Structural and morphological investigations of the formation of quasi-crystalline phases of 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG)

Abstract

1,2-Dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) in 100 mM NaCl at pH 7 forms a quasi-crystalline gel phase (L_c phase) when stored for several days or longer at 4°C. We studied the structure and morphology of this phase by freeze–fracture electron microscopy, synchrotron X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR). The phase behavior was also followed by differential scanning calorimetry. Freeze–fracture electron microscopy revealed two different distinct morphological aggregates for the L_c phase, namely flat multilamellar sheets and cochleate cylinders in an approximately 1:1 ratio. FTIR investigations showed that the hydrocarbon chains are tightly packed within an orthorhombic subcell lattice. The frequency shift of the ester carbonyl bands to lower wavenumbers (1732 cm⁻¹) indicates a direct involvement of the carbonyl groups in hydrogen bonding with hydroxy groups. The freeze–fracture electron microscopic and X-ray investigation reveal very tightly packed lamellar aggregates. The small lamellar repeat distance of 4.75 nm indicates that only a thin water layer is present between the lipid bilayers. The set of reflections observed in the wide angle X-ray region resembles that of the crystalline L_c phase observed in phosphatidylcholines. Upon cooling, the lamellar repeat distance observed in the liquid-crystalline phase increases drastically until the transition into the lamellar gel phase occurs. Then, the reflection in the low angle region becomes broad and diffuse. This finding agrees with the electron microscopic results that upon cooling multilamellar aggregates are destroyed and unilamellar DMPG vesicles are formed below the main phase transition. The unilamellar vesicles fuse to lamellar stacks upon low temperature storage and these then roll up into cochleate cylinders. The observed behavior is attributed to the unique properties of the glycerol headgroup in being able to replace water molecules in the hydration shell.