Water Mediates the Metastable Crystal-to-Stable Crystal Phase Transition Process in Phospholipid Aqueous Dispersion

The metastable crystal usually appears first during crystallization, from solution, and it will then transform into the stable form. This metastable crystal-to-stable crystal phase transition process is important in the field of crystal growth and engineering. Crystallization behaviors of amphiphiles such as phospholipids are special as they have preassembled structures prior to crystallization, and the fundamental mechanisms regarding the transformation details of their polymorphic forms at the molecular level are not well understood. In this work, by using Fourier transform infrared spectroscopy combined with the hydrogen/deuterium (H/D) exchange method, we have demonstrated that water plays a critical role in mediating the metastable crystal-to-stable crystal phase transition process of a representative phospholipid, dilauroylphosphoethanolamine (DLPE). The H/D exchange method applied here is a powerful and simple approach to probe if water is involved in the crystal-to-crystal transformation process. The results indicate that water works like a catalyst for the physical process. Furthermore, the conformational changes of the lipid molecules during the crystal-to-crystal transition process have been examined. Understanding the details of the transition from a metastable crystal to a stable crystal will shed light on the crystal growth theory and engineering of the self-assembled amphiphiles in aqueous solutions.