The phospholipid head-group orientation: effect on hydration and electrical conductivity.

The water adsorption isotherms have been obtained for egg phosphatidyl-ethanolamine when it is complexed to egg phosphatidylcholine and cholesterol, respectively. In the presence of phosphatidylcholine, the phosphatidylethanol amine water binding is changed to a strong binding as compared to when the phospholipid is in its uncomplexed form. Cholesterol increases the water adorbed by the phospholipid, however, it does not change the nature of the isotherm. Phosphatidylmonomethylethanolamine also exhibits a strong water binding. The electrical conductivity of these phospholipids has been measured concurrently with their hydration. Electrical activation energies have been obtained for the fully hydrated phospholipids and are a function of both the amount of water adsorbed and the orientation of the polar head-group. The results are discussed in terms of a model for water adsorption, previously put forth by the authors.