The interaction of water with the phospholipid head group and its relationship to the lipid electrical conductivity.

We have studied the interaction of water with the lipid head group by gravimetrically measuring the lipid water adsorption and the lateral dc electrical conductivity increase resulting from this hydration. We have done this for dimyristoyl phosphatidylcholine (DMPC) having protonated or deuterated hydrocarbon chains. These studies were also done for two cationic lipids having rather different polar head groups. All three lipids behave as strong water adsorbers and all three display a steep, logarithmic increase in the conductivity as the first 1-3 waters per lipid are adsorbed. This increase is usually 5-6 orders of magnitude. After the initial 1-3 waters are adsorbed, the conductivity increases much more gradually, upon additional water adsorption. This electrical behavior is also found for weak water adsorbers and appears to be independent of the head group composition. The conductivity behavior suggests two types of water interacting with the head group. Our studies also indicate that a choline-like component is responsible for the strong water binding nature of the lipids, although, both phosphate and choline make significant contributions to the total amount of adsorbed water. The conductivity behavior, however, does not depend on the presence of both these head group components.