THERMOTROPIC AND MIXING BEHAVIOR OF MIXED-CHAIN PHOSPHATIDYLCHOLINES WITH MOLECULAR-WEIGHTS IDENTICAL WITH THAT OF L-ALPHA-DIPALMITOYLPHOSPHATIDYLCHOLINE

The thermotropic phase behavior of 10 mixed-chain phosphatidylcholines, in excess water, has been examined and compared with that of identical-chain C(16):C(16)PC by using high-resolution differential scanning calorimetry (DSC). The molecular weights (MW) of these 11 molecular species are the same, but their DELTA-C/CL values, or the normalized chain length differences, vary considerably, ranging from 0.035 to 0.540. The thermodynamic parameters (T(m), DELTA-H, and DELTA-S) associated with the main phase transitions for these lipid dispersions exhibit biphasic V-shaped curves, when plotted against DELTA-C/CL. Similar characteric curves have been reported previously for aqueous dispersions of mixed-chain phosphatidylcholines with MW identical with that of C(17):C(17)PC [Lin et al. (1990) Biochemistry 29, 7063-70721. The initial decrease in T(m) (DELTA-H or DELTA-S) with increasing values of DELTA-C/CL is attributed to the progressive increase in the magnitude of the chain-terminal perturbations on the conformational statistics of the adjacent hydrocarbon chains and hence the lateral chain-chain contact interactions of these mixed-chain phosphatidylcholines in the gel-state bilayer. At DELTA-C/CL congruent-to 0.42, the chain-end perturbation is presumably at its maximum; beyond this point, the highly asymmetric phosphatidylcholines are proposed to pack, at T < T(m), into the mixed interdigitated bilayer. In this new packing mode, the methyl ends of the longer acyl chains are relocated at the interfaces between the hydrocarbon core of the bilayer and the aqueous medium. This disposition of the bulky chain ends releases a certain degree of chain-chain packing disorders, leading to an increase in T(m) (DELTA-H or DELTA-S) with increasing DELTA-C/CL. The mixing behavior of C(12):C(20)PC/C(16):C(16)PC, C(20):C(12)PC/C(10): C(22)PC, and C(10):C(22)PC/C(16):C(16)PC binary lipid systems is also studied. The results, in terms of the phase diagram, support the molecular interpretations given for the biphasic V-shaped curve observed in the T(M) (DELTA-H or DELTA-S) vs DELTA-C/CL plot.