Hydrophobic thickness

No impact on the hydrophobic thickness of the lipid membrane occurs [141,142]... 

Ipsen, J. H., Mouritsen, O. G. and Bloom, M. (1990). Relationships between lipid membrane area, hydrophobic thickness and acyl-chain orientational order. The effects of cholesterol. Biophys. J. 57 405. 

Cybulski LE, de Mendoza D (2011) Bilayer hydrophobic thickness and integral membrane protein function. Curr Protein Pept Sci 12 760-766... 

Because of the fluctuations of the flexible biological membranes, the structural characterization of lipid bilayers is an arduous task when atomic details are sought. Indeed, structural information about the membrane thicknesses, such as the hydrophobic thickness and head group separation, as well as the lipid density, are very difficult to quantify. This results in a large uncertainty in the experimentally determined structural parameters of lipid bilayers found in the literature. For example, values of the average area per phospholipid molecule measured in a single lipid system can vary by nearly 30... 

The corresponding values for lipid bilayers in a LC state indicated in Tieleman s web site are as follows. The hydrophobic thickness of bilayers is 2.97 nm for DPPC and 2.77 nm for DMPC, respectively. The areas per lipid are 0.629 nm /DPPC and 0.596 nm /DMPC, respectively. 

This value is shorter than hydrophobic thickness of the bilayer in a gel state (3.44 for DPPC and 3.2 nm for DMPC), but longer for BLM in a liquid-crystalline state (2.85 nm for DPPC and 2.62 nm for DMPC) [85]. Therefore we have analyzed the geometry of the systems in both gel and LC phases. Two examples of starting state and at the end of simulation (after 40 ns) are presented on Fig. 8.2. On beginning are lipids oriented perpendicular to membrane, but one example showed tilted linear helix, the other tilted but bended helix. 

Intrinsically, d is not equal to the hydrophobic thickness of the bilayer h. It is related to the distance spanning between the neutral surfaces close to the interfacial regions. It is very close to r long enough lipid molecules. As for saturated PCs the molecular area seems to be almost independent of die chain length in the fluid state [11], a linear increase of with was obtained (Figure 4b). It can be noted that such a simple scale dependence of as a function of the membrane thickness is the first clear evidence that mechanical properties are closely related to the molecular dimensions. 

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