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 ph ospholipids, infrared (1R) linear dichroism, NMR, and x-ray diffraction stu dies have been conducted on the lyotropic phase behavior and bilayer dimens ions 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 temperat ure (T) and relative humidity (RN). In excess water, the lipid forms exclus ively lamellar phases in the temperature range 0-50 degreesC. Upon dehydrat ion the lipid undergoes the main phase transition between the liquid-crysta lline (L-alpha) and gel (L-beta) phase at T < 15 degreesC. Both the saturat ed and polyunsaturated chains adopt a stretched conformation in the La phas e, presumably the all-trans (stearoyl) and angle iron or helical (docosahex aenoyl) one. A new fluid lamellar phase (L-alpha(')) was found in partially hydrated samples at T > 15 degreesC. SDPC membranes expand laterally and c ontract vertically in the L-alpha(') phase when water was removed. This ten dency is in sharp contrast to typical dehydration-induced changes of membra ne dimensions. The slope of the phase transition lines in the RH-T phase di agram reveal that the lyotropic L-alpha(')-L-alpha and L-beta-L-alpha trans itions are driven by enthalpy and entropy, respectively The possible molecu lar origin of the phase transitions is discussed. The properties of SDPC ar e compared with that of membranes of monounsaturated 1-palmitoyl-2-oleoyl-s n-glycero-3-phosphocholine (POPC-d31).