THERMODYNAMIC, THERMOMECHANICAL, AND STRUCTURAL-PROPERTIES OF A HYDRATED ASYMMETRIC PHOSPHATIDYLCHOLINE

1-Behenyl-2-lauryl-sn-glycero-3-phosphocholine (22/12PC) belongs to a unique group of phospholipids in which the molecule has one acyl chain almost twice as long as the other. The temperature-composition phase diagram for this lipid in the range of 25-65-degrees-C, and 0 to 84.3% (w/w) water has been constructed by using the isoplethal method in th e heating direction and x-ray diffraction for phase identification and structure characterization. At water contents between 10.3 and 34% (w /w) and at temperatures below 43-degrees-C, a single mixed interdigita ted lamellar gel phase (L(beta)m, [GRAPHICS] of the type described by Hui et al. (1984. Biochemistry. 23:5570-5577) and McIntosh et al. (198 4. Biochemistry. 23:4038-4044) was found. A second phase consisting of bulk aqueous solution coexists with the Lm phase at hydration levels above 34% (w/w) water in the temperature range between 25 and 43-degre es-C. Above 43-degrees-C, a partially interdigitated lamellar liquid c rystalline (L(alpha)p) phase [ GRAPHICS] is seen in the water concentr ation range extending from 0 to 84.3% (w/w). The pure LP phase is foun d below 43% (w/w) water, while coexistence of the LP phase and the bul k aqueous solution is observed above this water concentration which ma rks the hydration boundary. Interestingly, the latter boundary for bot h Lm and LP phases is nearly vertical in the temperature range studied . Furthermore, the lamellar chain-melting transition temperature appea rs to be relatively insensitive to hydration in the range 0-85% (w/w) water. We have confirmed the identity of the L(beta)m phase by constru cting a 5.7-A resolution electron density profile on oriented samples by the swelling method. Temperature-induced chain melting effects an i ncrease in lipid bilayer thickness suggesting that the LP phase has ch ains packed in the partially as opposed to the mixed interdigitated co nfiguration. Unlike the symmetric phosphatidylcholines a ripple (P(bet a')) phase was not found as an intermediate between the low and high t emperature lamellar phases of 22/12 PC. The specific volume of 22/12 P C is 940 (+/- 1) mul/g and 946 (+/- 1) mul/g in the hydrated lamellar gel state at 28 (+/- 2) and 40 (+/- 2)-degrees-C, respectively, from n eutral buoyancy experiments. Based on measurements of the temperature dependence of the various lattice parameters of the different phases e ncountered in this study the corresponding lattice thermal expansion c oefficients have been measured. These are discussed and their dependen ce on lipid hydration is reported.