Chain asymmetry alters thermotropic and barotropic properties of phospholipid bilayer membranes

The alignment of the sn-1 and sn-2 acyl chains at the terminal methyl ends generally produces significant influence on the thermodynamic properties of the bilayer phase transitions. We investigated the bilayer phase behavior of asymmetric phospholipids, myristoylpalmitoylphosphatidylcholine and palmitoylmyristoylphosphatidylcholine, by high-pressure light-transmittance and Prodan-fluorescence techniques and differential scanning calorimetry. Constructed temperature-pressure phase diagrams revealed that no stable L′β phase can exist in the whole pressure range because of the formation of the most stable Lc phase. Nevertheless, the pretransition, the detection of which is severely hampered by the exceptionally prompt formation of the Lc phase, was successfully observed. Moreover, the effect of the total and difference of the sn-1 and sn-2 acyl chain lengths on minimal interdigitation pressure (MIP) was summarized in a MIP vs. chain-length inequivalence parameter plot, where the effect was proved to be classified in three zones depending on the alignment of both terminal methyl ends.