DMSO PRODUCES A NEW SUBGEL PHASE IN DPPC - DSC AND X-RAY-DIFFRACTION STUDY

Equilibrium phases and the kinetics of subgel phase transformation of dipalmitoylphosphatidylcholine (DPPC) hydrated with mixtures of dimeth ylsulfoxide (DMSO)/water have been studied using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The rate of gel-to-sub gel transformation is decreased with a small increase in X, the DMSO/w ater mole fraction, but then speeds up and becomes faster than in pure water by X = 0.16. The DSC scans show multiple subgel peaks, some of which can be attributed to impacted domain growth. For X greater than 0.10, XRD shows that there is a new, stable subgel phase, S, which als o accounts for some of the multiplicity of DSC peaks. Our electron den sity profiles show that the thickness of the bilayer in the S phase is greater than in the usual C subgel phase. We suggest that the S subge l phase is characterized by different headgroup ordering and smaller c hain tilt angle than in the C subgel phase. Electron density profiles show that increasing X decreases the water space between bilayers in a ll phases, subgel, gel and fluid (L-alpha). For X = 0.20, a different gel phase is also observed that may be due to subtle changes in the or ientation of chain tilt first observed in partially dehydrated DMPC. T he dehydrating effect of DMSO explains the results of a previous study , confirmed in this study, that increasing the concentration of DMSO r aises the main transition temperature and eliminates the ripple phase. (C) 1998 Elsevier Science B.V.