Mesoscopic models of lipid bilayers and bilayers with embedded ...

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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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Mesoscopic models of lipid bilayers
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Promotiecommissie: Promotor: • pr
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ii CONTENTS 5.3 Double-tail lipid b
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2 Introduction 1.1 The cell membran
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4 Introduction between the beads, a
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6 Introduction whether the preferre
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8 Simulation method for coarse-grai
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10 Simulation method for coarse-gra
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Page 24 and 25: 18 Simulation method for coarse-gra
Page 27 and 28: III Surface tension in lipid bilaye
Page 29 and 30: 3.2 Method of calculation of surfac
Page 31 and 32: 3.2 Method of calculation of surfac
Page 33 and 34: 3.3 Constant surface tension ensemb
Page 35 and 36: 3.3 Constant surface tension ensemb
Page 37 and 38: 3.4 Surface tension in lipid bilaye
Page 43 and 44: IV Structural characterization of l
Page 45 and 46: 4.2 Structural quantities 39 been r
Page 47 and 48: 4.3 Computational details 41 lipid
Page 49 and 50: 4.4 Results and discussion 43 ρ(z)
Page 51 and 52: 4.4 Results and discussion 45 one l
Page 53 and 54: 4.4 Results and discussion 47 WH HT
Page 55 and 56: 4.4 Results and discussion 49 Shill
Page 57 and 58: 4.4 Results and discussion 51 chain
Page 59 and 60: 4.4 Results and discussion 53 4.4.4
Page 61: 4.4 Results and discussion 55 headg
Page 64 and 65: 58 Phase behavior of coarse-grained
Page 85 and 86: VI Interaction of small molecules w
Page 87 and 88: 6.2 Computational details 81 For re
Page 89 and 90: 6.3 Results and Discussion 83 withi
Page 91 and 92: 6.3 Results and Discussion 85 ρ(z)
Page 93 and 94: 6.3 Results and Discussion 87 ρ(z)
Page 95 and 96: 6.3 Results and Discussion 89 S m 0
Page 97 and 98: 6.3 Results and Discussion 91 π(z)
Page 99: 6.3 Results and Discussion 93 the l
Page 102 and 103: 96 Mesoscopic model for lipid bilay
Page 104 and 105: 98 Mesoscopic model for lipid bilay
Page 106 and 107: 100 Mesoscopic model for lipid bila
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References [1] Tanford, C. Science
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7.4 Conclusion 121 [68] Ono, S.; Ko
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7.4 Conclusion 123 [139] Lee, A. Bi
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Summary Biological membranes, as th
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agreement we find gives us confiden
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Samenvatting Biologische membranen,
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de lage temperatuur gel fase of Lβ
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ied dat het dichtst bij het hydrofo
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This thesis is based on the followi
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