Liquid crystal glycolipids - Carbohydrate Chemistry: Chemical and Biological Approaches - page 330

The interesting point here about both the sugar units and the lipid
chains is that they have been designed to be stable under extreme con-
ditions, e.g. the ester groups and alkenyl units have been replaced by
moieties that function in a similar way. For example, the use of
cis-alkenyl groups in the lipid chains helps to prevent recrystallization,
whereas for the lipids of archael bacteria a similar effect is achieved via
the isoprenyl moieties. All of the compounds a to g were found to exhibit
columnar phases. 29 It is interesting to note that only the glycolipid e,
bearing two linear saturated chains, exhibits a defined melting point in
addition to the clearing point. All of the other glycosides with two methyl
branched aliphatic moieties exhibit no detectable melting points, instead
they all form glassy phases at very low temperatures. The lack of a
melting point transition can be explained by the steric hindrance of the
methyl groups to ordered packing of the hydrocarbon residues. Con-
sequently, these materials are in their liquid crystal phases at room
temperature and can be cooled down to 50 1C without recrystallization
occurring. On cooling, compound f, which possesses mixed linear and
branched chains and with a sn-2 stereochemistry for the glycerol unit,
was found to give a glass transition below 25 1C. The clearing point
enthalpies for all compounds were found to be relatively small ( o 1Jg 1 )
in comparison with the values obtained for carbohydrates with a single
lipophilic chain (around 5 Jg 1 ). These results indicate that, near to the
transition from the isotropic liquid state to the columnar phase, the
structure of the liquid and mesophase are similar. Thus, for most of
the materials, they exist in columnar mesophases at room temperature,
but when they glassify upon cooling, their defect textures appear the
same in the glass state as in the liquid crystal phase. Thus when warmed
up to room temperature the glass returns to the columnar phase without
apparent change, ie the materials self-heal.
The identification of hexagonal columnar phases is relatively
straight-forward. The defect textures seen in the microscope at 100
magnification show fan-like domains, however, they lack elliptical and
hyperbolic disclinations that are present for lamellar phases, see Fig. 19.
Unlike the glycolipids that have one head group and two aliphatic tails,
there are relatively few naturally occurring mesomorphic materials that
Fig. 19 Texture of the columnar phase of a synthetic diether-type glycolipid under
crossed polars ( 100).