Stereochemical effect revealed in self-assemblies based on archaeal lipid analogues bearing a central five-membered carbocycle: a SAXS study.

The relative stereochemistry (cis or trans) of a 1,3-disubstituted cyclopentane unit in the middle of tetraether archaeal bipolar lipid analogues was found to have a dramatic influence on their supramolecular self-assembly properties. SAXS studies of two synthetic diastereomeric archaeal lipids bearing two lactosyl polar head groups at opposite ends revealed different lyotropic behaviors. The cis isomer led to L(c)-L(α)-Q(II) transitions whereas the trans isomer retained an L(α) phase from 20 to 100 °C. These main differences originate from the conformational equilibrium (pseudorotation) of 1,3-disubstituted cyclopentanes. Indeed, this pseudorotation exhibits quite similar orientations of the two substituents in a trans isomer whereas several orientations of the two alkyl chains are expected in a cis-1,3-dialkyl cyclopentane, thus authorizing more conformational flexibility in the lipid packing.