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

92 Interaction of small molecules with bilayers
pensated by an opposite change of pressure at the water/headgroups interface (first
maximum in figure 6.8).
Also the shift in lateral pressure in the bilayer hydrophobic core depends on the
bilayer topology and solute characteristics. In the ht (L) t (K)
4 t(L) t and ht7 bilayers, the
hydrophilic (and the neutral) dumb-bells have little effect in the bilayer center (z =
0). In the ht (L)
6 t bilayer, because of the decrease in interdigitation, and of packing
density, induced by the interface-adsorbed solutes, the effect is a decrease of the local
pressure in the bilayer center.
To further characterize the changes in the local pressure induced by the solute
molecules we define the following quantity
∆π(z) = π db(z) − πo(z) (6.2)
where π db(z) is the pressure for a bilayer with dumb-bells and πo(z) is the pressure
for the pure bilayer.
Since the most common organization of lipid bilayers is the non interdigitated
state, we focus on the pressure shift ∆π(z) for the lipid type ht (L) t (K)
4 t(L) t, as shown in
figure 6.9. The shift in pressure for the fully flexible bilayer ht7 is very similar to the
one shown. Because of symmetry, just half of the pressure profile is represented, with
the bilayer center at z = 0. Also, z has been normalized by the hydrophobic thickness
of the bilayer. From the figure it can be seen that the hydrophobic dumb-bells shift
Figure 6.9: Difference in pressure profiles ∆π(z) (equation 6.2) in a ht (L) t (K)
4 t (L) t bilayer with
added different dumb-bells respect to the pure bilayer. Only half of the pressure profile is
shown. The bilayer center is at z = 0, and z has been rescaled by the bilayer hydrophobic thickness,
Dc. A schematic representation of the lipids is also shown for a better characterization
of the position of the peaks of ∆π(z). The first peak in the graphs (at z ≈ −0.6) corresponds
to the water/headgroups interface, and the second peak (z ≈ −0.5) to the headgroups/tails
interface.