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 thro ugh the studies of solubilization and reconstitution processes. The so lubilization of large unilamellar liposomes was analyzed through chang es in turbidity and cryo-transmission electron microscopy. Solubilizat ion was well described by the three-stage model previously reported fo r other detergents, and the critical detergent/phospholipid ratios at which lamellar-to-micellar transition occurred (R-sat = 1 mol/mol) and finished (R-sol = 1.6 mol/mol) were determined. The vesicle-micelle t ransition was further observed in the vitrified hydrated state by cryo -transmission electron microscopy. A striking feature of the solubiliz ation process by dodecyl maltoside was the discovery of a new phase co nsisting 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 up on solubilization of sonicated liposomes, multilamellar liposomes, or biological Ca2+ ATPase membranes. This ''gel-like'' phase was also vis ualized during the process of liposome reconstitution after detergent removal from lipid-dodecyl maltoside micelles. The rate of detergent r emoval, controlled through the use of SM2 Bio-Beads, was demonstrated to drastically influence the morphology of reconstituted liposomes wit h a propensity for multilamellar liposome formation upon slow transiti on through the ''gel-like'' phase. Finally, on the basis of these obse rvations, the mechanisms of dodecyl maltoside-mediated reconstitution of bacteriorhodopsin were analyzed, and optimal conditions for reconst itution were defined.