X-ray Diffraction Study of the Effect of the Detergent Octyl Glucoside on the Structure of Lamellar and Nonlamellar Lipid/Water Phases of Use for Membrane Protein Reconstitution
The phase behavior and structural parameters of dioleoylphosphatidylethanolamine (DOPE), monoolein (MO), and dipalmitoylphosphatidylcholine (DPPC) hydrated in excess aqueous phase containing micellar octyl glucoside (OG) were investigated by means of synchrotron X-ray diffraction in a wide range of temperatures. The lipid-to-detergent molar ratios were selected to be equal to those in a recent membrane-protein-reconstitution study. It was established that lipids displaying either nonlamellar (DOPE and MO) or lamellar propensities (DPPC) form mixed supramolecular structures with the detergent OG. The phase states and structures of these mixed assemblies are essentially different from those of the pure lipid components. At full hydration and at the investigated lipid-to-detergent molar ratios, OG destabilizes the inverted-hexagonal (HII) phase organization of DOPE and the inverted cubic (Q224) structure of MO, transforming them into a lamellar liquid crystalline (Lα) phase. The fluid DOPE/OG and MO/OG bilayers totally accommodate the micellar OG from the aqueous phase, as evidenced by the absence of diffuse scattering of OG or lipid/OG micelles. In the temperature interval from 0 to 52 °C, the investigated DOPE/OG/water and MO/OG/water systems are in the lamellar Lα state and do not undergo structural phase transitions. The DPPC/OG/water system undergoes a phase transition from a gel to a liquid-crystalline phase state. Due to the particular molar amount of OG in this system, a coexistence of mixed DPPC/OG bilayers and DPPC/OG micelles was found. The structural results suggest that the DOPE/OG and MO/OG systems, which are of lamellar rather than of nonlamellar organizations, are suitable for reconstitution of membrane proteins, as was established for DPPC/OG membranes.