Anesthetics release unfreezable and bound water in partially hydrated phospholipid lamellar systems and elevate phase transition temperature.

A dimyristoylphosphatidylcholine multilamellar system with varied water content was prepared by dessiccating sonicated vesicles in vacuo. The water content in the sample was determined by gas chromatography after dissolving the multilamellar system in water-free benzene. Differential scanning microcalorimetry revealed several endothermic peaks in the heating scan at subzero temperature, ranging from -25 to -3 degrees. The peaks that appeared in the subzero temperature range indicate the existence of water molecules bound to the lipid head groups, differing from free water that freezes at 0 degrees. The difference between the amount of water molecules that froze in calorimetry and the total amount of water detected by gas chromatography indicates the presence of unfreezable, tightly bound water molecules. The relative amount of free, intermediate, and unfreezable water was estimated by comparing the differential scanning microcalorimetry data with gas chromatography measurements. The addition of halothane and 1-hexanol significantly decreased the intermediately bound water peaks. The anesthetics dehydrated the lamellar system. The phase polymorphism of partially hydrated phospholipid multilayers is well known, and the temperature that corresponds to the main phase transition of fully hydrated lipid membranes shifts to a higher temperature. The addition of anesthetics increased the phase transition temperature when the water content was less than 18 wt%. This result is the complete reverse of the depressant action of anesthetics in fully hydrated lipid membranes. The present anesthetic effect upon the elevation of the transition temperature is apparently caused by anesthetic-induced dehydration of the lipid-water interface at the present experimental condition.