On the position of the hydro-phobic/philic boundary in lipid bilayers.

The sensitivity of calculated structural dimensions of hydrated lipids to the position of the hydrophobic/hydrophilic boundary is reviewed. The position of this boundary is critical in determining the extent of hydration and location of water in the bilayer. A calculation of the dimensions of the hydrophilic and hydrophobic parts of the phosphatidylcholine and ethanolamine bilayer from literature values of the x-ray long spacing shows that the choice of boundary in phospholipids is not arbitrary and is best placed at the average position of the first CH(2) group in the hydrocarbon chains. Calculated dimensions of the hydrocarbon core and the water accessible regions agree with neutron and x-ray diffraction measurements. Hydration differences between phosphatidylcholines and phosphatidylethanolamines are readily explained from derived estimates of the layers of water which cover these headgroups.