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44 Structural characterization of lipid bilayers bution presents a clear minimum in the center of the bilayer. This void is filled-in by the non-terminal tails beads, whose density distribution shows a maximum in the bilayer center. This distribution of densities indicates that the stiff bilayer is largely interdigitated: the molecules in one monolayer are not confined on one side of the bilayer midplane, but extend much further into the opposite monolayer. By decreasing the headgroup repulsion parameter to a hh = 15 the two monolayers separate, as can be seen from figure 4.3. The interdigitated phase is experimentally known to occur in lipid membranes below the melting temperature, although it does not spontaneously occur for double-tail lipids with symmetric tails, but it has to be induced, for example, by changes in the environment or in the molecular structure of the lipids [90, 91]. In section 5.2 of Chapter 5, where we compute the phase diagram for single-tail lipids, we will discuss the interdigitated phase in more detail. The effect of increasing the chain stiffness on the structure of the bilayer can be seen by comparing figure 4.3 with figure 4.2(a). Increasing the chain stiffness decreases the disorder in the lipid tails as it is also shown by the increase in the value of the order parameter S tail which is reported in table 4.2. The stiff lipids are more aligned along the bilayer normal, and the terminal tail bead is more localized in the bilayer center, although still with a rather broad distribution. Also, the minimum in the distribution of the other tail beads (dashed line in figures 4.2 and 4.3) at the bilayer center is deeper than in the case of flexible lipids. ρ(z) 3 2 1 0 −6 −4 −2 0 Z 2 4 6 w h t (1,..,n−1) t n ρ tot Figure 4.3: Density profiles for a bilayer formed of stiff lipids with headgroup repulsion parameter ahh = 15. See also caption of figure 4.2. In table 4.2 we summarize the values of the structural properties of the three different bilayers considered. The bilayers of stiff lipids have a larger hydrophobic thickness compared to the bilayer of flexible lipids. Note, however, that, despite stiff lipids have the same end-to-end length, because of interdigitation, the bilayer with a hh = 35 has a smaller thickness than the bilayer with a hh = 15. Since the volume occupied by
4.4 Results and discussion 45 one lipid molecule is about the same in all cases, the area per lipid of the stiff lipids is smaller than the area of the flexible lipids, and the area in the interdigitated bilayer is larger than in the stiff but non-interdigitated one. lipid type S tail Dc L n ee AL ht5 (a hh = 35) 0.2 ± 0.1 2.68±0.05 0.99± 0.03 1.01±0.02 ht (L) 4 t (a hh = 35) 0.4 ± 0.1 3.55±0.07 2.04± 0.04 0.82±0.02 ht (L) 4 t (a hh = 15) 0.4 ± 0.1 4.25±0.10 2.08± 0.03 0.68±0.02 Table 4.2: Values of bilayer structural properties as function of lipid type. 4.4.2 Effect of chain length By varying the number of beads in the tail of the lipids we can investigate the effect of chain length on the bilayer structural properties. We consider flexible and stiff lipid with hydrophobic chain lengths of 5,6,7,8, and 9 beads. The dependence of bilayer area and thickness on the lipid hydrophobic chain length has been investigated by Petrache and co-workers [92] in a 2 H NMR spectroscopy study of saturated phosphocholines (PC). These authors found that, at fixed temperature, the area per lipid slightly decreases with increasing acyl-chain length. The main effect of increasing chain length is on the bilayer thickness which increases with increasing number of hydrophobic segments in the tail. Figure 4.4 shows our results on the dependence of the area per lipid and the bilayer hydrophobic thickness on lipid chain length for the fully flexible lipids and the stiff ones with the two headgroup repulsion parameters (a hh = 15 and a hh = 35). In the case of the flexible model, the area per lipid increases with increasing chain length. This behavior does not reproduce the experimental observed trend. With the addition of chain stiffness, however, the area per lipid for a non-interdigitated bilayer is slightly decreasing with increasing chain length, in agreement with the results in [92]; while it is approximately constant for the interdigitated bilayer. The decrease in area for longer lipids is due to an increase of the effective packing interactions between the tails. In the case of flexible lipids this effect is counterbalanced by the entropic effect that disorders the tails, leading to an increase in the chain cross sectional area with increasing chain length. As a consequence of the larger cross sectional area, at fixed chain length, the bilayer thickness for flexible lipids is smaller than the thickness for stiff lipids. The increase in bilayer thickness with increasing chain length is in agreement with the results of Petrache et al. [92]. 4.4.3 Lateral pressure profiles in tensionless bilayers In this section we discuss the shape of the lateral pressure profile in tensionless bilayers. We use the definition of the lateral pressure as given in equation 3.13 of Chapter
Page 1: Mesoscopic models of lipid bilayers
Page 4 and 5: Promotiecommissie: Promotor: • pr
Page 6 and 7: ii CONTENTS 5.3 Double-tail lipid b
Page 8 and 9: 2 Introduction 1.1 The cell membran
Page 10 and 11: 4 Introduction between the beads, a
Page 12 and 13: 6 Introduction whether the preferre
Page 14 and 15: 8 Simulation method for coarse-grai
Page 16 and 17: 10 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: 4.4 Results and discussion 43 ρ(z)
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
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96 Mesoscopic model for lipid bilay
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98 Mesoscopic model for lipid bilay
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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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