Infrared spectroscopy reveals the nonsynchronicity phenomenon in the glassy to fluid micellar transition of DSPE-PEG2000 aqueous dispersions.

One challenging question regarding the phase transition mechanism of amphiphiles is to seek the roles individual groups/portions of the amphiphilic molecule play during the transformation. To address this question, we selected a poly(ethylene glycol)-grafted phospholipid, distearoylphosphatidylethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE-PEG2000), to study its glassy to fluid micellar phase transition by using differential scanning calorimetry and Fourier transform infrared (FTIR) spectroscopy. FTIR results revealed that during the glassy to fluid micellar transition, the lipid acyl tails have evident conformational rearrangements but undergo only slight modifications in the packing state. For the lipid interface region, small changes in the hydration state of C=O groups were observed, whereas for the lipid headgroups (NHCO and PO(4)(-)), their conformation and hydration states remain unchanged. Thus, the head, the interface, and the tail regions of DSPE-PEG2000 molecules change nonsynchronously during the transition. As to the bulky PEG corona residing at the outer micellar surface, no evident hydration state change was observed upon heating, and its behavior is almost the same as that of the hydrated free PEG2000 molecules. Such a nonsynchronous change in different parts of the self-assembled amphiphilic aggregates undergoing phase transition could be a common phenomenon that needs to be widely recognized.