Comparison of SCF and MD for SOPC Membranes

In biological membranes, the class of fully saturated PC lipids is just one of many. Indeed, variations in the lipid architecture are frequently encountered. There are variations with respect to the head-group structure as well as variations in the tail architecture. For example, in the tails, single or multiple unsaturated bonds are very common. There is relatively little knowledge about the role of lipid unsaturation in biomembranes. There are relatively few MD simulations on bilayers composed of lipids with unsaturated bonds. Nevertheless, single unsaturation as well as multiple unsaturated chains have been of some interest. For a recent review see reference [102], We will now present a selection of the results. We will also take this opportunity to compare the results from the MD simulations with those from the SCF calculations. 

The MD trajectories of 1018 ps were computed at T = 303 K. The zero point (z = 0) is the centre of the bilayer it was calculated during the MD simulations as the middle point between the centres of P-N vectors of the two monolayers (the centre of the P-N vector is the middle of P-N distance). The ratio between the CPU times (MD/SCF) needed to obtain these results is in the order of 105 

From the modelling results for bilayers composed of unsaturated lipids one can begin to speculate about the various roles unsaturated lipids play in biomembranes. One very well-known effect is that unsaturated bonds suppress the gel-to-liquid phase transition temperature. Unsaturated lipids also modulate the lateral mobility of molecules in the membrane matrix. The results discussed above suggest that in biomembranes the average interpenetration depth of lipid tails into opposite monolayers can be tuned by using unsaturated lipids. Rabinovich and co-workers have shown that the end-to-end distance of multiple unsaturated acyl chains was significantly less sensitive to the temperature than that of saturated acyls. They suggested from this that unsaturated 

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