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72 Phase behavior of coarse-grained lipid bilayers As we increase the tail length the gel phases are stabilized and the transition shifts to higher temperatures. The effect of increasing the head-head repulsion on the gel to liquid crystalline transition temperature is much more pronounced for the Lβ → Lα compared to LβI → Lα. This can be understood from the fact that in the interdigitated phase the average distance between the heads is already much larger compared to the non-interdigitated phase, and a further increase in this distance does not have a dramatic effect on the stability of the gel phase. For lipids ht8 and ht9 the LβI phase occurs at slightly lower repulsion parameters than for lipids ht6 and ht7. This is consistent with experimental results [91]. Since the interdigitated phase is more closely packed than the non-interdigitated phase, the van der Waals energy is greater. This energy gain is proportional to the number of carbon atoms in the phospholipid chain and thus interdigitation becomes energetically more favorable for longer chains. Also in our simulations we observe that the interdigitated phase is more compact and hence a∗ hh decreases slightly with increasing tail length. It is interesting to compare these results with the experimental data. Misquitta and Caffrey in [108] systematically investigate the phase diagrams of monoacylglycerols, a single-tail lipid, and show a similar tail length dependence for the Lβ →Lα transition. Interestingly, as we will show later, for a similar model of a double-tail lipid we do not observe the spontaneous formation of an interdigitated phase. This corresponds to the experimental observation that for the most common double-tail lipids the interdigitated phase does not form spontaneously, but should be induced by the addition of, for example, alcohol [98]. The effect of adding salt on the gel to liquid crystalline transition has been studied for double-tail lipids [109] and recently for single-tail lipids [110]. These studies show that adding so-called kosmotropic salts increases the Lβ →Lα transition temperature, while chaotropic salts decrease this transition temperature. Similar effects have been observed for nonionic single-tail lipids [111]. Takahashi et al. [110] explain these observations by assuming that kosmotropes tend to be excluded from the interfacial region and hence reduce the amount of interfacial water, while chaotropic salts have the inverse effects, i.e. are adsorbed at the interfacial region and increase the amount of interfacial water. In our model a similar effect can be achieved by changing the head-head interactions; increasing or decreasing a hh corresponds to adding chaotropes or kosmotropes, respectively. Our simulations show that decreasing the head-head repulsion stabilizes the Lβ phase, which corresponds to the case that water is excluded from the interface. Adding chaotropic salts has the reverse effect: it increases the head-head repulsion and stabilizes the Lα phase. Our simulations show that at sufficiently high head-head repulsion the interdigitated phase (LβI) is stable. This suggests that it might be possible in experiments to induce the Lβ →LβI phase transition by adding chaotropic salts to the systems.
5.3 Double-tail lipid bilayers 73 5.3 Double-tail lipid bilayers In the previous section we have discussed the phase behavior of single-tail lipid bilayers. In this section we extend the model and investigate the phase behavior of double-tail lipids. In particular, we consider a coarse-grained lipid with three headbeads and two tails of five beads each. This lipid will be denoted as h3(t5)2. As discussed in section 2.2 of Chapter 2, if a DPD bead is taken to represent the volume of three water molecules, i.e. 90 ˚A 3 , then the coarse-grained lipid h3(t5)2 can be considered as a model for dimyristoylphosphatidylcholine (DMPC) (see figure 2.2 in Chapter 2). 5.3.1 Computational details For this study we consider bilayers of 900 h3(t5)2 lipids, and 25 water particles per lipid, corresponding to fully hydrated conditions (i.e. no interaction of the bilayer with its periodic images). The values of the interaction parameters are aww = att = 25, a hh = 35, awt = a ht = 80. Two consecutive beads in the lipid chain are connected by a harmonic spring, with equilibrium distance ro = 0.7, and elastic constant Kr = 100. Consecutive bonds in the lipid chain are subjected to a harmonic bond-bending potential. The values of the parameters related to this bond-bending potential were derived from the comparison of the CG model with MD simulations on all-atom model for a DMPC lipid bilayer [112]. The resulting values for the bending constant and the equilibrium angle in the lipid tails are Kθ=6 and θo=180 o , respectively. About the bond-bending potential between the head-bead connected to the lipid tails and the first beads in the tails values of Kθ=3 and θo=90 o were found to reproduce the correct configurational distribution and structure of the atomistic detailed phospholipid. All the simulations were carried at zero surface tension conditions. To explore the phase diagram of the bilayer, the temperature of the system was gradually decreased from T ∗ = 1.0 to T ∗ = 0.2 . At each temperature, a total of 100,000 DPD-MC cycles was performed, of which the first 20,000 cycles were needed for equilibration. Statistical averages were then collected over the next 80,000 DPD-MC cycles. The phase boundaries were detected, as described in the previous sections for the single-tail lipids, by monitoring the temperature behavior of the area per lipid, the bilayer thickness and the order of the tails. 5.3.2 Results and Discussion The area per lipid, AL, bilayer hydrophobic thickness, Dc, and tail order parameter S tail, as function of reduced temperature, T ∗ , are shown in Figure 5.15.
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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 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
Page 125 and 126: References [1] Tanford, C. Science
Page 127 and 128: 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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