Structure and thermotropic properties of 1,3-dipalmitoyl-glycero-2-phosphocholine.

The structure and thermotropic properties of hydrated 1,3-dipalmitoyl-glycero-2-phosphocholine (beta-DPPC) have been studied by X-ray diffraction and differential scanning calorimetry. After prolonged storage at -3 degrees C, differential scanning calorimetry heating scans exhibit endothermic transitions at 27 degrees C and 37 degrees C, with transition enthalpies, delta H = 9.1 and 10.5 kcal/mol beta-DPPC, respectively (1 cal = 4.184 J). Upon cooling, the high temperature transition is completely reversible, whereas the low temperature transition is not. Prolonged incubation of hydrated beta-DPPC at low temperatures is necessary in order to regain the full enthalpy of the low temperature transition, indicating metastability of the low temperature form. X-ray diffraction studies indicate three different lamellar phases upon heating equilibrated beta-DPPC from -3 degrees C: (1) below 18 degrees C, a hydrated (14 mol water/mol beta-DPPC) "crystalline" bilayer phase, Lc, with an ordered hydrocarbon chain-packing mode and a bilayer periodicity d = 58 A; (2) between 30 degrees C and 35 degrees C, a hydrated (22 mol water/mol beta-DPPC) gel phase, L beta, with hexagonal chain-packing and d = 47 A; hydrocarbon chain interdigitation in this phase is suggested by the small bilayer periodicity, a sharp, symmetric wide-angle reflection at 1/4.2 A-1, an area per mol beta-DPPC at the interface of approximately 80 A2, electron density profiles and structure factor calculations using strip electron density models; (3) above 37 degrees C, a highly hydrated (48 mol water/mol beta-DPPC) liquid crystalline bilayer phase, L alpha, with d = 65 A. Previous nuclear magnetic resonance and neutron diffraction studies have suggested that in beta-DPPC the glycerol backbone adopts an orientation parallel to the bilayer surface, in contrast to its usual perpendicular orientation in alpha-DPPC. This conformation presumably results in an increased intramolecular chain separation, with consequent changes in the molecular packing, hydration and thermotropic behavior of beta-DPPC, compared to its positional isomer alpha-DPPC.