Nonsynchronicity phenomenon observed during the lamellar-micellar phase transitions of 1-stearoyllysophosphatidylcholine dispersed in water.

Knowledge on the synchronicity or cooperativity of changes in different parts of the amphiphilic molecules is important to understand the molecular mechanisms of phase transformations of self-assembled aggregates. A long-standing and challenging question is to understand the roles individual groups/portions in an amphiphilic molecule play during phase transitions. To address this question, we selected a lysophospholipid, 1-stearoyllysophosphatidylcholine (SLPC), to study the transition mechanisms between its lamellar phase and micellar phase by using differential scanning calorimetry, small-angle X-ray scattering, Fourier transform infrared spectroscopy, and two-dimensional correlation analysis. It was found that during the lamellar to micellar transition the interfacial C=O groups and the lipid acyl tails change evidently with the former changing a little earlier, but the lipid headgroups remain unchanged in the hydration and conformation state. This means that the head, interface, and tail of SLPC molecules change nonsynchronously. Moreover, the results show that the lamellar to micellar transition is initiated by the interfacial groups. The molecular mechanism of the slow formation kinetics of the lamellar state from the micellar state was also discussed in the context of the nonsynchronicity phenomenon. The markedly different behaviors of the head and interface/tail groups during phase transitions are explained as the retention of the intermolecular attractive forces between the neighboring polar headgroups of the amphiphiles.