Charged Lipid Vesicles: Effects of Salts on Bending Rigidity, Stability, and Size

• Published in: Biophysical journal 2004-12, Vol.87 (6), p.3882-3893
• Main Authors: Claessens, M.M.A.E., van Oort, B.F., Leermakers, F.A.M., Hoekstra, F.A., Cohen Stuart, M.A.
• Format: Article
• Language: English
• Subjects: association colloids; behavior; bilayer-membranes; Computer Simulation; curvature elasticity; dilute; dispersions; Elasticity; Entropy; equilibrium; Lipid Bilayers - chemistry; Lipids; Liposomes - chemistry; mechanics; Membrane Fluidity; Membranes; Models, Chemical; Models, Molecular; Molecular Conformation; Particle Size; phase-transition; Phosphatidylglycerols - chemistry; Salt; Sodium Chloride - chemistry; Static Electricity; statistical thermodynamics; Stress, Mechanical; Surface Properties; Temperature; Water
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1529/biophysj.103.036772

The swelling behavior of charged phospholipids in pure water is completely different from that of neutral or isoelectric phospholipids. It was therefore suggested in the past that, instead of multilamellar phases, vesicles represent the stable structures of charged lipids in excess water. In this article, we show that this might indeed be the case for dioleoylphosphatidylglycerol and even for dioleoylphosphatidylcholine in certain salts. The size of the vesicles formed by these lipids depends on the phospholipid concentration in a way that has been predicted in the literature for vesicles of which the curvature energy is compensated for by translational entropy and a renormalization of the bending moduli (entropic stabilization). Self-consistent field calculations on charged bilayers show that the mean bending modulus k c and the Gaussian bending modulus k ¯ have opposite sign and | k ¯ | > k c , especially at low ionic strength. This has the implication that the energy needed to curve the bilayer into a closed vesicle E ves = 4 π ( 2 k c + k ¯ ) is much less than one would expect based on the value of k c alone. As a result, E ves can relatively easily be entropically compensated. The radii of vesicles that are stabilized by entropy are expected to depend on the membrane persistence length and thus on k c. Experiments in which the vesicle size is studied as a function of the salt and the salt concentration correlate well with self-consistent field predictions of k c as a function of ionic strength.