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

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90 Interaction of small molecules with bilayers from figure 6.7). In the ht (L) 6 t bilayer, this decrease in chain order leads to a decrease in interdigitation, and the two monolayers become more separated. This results in an increase of the bilayer thickness, even if the end-to-end distance decreases. In the case of the flexible and unsaturated bilayer, the increase in thickness is much smaller, and it can be accounted for by both the presence of the solute molecules in the bilayer interior and by a (small) increase in the end-to-end distance. Since the lipids in these bilayer types are already much less ordered than in the stiff bilayer, the addition of the solutes does not have a significant effect on the order parameters (data not shown). It is important to observe that the effect of the neutral dumb-bells on the bilayer properties is the same than the effect of the amphiphilic dumb-bells, because they both absorb at the bilayer interface. However, since the neutral molecules largely diffuse in the water phase (and sometimes even in the bilayer inner core) they have an effective lower concentration in the bilayer compared to the amphiphilic ones. For this reason, they have a lesser effect on the bilayer structure compared to the amphiphilic molecules. 6.3.3 Effect on the pressure distribution The effect of addition of solute molecules on the lateral pressure profile across the bilayer is shown in figure 6.8, where we plot the pressure profiles in the bilayers with added the solutes to compare them to the pressure profile in the pure bilayers. As a general observation, the peaks (both positive and negative) of the lateral pressure decrease in magnitude by addition of solutes, irrespective of the bilayer type, and of the nature of the added molecules. In this sense, the solutes can be seen as interfacial active molecules, that, like soap, have the effect of shifting to zero the local pressure (and the surface tension) of the interface where it locates. However, since a bilayer is not a simple interface, but has a complex structure, different molecules, at different position within the bilayer, change the lateral pressure in different ways. To describe the characteristics of the pressure profile, we make use of the interfaces defined in Chapter 4 (see figure 4.5 therein). It is important to remind that the integral of the lateral pressure across the bilayer is always zero, since the bilayer is in a tensionless state. Hence, a positive or negative change in the lateral pressure in one region of the bilayer, should be compensated by opposite changes in other regions of the bilayer. The largest shift in the pressure is at the headgroup/tails interface (first negative peak in figures 6.8), but the magnitude of this shift largely depends on the bilayer structure. For the stiff bilayer (figure 6.8(a)) both hydrophilic and hydrophobic solutes increase the local pressure by approximately the same amount, while for the more flexible bilayers (figure 6.8(b) and 6.8(c)) the amphiphilic molecules have a much larger effect than the hydrophobic ones. This shift of lateral pressure is com-
6.3 Results and Discussion 91 π(z) 0.1 0.0 −0.1 −0.2 −5 −3 −1 z 1 3 5 (a) ht (L) 6 t π(z) 0.1 0.0 −0.1 pure dbA dbH dbN π(z) 0.1 0.0 −0.1 −0.2 −5 −3 −1 z 1 3 5 (c) ht7 −0.2 −5 −3 −1 z 1 3 5 (b) ht (L) t (K) 4 t (L) t Figure 6.8: Pressure profiles in bilayers with added dumb-bells compared to the pure bilayer, for the three bilayer types, and the three dumb-bell types considered in this study. pure dbA dbH dbN pure dbA dbH dbN
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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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III Surface tension in lipid bilaye
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3.2 Method of calculation of surfac
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3.2 Method of calculation of surfac
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3.3 Constant surface tension ensemb
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3.3 Constant surface tension ensemb
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3.4 Surface tension in lipid bilaye
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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: 6.3 Results and Discussion 89 S m 0
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
Page 129 and 130: 7.4 Conclusion 123 [139] Lee, A. Bi
Page 131 and 132: Summary Biological membranes, as th
Page 133 and 134: agreement we find gives us confiden
Page 135 and 136: Samenvatting Biologische membranen,
Page 137 and 138: de lage temperatuur gel fase of Lβ
Page 139: ied dat het dichtst bij het hydrofo
Page 143: This thesis is based on the followi
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