Lipid chain dynamics and molecular location of diacylglycerol in hydrated binary mixtures with phosphatidylcholine: Spin label ESR studies

The lipid chain motions in hydrated binary mixtures of dimyristoylglycerol (DMG) with dimyristoylphosphatidylcholine (DMPC) have been studied by using ESR spectroscopy of analogues of both components that are spin-labeled at one of eight different positions along the sn-2 chain. The phase diagram of the binary mixtures divides into three separate regions along the composition axis, corresponding to the formation of isothermally melting compounds with DMPC/DMG stoichiometries of approximately 1:1 and 1:2 mol/mol in the gel phase [Heimburg, T,, Wurz, U., & Marsh, D. (1992) Biophys. J. 63, 1369-1378]. In the first region (up to 50 mol % DMG), comparison of the chain flexibility profiles, and the chain profiles of the polarity-dependent isotropic hyperfine coupling constant, of the two different spin-labeled components indicates that DMG is incorporated in the fluid lipid bilayer in a manner similar to that of the host DMPC but is situated approximately two CH2 groups deeper into the hydrophobic interior. At lower contents of DMG, the chain packing is increased by the addition of DMG, whereas at higher DMG contents the lipid chain order decreases rapidly, on reaching the inverted hexagonal phase of the second region of the phase diagram. In the second region of the phase diagram (50-67 mol % DMG), the DMG fits better into the fluid inverted hexagonal phase than into the fluid lamellar phase of the first region and is located only approximately one CH2 group deeper than the corresponding DMPC. In these first two regions of the phase diagram, the ESR spectra of both spin-labeled components display an axial anisotropy that evidences the increasing angular amplitude of motion with position down the chain that is characteristic of liquid crystalline fluid phases. In the third region of the phase diagram (above 67 mol % DMG), the fluid phase consists of isotropically tumbling DMG molecules in which the DMPC molecules are incorporated as inverted micelles as indicated by the residual anisotropic motion of the spin-labeled phosphatidylcholine analogues.