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

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-