Regional Cooperativity in the Phase Transitions of Dipalmitoylphosphatidylcholine Bilayers: The Lipid Tail Triggers the Isothermal Crystallization Process

We have a long-standing interest to explore the answer of the question: Which part of the amphiphilic molecule triggers the phase transition of the self-assembled aggregates consisting of these amphiphiles? This is an important issue regarding the phase transition kinetics of amphiphiles. To this end, we studied the phase transition behaviors of dipalmitoyl-phosphatidylcholine (DPPC) by differential scanning calorimetry, synchrotron X-ray scattering, Fourier transform infrared spectroscopy, and image analysis. We found that different parts (head, interface, and tail) of DPPC molecules all exhibit nonsynchronous changes during the sub-, pre-, and main transitions. Particular efforts have been devoted to studying the isothermal subgel (L-c') formation process. It was found that only the lipid interface and tail regions change, and only when the rearrangement of the lipid hydrocarbon chain packing reaches a certain extent can the interfacial C=O groups be induced to undergo vibrational environment changes. The result means that the hydrocarbon tail is the part that triggers the gel (L-beta') to L-c' phase transition. The present work deepens our understanding on the phase transition mechanisms of DPPC and may shed light on those of other phospholipids and other types of amphiphiles.