A new "gel-like" phase in dodecyl maltoside-lipid mixtures: implications in solubilization and reconstitution studies

The interaction of dodecyl maltoside with lipids was investigated through the studies of solubilization and reconstitution processes. The solubilization of large unilamellar liposomes was analyzed through changes in turbidity and cryo-transmission electron microscopy. Solubilization was well described by the three-stage model previously reported for other detergents, and the critical detergent/phospholipid ratios at which lamellar-to-micellar transition occurred (Rsat = 1 mol/mol) and finished (Rsol = 1.6 mol/mol) were determined. The vesicle-micelle transition was further observed in the vitrified hydrated state by cryo-transmission electron microscopy. A striking feature of the solubilization process by dodecyl maltoside was the discovery of a new phase consisting of a very viscous "gel-like" sample. It is shown that this equilibrium cohesive phase is composed of long filamentous thread-like micelles, over microns in length. Similar structures were observed upon solubilization of sonicated liposomes, multilamellar liposomes, or biological Ca2+ ATPase membranes. This "gel-like" phase was also visualized during the process of liposome reconstitution after detergent removal from lipid-dodecyl maltoside micelles. The rate of detergent removal, controlled through the use of SM2 Bio-Beads, was demonstrated to drastically influence the morphology of reconstituted liposomes with a propensity for multilamellar liposome formation upon slow transition through the "gel-like" phase. Finally, on the basis of these observations, the mechanisms of dodecyl maltoside-mediated reconstitution of bacteriorhodopsin were analyzed, and optimal conditions for reconstitution were defined.