Lipid bilayer phase behavior

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One property of a

diffuse freely within this plane. Thus, in a liquid bilayer a given lipid will rapidly exchange locations with its neighbor millions of times a second and will, through the process of a random walk, migrate over long distances.[1]

Motion constraints

In contrast to this large in-plane mobility, it is very difficult for lipid molecules to flip-flop from one side of the

integral membrane proteins
.

Physical origins

Diagram showing the effect of unsaturated lipids on a bilayer. The lipids with an unsaturated tail (blue) disrupt the packing of those with only saturated tails (black). The resulting bilayer has more free space and is consequently more permeable to water and other small molecules.

The phase behavior of lipid bilayers is largely determined by the strength of the attractive Van der Waals interactions between adjacent lipid molecules. The extent of this interaction is in turn governed by how long the lipid tails are and how well they can pack together. Longer tailed lipids have more area over which to interact, increasing the strength of this interaction and consequently decreasing the lipid mobility. Thus, at a given temperature, a short-tailed lipid will be more fluid than an otherwise identical long-tailed lipid.[3] Another way of expressing this would be to say that the gel to liquid phase transition temperature increases with increasing number of carbons in the lipid alkane chains. Saturated phosphatidylcholine lipids with tails longer than 14 carbons are solid at room temperature, while those with fewer than 14 are liquid. This phenomenon is analogous to the fact that paraffin wax, which is composed of long alkanes, is solid at room temperature, while octane (gasoline), a short alkane, is liquid.

Aside from chain length, transition temperature can also be affected by the

saturated fats, is solid at room temperature while vegetable oil
, which is mostly unsaturated, is liquid.

Transition temperature (in °C) as a function of tail length and saturation. All data are for lipids with PC headgroups and two identical tails.[4]
Tail Length Double Bonds Transition Temperature
12 0 -1
14 0 23
16 0 41
18 0 55
20 0 66
22 0 75
24 0 80
18 1 1
18 2 -53
18 3 -60

Mixed systems

Bilayers need not be composed of a single type of lipid and, in fact, most natural membranes are a complex mixture of different lipid molecules. Such mixtures often exhibit properties intermediate to their components, but are also capable of a phenomenon not seen in single component systems:

proteins can partition into one or the other phase [5]
and thus be locally concentrated or activated.

Cholesterol

The chemical structure of cholesterol, which differs greatly from a standard phospholipid.

The presence of

Lipid rafts are cholesterol-enriched gel domains that have been potentially implicated in certain cell signaling processes,[12] but the subject remains controversial, with some researchers doubting even their existence in vivo.[13]

Lipid polymorphism

Example of lipid polymorphism as bilayer (le), reverse spherical micelles (M) and reverse hexagonal cylinders H-II phase (H) in negatively stained transmission electron micrograph of spinach thylakoid lipid-water dispersions.

Mixed lipid liposomes can undergo changes into different phase dispersion structures, called

micelles, lipid bilayer lamellae and hexagonal phase cylinders, depending on physical and chemical changes in their microenvironment.[14]
magnetic resonance spectroscopy and other techniques.[15]

See also

References

  1. ^ H. C. Berg, "Random Walks in Biology". Extended Paperback Ed. ed. 1993, Princeton, NJ: Princeton University Press.
  2. ^ R. Homan and H. J. Pownall."Transbilayer diffusion of phospholipids: dependence on headgroup structure and acyl chain length." Biochimica et Biophysica Acta 938. (1988) 155 -166.
  3. ^ a b W. Rawicz, K. C. Olbrich, T. McIntosh, D. Needham and E. Evans."Effect of chain length and unsaturation on elasticity of lipid bilayers." Biophysical Journal. 79. (2000) 328-39.
  4. ^ J. R. Silvius. Thermotropic Phase Transitions of Pure Lipids in Model Membranes and Their Modifications by Membrane Proteins. John Wiley & Sons, Inc., New York. (1982)
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  9. ^ D. Boal, "Mechanics of the Cell". 2002, Cambridge, UK: Cambridge University Press
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