Mesoscopic models of lipid bilayers and bilayers with embedded ...
Mesoscopic models of lipid bilayers and bilayers with embedded ... Mesoscopic models of lipid bilayers and bilayers with embedded ...
68 Phase behavior of coarse-grained lipid bilayers We have shown that the bilayer structure in the low temperature region depends on the repulsion between the lipid headgroups. By tuning this parameter, we can obtain both the gel phase Lβ and the interdigitated gel phase LβI. Experimentally, both in the liquid crystalline phase [92] and in the gel phase [107], a monotonic increase of the area per lipid is observed when the temperature is increased. This is caused by an increase in the disorder of the tails [92]. For the low repulsion parameter of ahh = 15 we reproduce the experimental observed trends. It is worth mentioning that, in most cases, in the gel phase the phospholipid chains are tilted with respect to the bilayer normal [96]. While for single tail lipids we do not observe any tilt, we will see in the next section that the double tail lipids are tilted in the gel phase (Lβ ′ phase). Phase behavior as a function of head-head repulsion It is now interesting to do a more systematic study of these phase transitions for a range of repulsion parameters. The phase transitions we consider are: 1. transition from interdigitated gel to gel (LβI → Lβ) 2. transition from interdigitated gel to liquid crystalline (LβI → Lα) 3. transition from gel to liquid crystalline (Lβ → Lα). As we have shown, the first transition is induced by a decrease in the repulsion parameter a hh, while the latter ones are temperature dependent. We use three quantities to distinguish among the different phases: the area per lipid AL, the extent of tail overlap D overlap, and the ordering of the tails S tail. By studying the behavior of these quantities as function of temperature and head-head repulsion parameter we can determine the phase diagram of ht9 as shown in figure 5.3. In figure 5.12 we plot the area per lipid AL, the extent of tail overlap D overlap, and the chain order parameter S tail as function of temperature and head-head repulsion parameter. For repulsion parameters a hh ≤ 18, the low temperature phase is the bilayer gel Lβ phase, while for repulsion parameters a hh > 18, the low temperature phase is the interdigitated gel LβI.
5.2 Single-tail lipid bilayers 69 A L 1.0 0.9 0.8 0.7 0.6 a hh =10 a hh =15 a hh =20 a hh =25 a hh =35 0.5 0.8 0.9 1.0 T* 1.1 1.2 (a) (b) (c) Figure 5.12: (a) Area per lipid AL, (b) extent of chain overlap Doverlap, and (c) tail order parameter Stail as function of reduced temperature T ∗ for different repulsion parameters ahh. Dashed curves show a transition from the Lβ to the Lα phase, solid curves show the transition from the LβI to the Lα phase. By increasing temperature all bilayers melt from an ordered into a disordered phase. For bilayers in the Lβ phase, the area per molecule and chain overlap increase upon melting, while for bilayers in the LβI phase the area per molecule and chain overlap decrease. The curves in figure 5.12(c) show that the transition from an ordered phase to a disordered one is very gradual. Much larger systems might be required to observe a sharp transition in these quasi two-dimensional systems. This gradual transition makes it difficult to determine the exact location of the phase boundaries and therefore we used the inflection point as our definition of the phase boundary. The temperature at which the chains get disordered is the same as the temperature of the inflection point in AL and D overlap. We define as the main transition temperature Tm the value of temperature at the inflection point of the shown curves. Tm is higher for
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- Page 43 and 44: IV Structural characterization of l
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68 Phase behavior <strong>of</strong> coarse-grained <strong>lipid</strong> <strong>bilayers</strong><br />
We have shown that the bilayer structure in the low temperature region depends<br />
on the repulsion between the <strong>lipid</strong> headgroups. By tuning this parameter, we can obtain<br />
both the gel phase Lβ <strong>and</strong> the interdigitated gel phase LβI. Experimentally, both<br />
in the liquid crystalline phase [92] <strong>and</strong> in the gel phase [107], a monotonic increase <strong>of</strong><br />
the area per <strong>lipid</strong> is observed when the temperature is increased. This is caused by an<br />
increase in the disorder <strong>of</strong> the tails [92]. For the low repulsion parameter <strong>of</strong> ahh = 15<br />
we reproduce the experimental observed trends. It is worth mentioning that, in most<br />
cases, in the gel phase the phospho<strong>lipid</strong> chains are tilted <strong>with</strong> respect to the bilayer<br />
normal [96]. While for single tail <strong>lipid</strong>s we do not observe any tilt, we will see in the<br />
next section that the double tail <strong>lipid</strong>s are tilted in the gel phase (Lβ ′ phase).<br />
Phase behavior as a function <strong>of</strong> head-head repulsion<br />
It is now interesting to do a more systematic study <strong>of</strong> these phase transitions for a<br />
range <strong>of</strong> repulsion parameters. The phase transitions we consider are:<br />
1. transition from interdigitated gel to gel (LβI → Lβ)<br />
2. transition from interdigitated gel to liquid crystalline (LβI → Lα)<br />
3. transition from gel to liquid crystalline (Lβ → Lα).<br />
As we have shown, the first transition is induced by a decrease in the repulsion parameter<br />
a hh, while the latter ones are temperature dependent.<br />
We use three quantities to distinguish among the different phases: the area per<br />
<strong>lipid</strong> AL, the extent <strong>of</strong> tail overlap D overlap, <strong>and</strong> the ordering <strong>of</strong> the tails S tail. By studying<br />
the behavior <strong>of</strong> these quantities as function <strong>of</strong> temperature <strong>and</strong> head-head repulsion<br />
parameter we can determine the phase diagram <strong>of</strong> ht9 as shown in figure 5.3.<br />
In figure 5.12 we plot the area per <strong>lipid</strong> AL, the extent <strong>of</strong> tail overlap D overlap, <strong>and</strong><br />
the chain order parameter S tail as function <strong>of</strong> temperature <strong>and</strong> head-head repulsion<br />
parameter. For repulsion parameters a hh ≤ 18, the low temperature phase is the<br />
bilayer gel Lβ phase, while for repulsion parameters a hh > 18, the low temperature<br />
phase is the interdigitated gel LβI.
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