We have investigated the physical and biomimetic properties of an L3 (sponge) phase composed of a non-ionic surfactant, pentaethylene glycol monododecyl ether (C12E5), water (or alternatively a buffer), and co-surfactant (1). L3 phase diagram domain was characterized by diffusion of a fluorescent probe and by freeze fracture electron microscopy (FFEM). For all investigated systems, L3 phase is found in regions ranging from 0.12 to 0.95 M C12E5 at 0.02 M of co-surfactant and from 0.12 to 0.71M C12E5 at 0.17 M of co-surfactant. The L3 phase is stable at temperatures ranging from 6 - 30℃. The bilayers of the phase can be swollen by the addition of dodecane and its bilayer distance can be adjusted by varying the water content. The characteristic distance (db) of the phase was obtained from small angle scattering spectra (SAXS/SANS) as well as from FFEM, which yielded complementary db values. The dilution behavior suggests that our sponge phase is symmetrical and that the surfactant surface area remains constant during the dilution rather than the bilayer thickness. The db values were neither affected by the nature of the co-surfactant nor by the addition of membrane proteins inserted into the L3 phase. All inserted proteins retain their structure in the L3 phase whereas active proteins such as Bacteriorhodopsin and Ca2+-ATPase also maintain their activity when embedded in the L3 bilayers. These findings illustrate that a biomimetic surfactant is a versatile medium for membrane protein studies.
(1) Co-surfactants investigated: n-octyl-ß-D-glucopyranoside; n-dodecyl-ß-D-maltopyranoside (DDM); 4-cyclohexyl-1-butyl-ß-D-maltoside ; and 5-cyclohexyl-1-pentyl-ß-D-maltoside .