-10%), display a low modulus (3 .times. 107 dyn/cm2), and burst at 3-4 mosM bilayer pressure difference. Vesicles suspended in media of constant osmolarity at various ionic strengths display properties with proportional values. Vesicles burst when diluted from 150 to 120 .+-. 5 mM KCl or from 250 to 215 .+-. 5 mM sucrose. This is a considerably narrower range of dilutions than has been examined by studies on the effects of osmotic stress on bilayers in the literature. A comparison of the mechanical properties of bilayers comprised of different lipid head groups shows that dioleoylphosphatidylethanolamine displays the properties of an acidic lipid with an unusually high modulus, low surface expansion, and high transbilayer pressure difference at bursting. This contrasts with dioleoylphosphatidylserine, which displays an unusually low modulus, moderate surface expansion, and a low transbilayer pressure difference at bursting. Vesicles of reconstituted E. coli lactose carrier into the cell's lipids display an even higher modulus than phosphatidylethanolamine, which dominates the bacterium's lipid composition.

Accession: 006670037

PMID: 3676261

DOI: 10.1021/bi00391a034

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