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

7.3 Results and discussion 113
retical predictions discussed here. If the coherence length associated to the proteininduced
perturbation is dependent on protein size, one would expect that bilayer
activities affected by changes of the coherence length, might be thus be affected by
protein sizes. This is indeed the case for the phenomenon of flip-flop of phospholipids
in bilayers. In fact, experimental data on flip-flop suggest that the larger the
protein size (hence the smaller its curvature at the interface with the lipid chains) the
more reduced is the ability of the protein to cause flip-flop [184].
It is worth mentioning that the larger the protein size is, the more the behavior
of dL(r) obtained from our DPD simulations differs from the one of the exponential
function used for the best fitting. This could already be seen by looking at figure 7.4f
(at a temperature well above the main-transition temperature) for NP=43 and in the
case of negative mismatch. Figure 7.7 illustrates more in details the non-exponential
dependence of the lipid bilayer thickness profile on r. The figure shows the calculated
values of dL(r) (open circles) and the fitting ones using expression 7.3 (solid line). The
30.0
d L (r) [Å]
25.0
20.0
15.0
10.0
5.0
0.0
eff
dP dP ∆d0
0 10 20 30 40 50 60 70 80 90
r [Å]
(b)
o
dL dL dL fit
Figure 7.7: The calculated values of dL(r) (open circles) and the fitting ones using expression
7.3 (solid line) as a function of the distance r from the protein surface. The data refer to a protein
size NP=43, and to the following cases: (a) ∆d=-12 ˚A (˜dP=14 ˚A) and T ∗ =0.5, and (b) ∆d=19
˚A (˜dP=41 ˚A) and T ∗ =1.0, where in both cases the temperature is above the melting temperature
of the pure system. The dashed line indicates the value of the pure lipid bilayer hydrophobic
thickness d o L at the considered temperature. Also shown are the calculated protein hydropho-
bic length dP (gray area), and the effective protein hydrophobic length d eff
P (white area), which
is defined as the projection of dP onto the normal to the bilayer plane.
data refer to a protein size NP=43, and to the following two cases: (a) ∆d=-12 ˚A (˜dP=14
˚A) and T∗ =0.5, and (b) ∆d=19 ˚A (˜dP=41 ˚A) and T∗ =1.0. In the case of positive and large
mismatch (dP > do L ) (but low enough to avoid protein tilting), figure 7.7(b) indicates
that the lipids in the layers closest to the protein surface are characterized by gellike
chain in order to minimize the hydrophobic mismatch; surprisingly, next to this