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

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62 Phase behavior of coarse-grained lipid bilayers In both figures 5.4(a) and 5.4(b) we can distinguish two regions: at low temperatures, the area per lipid decreases with increasing temperature and the thickness is increasing, while at high temperatures the area is increasing with increasing temperature, and the thickness is decreasing. At the lowest temperature studied (T ∗ = 0.8) the area is larger than the area at the highest temperature studied (T ∗ = 1.5) while the thickness at T ∗ = 0.8 is smaller than the thickness at T ∗ = 1.5. This different temperature dependence of AL and Dc suggests that the bilayer undergoes a phase transition. Before discussing this transition in detail we will first characterize the low and the high temperature phases. To characterize the ordering of the lipids in the bilayer we use the order parameters S tail and Sn. In figure 5.5 the values of both S tail and Sn are plotted as a function of temperature. The high values of Sn at temperatures below T ∗ = 0.95 indicate that the bonds are ordered along the bilayer normal. This order persists even for bonds far from the head-group region, decreasing slightly with increasing temperature. Above T ∗ = 0.95 the values of Sn further decrease with increasing temperature, and the order along the chain is lost. (a) (b) Figure 5.5: (a) Local order parameter Sn and (b) tail order parameter Stail, as function of reduced temperature T ∗ . The overall order of the tails (S tail) shows a similar behavior (figure 5.5(b)). Also here we can distinguish two regions: below T ∗ = 0.95 where S tail has values higher than 0.5 indicating that the chains are ordered along the bilayer normal, and above T ∗ = 0.95 where the values of S tail decrease below 0.5, showing an increase in the disorder of the chains. To further characterize the structure of the bilayer in the low and high temperature regions, we compare in figure 5.6 the in-plane radial distribution function g(r) of the head beads of the lipids at one interface, for two different temperatures: T ∗ = 0.8 and T ∗ = 1.5.At T ∗ = 0.8, the radial distribution function shows more pronounced
5.2 Single-tail lipid bilayers 63 peaks compared to the g(r) at T ∗ = 1.5, which corresponds to a more structured organization of the lipids headgroups in the bilayer plane. The structure in the radial distribution function and the high values of the order parameters for low temperatures, suggest that the low temperature phase is the ordered gel phase, while at high temperatures the bilayer is in the disordered liquid crystalline phase. Figure 5.6: Two dimensional radial distribution function g(r) in the bilayer plane for the headgroups at T ∗ = 0.8 and T ∗ = 1.5. In figure 5.7 we show the density profiles in the direction normal to the bilayer for the system components at different reduced temperatures. Figures 5.7(a) and 5.7(b) correspond to a bilayer in the gel phase, while 5.7(c) and 5.7(d) correspond to a bilayer in the liquid crystalline phase. It is clearly visible that, in the low temperature region, the two monolayers are interdigitated. At T ∗ = 0.8 the overlap extends up to the 8 th bead in the tail and the peaks of the density profiles for the lipids tail beads in one monolayer (black full lines) are exactly alternating with the peaks of the opposite monolayer (gray full lines), showing an optimal packing of the tails. This structure resembles the experimentally observed interdigitated phase LβI. We can now explain the temperature dependence of the area per lipid (figure 5.4). The low temperature phase is the interdigitated gel LβI. In this phase the ordering of the chains is the dominating effect. The lipids stretch out in the direction normal to the bilayer, inducing interdigitation. This packing results in a larger average distance between the lipids headgroups in each monolayer and in a larger area. In this region an increase of temperature reduces the values of the order parameter (figure 5.5(b)), but along the chain the order persists (figure 5.5(a)). Thus interdigitation is still present, but is decreasing in depth, resulting in an increase of the bilayer thickness and a decrease of the area per lipid. Above the transition temperature, the chains loose the persisting order and are not interdigitated. Only the terminal tail beads overlap, due to thermal disorder. In this temperature region an increase in temperature increases the effective volume occupied by the molecules, but the extent of tail overlap does not depend significantly of temperature. As a result the area
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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
Page 18 and 19: 12 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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112 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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