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

58 Phase behavior of coarse-grained lipid bilayers
5.1 Introduction
As we have discussed in the previous Chapter, an important question in the development
of a mesoscopic model is how much chemical detail should be included in the
model. We have described in some detail the structural characteristics of bilayers of
single-tail lipids. In this Chapter, we investigate if such a model correctly describes
the (phase) behavior of double tail phospholipids, and we compare the results of our
simulations with experiments that are performed on single-tail lipids.
In the second part of this Chapter we increase the complexity of the model lipids,
by studying the phase behavior of a double-tail lipid with three hydrophilic headbeads
and five hydrophobic beads in each of the tails. Furthermore, we show that
a correspondence can be made between reduced and physical temperature. With
this mapping, and the mapping of reduced length scales onto physical lengths, the
typical bilayer characteristics, like the area per lipid and bilayer thickness, computed
in the simulations are found to be in good quantitative agreement with experimental
results. In particular, we show that this lipid model correctly reproduces the phase
behavior of dimyristoylphosphatidylcholine (DMPC) lipid bilayers.
The phase behavior of different phosphocholines (PC’s) has been determined experimentally
(see [96] for a review). All PC’s have a low temperature Lβ ′ phase (see
figure 5.1(a)). In this phase the bilayer is a gel: the chains of the phospholipids are
ordered and show a tilt relative to the bilayer normal. At higher temperature the Lα
phase is the stable phase. This phase is the liquid crystalline state of the bilayer in
which the chains are disordered and tail overlap due to this thermal disorder is possible.
This phase is physiologically the most relevant [97].
(a) L β ′ (b) P β ′ (c) Lα (d) LβI
Figure 5.1: Schematic drawings of the various bilayer phases. The characteristics of these
phases are explained in the text. The filled circles represent the hydrophilic headgroup of a
phospholipid and the lines represent the hydrophobic tails.
Under normal conditions the two monolayers of a bilayer contact each other
at the terminal methyl group of their hydrophobic chains, while their hydrophilic
headgroups are in contact with water. However, it is known experimentally that at
low temperatures an interdigitated state, in which the terminal methyl groups of
one monolayer interpenetrate the opposing layer, is also possible. This LβI phase