Microscopic origin of the chirality driven morphologies of the amphiphilic monolayers and bilayers

Abstract

It is widely known that the compressed monolayers and bilayers of chiral lipids or fatty acids form helical morphologies, while the corresponding racemic modification gives only flat platelets without twist. No molecular explanation of this phenomenon is yet available, although subtle interactions at the chiral centers have often been proposed as the driving force behind the morphology of the aggregate to form a particular shape. In the present study, the morphologies of the chiral amphiphilic assemblies have been predicted on the basis of an effective pair potential between the molecules, which depends on the relative sizes of the groups attached to the chiral centers, the orientation of the amphiphilic molecules and also on the distance between them. It is shown that for a pair of same kind of enantiomers, the minimum energy conformation favours a twist angle between them. This twist between the neighbouring molecules gives rise to the helicity of the aggregate. The present theory also shows from the molecular considerations that for a pair of mirror-image isomers (i.e. the racemic modification) the minimum energy conformation corresponds to the zero angle between the molecules, thus giving rise to flat platelets as observed in experiments. Another fascinating aspect of such chirality driven helical structures is that the sense (or the handedness) of the helix is highly specific about the chirality of the monomer concerned. The molecular theory shows, for the first time, that the sense of the helical structures in many cases is determined by the sizes of the groups attached to the chiral centers and the effective potential between them. The predicted senses of the helical structures are in complete agreement with the experimental results.