Subgel phases of n-saturated diacylphosphatidylcholines: a Fourier-transform infrared spectroscopic study

The subgel phases of a homologous series of saturated straight-chain diacylphosphatidylcholines with hydrocarbon chains consisting of 10-18 carbon atoms were studied by Fourier-transform infrared spectroscopy. All of these lipids initially form a subgel phase which is spectroscopically similar to that obtained when fully hydrated multilamellar dispersions of dipalmitoylphosphatidylcholine are incubated at 0-4 degrees C for 2-4 days. However, further low-temperature incubation of those phosphatidylcholines with acyl chains of 16 or fewer carbon atoms results in the sequential formation of 1 or more additional, spectroscopically distinct subgel phases, with the number of such phases increasing as hydrocarbon chain length decreases. Our data indicate that the formation of all of these subgel phases involves both reorientation of the acyl chains and major changes in hydration and/or hydrogen-bonding interactions at the polar/apolar interfacial region of the lipid bilayer. We suggest that the driving force behind the formation of these Lc phases is the formation of an extended hydrogen-bonding network in the interfacial region of the bilayer and that the optimization of this network probably requires some distortion of the optimal packing of the acyl chains. As a result, an increase in acyl chain length makes the formation of these Lc phases less favorable and eventually prevents optimization of the hydrogen-bonding network at the bilayer polar/apolar interface.