The kinetics and mechanism of the formation of crystalline phase of dipalmitoylphosphatidylethanolamine dispersed in aqueous dimethyl sulfoxide solutions

The phase transition kinetics and mechanism of formation of a lamellar-crystalline phase of dipalmitoylphosphatidylethanolamine (DPPE) dispersed in different concentrations of aqueous dimethyl sulfoxide (DMSO) during cooling have been examined by differential scanning calorimetry and synchrotron X-ray diffraction techniques. In dispersions containing mole fractions of DMSO (x <0.22), the phase transition sequence of the phospholipid is from lamellar liquid-crystal phase to lamellar-gel phase. Increasing the mole fraction of DMSO to 0.22 <x <0.50 induced the formation of a lamellar crystal phase from the gel phase. The proportion of gel phase converted to the lamellar crystal phase increased with DMSO concentration. Dispersion of phospholipid in aqueous DMSO solutions when x > 0.5 resulted in a direct transition from liquid-crystal phase to lamellar crystal phase with no detectable intermediate gel phase. A temperature versus DMSO concentration phase diagram was constructed based on calorimetric data with phase assignments made using synchrotron X-ray diffraction measurements. The non-isothermal formation kinetics of the lamellar crystal phase, which is expressed as the half time of the transformation process, was found to depend on DMSO concentration. The inducement of lamellar crystal phase in DPPE by DMSO is discussed in terms of the dehydration effect of DMSO and competitive molecular interactions between DMSO, water, and the phospholipid. (C) 2003 Elsevier Ireland Ltd. All rights reserved.