Lipid, gel phase

Figure 5 A schematic model of the formation of lipid gel phase by hydration of the polar groups in crystalline regions of emulsifiers, d = interplanar Bragg spacing d, = thickness of lipid bilayer dw = thickness of water iayer. Redrawn from reference 15, courtesy of Marcel Dekker Inc.

The disordering effects of cholesterol on the lipid gel phase are much more difficult to measure because of the veakness of the gauche marker bands. Discussion of the 4 position vill be deferred because of the anomalously high disorder observed in the gel phase of the cholesterol free system. The 6-d DPPC derivative shoved... 

Some important aspects of the mixing behavior of selected systems should be mentioned. Non-ideal mixing is not only found for lipids in the ordered gel phases but also for liquid-crystalline lipids. Gel phase immiscibility and phase diagrams which are of the peritectic or eutectic type are regularly observed when the two lipids in the mixture have differences in chain length of 4 or more CH2-groups or when the gel phase stnictures are different. For instance, lipids that form inter-... 

Quantity Gel phase (19°C, 12 water molecules/lipid) Liquid crystal phase (50°C, 28 water molecules/lipid) ... 

Lipid assemblies of the lamellar type, such as lipid bilayers, can feature a true phase transition in which the topology does not change. Upon cooling, the bilayer goes from the fluid phase to the gel phase. In the fluid phase, the acyl chains are disordered, in the sense that there is enough free volume around the chains to allow for chain conformation variations. In the gel phase, the acyl chains are more densely packed and believed to be ordered in an all-trans (straight) configuration. For very pure systems, at temperatures below this sharp gel-to-liquid phase transition, there are several other states and distinct transitions detectable (pre-transition, ripple phase, etc.). These phases will not be reviewed here. In biomembranes, many type of lipids (and other molecules) occur, and it is known that for this reason the gel-to-liquid phase transition is... 

Below we will see that a sufficiently general model for the lipid bilayer system should account for some free volume (unoccupied lattice sites) in the bilayer core. If this is not done, then the lipid bilayer system will change spontaneously into the gel phase (at all reasonable temperatures). 

Venable, R. M., Brooks, B. R. and Pastor, R. W. (2000). Molecular dynamics simulations of gel phase lipid bilayers in constant pressure and constant surface area ensembles, J. Chem. Phys., 112, 4822-4832. 

Pressure was applied in this study to fine tune the lipid chain-lengths and conformation and to select specific lamellar phases. For example, the phospholipid bilayer thickness increases by 1 A/kbar in the liquid-crystalline phase, and up to six gel phases have been found in fully hydrated DPPC dispersions in the pressure-temperature phase space up to 15 kbar and 80 °C, respectively. NMR spectral parameters were used to detect structural and dynamic changes upon incorporation of the polypeptide into the lipid bilayers. 

The vast majority of biological membranes are in the liquid-crystalline phase. There are many experimental studies on model bilayer phase behavior [3]. Briefly, at low temperatures lipid bilayers form a gel phase, characterized by high order and rigidity and slow lateral diffusion. There is a main phase transition, as the temperature is increased, to the liquid-crystalline phase. The liquid-crystalline phase has more fluidity and fast lateral diffusion. 

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