Dehydration induces lateral expansion of polyunsaturated 18:0-22:6 phosphatidylcholine in a new lamellar phase

To gain a better understanding of the biological role of polyunsaturated phospholipids, infrared (1R) linear dichroism, NMR, and x-ray diffraction studies have been conducted on the lyotropic phase behavior and bilayer dimensions of sn-1 chain perdeuterated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (SDPC-d35), a mixed-chain saturated (18:0)-polyunsaturated (22:6 omega3) lipid. SDPC films were hydrated at definite values of temperature (T) and relative humidity (RN). In excess water, the lipid forms exclusively lamellar phases in the temperature range 0-50 degreesC. Upon dehydration the lipid undergoes the main phase transition between the liquid-crystalline (L-alpha) and gel (L-beta) phase at T < 15 degreesC. Both the saturated and polyunsaturated chains adopt a stretched conformation in the La phase, presumably the all-trans (stearoyl) and angle iron or helical (docosahexaenoyl) one. A new fluid lamellar phase (L-alpha(')) was found in partially hydrated samples at T > 15 degreesC. SDPC membranes expand laterally and contract vertically in the L-alpha(') phase when water was removed. This tendency is in sharp contrast to typical dehydration-induced changes of membrane dimensions. The slope of the phase transition lines in the RH-T phase diagram reveal that the lyotropic L-alpha(')-L-alpha and L-beta-L-alpha transitions are driven by enthalpy and entropy, respectively The possible molecular origin of the phase transitions is discussed. The properties of SDPC are compared with that of membranes of monounsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31).