Biophysical Journal 55: 957-964 (1989)
© 1989 the Biophysical Society

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

ABSTRACT

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 CH₂ 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.