Direct measurement of headgroup hydration of polar lipids in inverted micelles.

The maximum hydration level, i.e. maximum number of water molecules bound per lipid molecule, has been measured for some common phospholipids, glycolipids and cholesterol. Maximum hydration level has been measured using a two-phase, hexane/water, partitioning technique using 3H-labelled water. This technique is rapid and provides an easy method for direct measurement of water binding. It is observed from these studies that there are two classes of lipids, one which binds more than 20 water molecules per lipid headgroup and the other which binds less than 20 water molecules. There appears to be a correlation between the number of water molecules a lipid binds to the phase preference of that lipid. The data suggest that the lipids, like phosphatidylcholine, which favours the lamellar phase bind far greater number of water molecules as compared to those, like phosphatidylethanolamine, which favour the hexagonal phase.