Dependence of lipid chain and head group packing of the inverted hexagonal phase on hydration

A model which positions the hydrophobic/hydrophilic boundary in phosphatidylethanolamine lipids at the first CH₂ group in the acyl or alkyl chain is used to calculate the surface area per lipid, the mean chain and head-group dimensions and diameters of the hydrophilic tubes of the inverted hexagonal phase of didodecylphosphatidylethanolamine. The calculated surface areas compare favorably with areas obtained for the lamellar liquid crystal phase of the same lipid using the same boundary. Placement of the boundary within the lipid structure permits a determination of the maximum headgroup packing at hydration levels down to complete dehydration. The headgroup dimensions are consistent with a 5 Å diam void at the center of a hydrophilic tube at zero hydration. The calculated mean fluid chain length is ~2 Å smaller than the mean chain length of the lamellar phase at comparable levels of hydration. Comparison of the calculated mean fluid chain length and distances between hydrophobic boundaries shows that the fluid chains are interdigitated between adjacent tubes, and not interdigitated in the central space between three tubes. At low hydration the chains interdigitate in both spaces. The number of lipids packed around a tube at low hydration is only a function of the headgroup geometry, whereas at high hydration, it is a function of the number of carbon atoms in the chains.