Comparative cryo-electron microscopy of noncovalent N-dodecanoyl- (D- and L-) serine assemblies in vitreous toluene and water

Hot solutions (> 80 degreesC) of N-dodecanoyl-(D- or L-) serine in toluene or water were cooled to room temperature to give gels and were then vitrified for cryo-TEM investigations. Electron micrographs showed multilamellar aggregates with a repetitive density profile of 33 Angstrom as constituents of tubules and vesicles in toluene as well as of twisted ribbons and tubules in water. In water the aggregates were about 10 times longer than those in toluene (up to 10 mum), and platinum shadowing showed inverse twists for ribbons of D- and L-enantiomers. The racemate gave planar platelets in both solvents. It is concluded that the high dielectric constant of bulk water is needed to stabilize the high surface energy of the edges in thin helical ribbons. The trace amounts of water stabilizing the reverse-micellar structures in toluene, on the other hand, do not have an ordering effect on the headgroup arrangement. Curvature is caused by chirality of the monomers (chiral bilayer effect) and is effective in both environments.