Acetonitrile induces nonsynchronous interdigitation and dehydration of dipalmitoylphosphatidylcholine bilayers

The formation mechanism of the interdigitated (L(β)I) phase and the responses of the individual groups of phospholipids to the phase transition are of basic concern within the community of lipid research. In this work, we studied the effect of acetonitrile (CH(3)CN) on the structure and phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers by using differential scanning calorimetry, synchrotron X-ray diffraction, and Fourier transform infrared spectroscopy. We found that the two processes (i.e., the interdigitation and dehydration of the DPPC bilayers) occur nonsynchronously at two different CH(3)CN concentrations (4 and 12 wt %). A detailed submolecular picture for the formation mechanism of the L(β)I phase was provided during the L(β') to L(β)I phase transition at c(CH(3)CN) = 4 wt %: the conformation state and the hexagonal packing mode of the lipid acyl chains and the hydration properties of the lipid polar groups do not change, and the only difference is that the formed L(β)I phase has a tighter lipid acyl chain packing than that of the L(β') phase. When c(CH(3)CN) > 12 wt %, the added CH(3)CN molecules selectively dehydrate the interfacial carbonyl groups. Thus, two different kinds of L(β)I phases differing only in the hydration states of the interfacial carbonyl groups of phospholipids exist in the c(CH(3)CN) regions of 5-12 and 13-40 wt %, respectively. The strong ability of acetonitrile to induce interdigitation in the lipid bilayers has been discussed in the viewpoint of its toxicity.