Materials Science

Electrostatic Stitching

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Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1871
DOI: 10.1126/science.310.5756.1871c

Supported lipid bilayers are often used as model systems for studying surface phenomena because of their well-defined planar geometry. One interesting phenomenon relates to the “fluid-to-gel” phase transition that occurs when the mobile liquid-crystalline ordering crystallizes as the temperature is lowered. However, it is difficult to study this phase transition in simple single-component bilayers, such as those composed of zwitterionic phospholipids, because of a high density of defects that form in the gel phase. The defects are most likely caused by shrinkage of the area occupied by a lipid molecule as the tilt angle of the head-group changes on cooling. Zhang et al. tested this hypothesis by studying the effects of adding cationic or anionic lipids to a bilayer composed of a zwitterionic phosphatidylcholine. With the addition of the cationic lipid, defects no longer formed on gelation. Measurements of the head-group orientation showed that the cationic lipid increased the tilt angle in the fluid phase, but that it no longer changed on cooling. The cationic lipid is expected to be well dispersed because of electrostatic repulsions, and they may act to stitch together the bilayer, thus giving it stability through the phase transition. The addition of anionic lipids, which is expected to be unfavorable, led to defects in the gel phase, supporting the stitching hypothesis. — MSL

J. Phys. Chem. B, 10.1021/jp055995s (2005).

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