Lipid Classification

Authored by: Marsh Derek

Handbook of Lipid Bilayers

Print publication date:  February  2013
Online publication date:  February  2013

Print ISBN: 9781420088328
eBook ISBN: 9781420088335
Adobe ISBN:

10.1201/9781420088335-2

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Abstract

Stable lipid bilayers are formed in water only from amphiphilic molecules, i.e., those lipid molecules which have a hydrophilic polar headgroup in addition to hydrophobic hydrocarbon chains. Such amphipathic lipids can be classified according to their interaction with water:

Insoluble, nonhydrated lipids. This class does not disperse in water and exists as microcrystals or a coagel. The microcrystals are almost certainly of a bilayer form. Of the biologically relevant lipids considered here, this class includes certain phosphatidylethanolamines and glycolipids below their chain-melting temperatures. Often in these cases a metastable, hydrated gel phase will form on cooling below the chain-melting temperature, which reverts spontaneously on incubation to the less hydrated, crystalline form.

Insoluble, hydrated lipids. This class forms lyotropic liquid crystals — the stable low-temperature form of which is the bilayer — on warming above the chain-melting temperature in water. On cooling below the chain-melting temperature, a hydrated gel phase is formed that may be stable or may be metastable and transform to a stable hydrated crystalline form on further cooling. On heating further above the chain-melting temperature, a transformation may take place from the bilayer to nonlamellar phases such as inverse cubic or inverse hexagonal. To this class belong all phospholipids and glycolipids with two long hydrocarbon chains (above their chain-melting temperature).

Soluble, hydrated lipids. This class forms micelles on solution in water, which in certain cases transform to bilayers after incubation below the chain-melting temperature. The principal biological examples of this class are single-chain (lysoderivatives) and short-chain phospholipids and glycolipids.

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