Liquid crystal glycolipids - Carbohydrate Chemistry: Chemical and Biological Approaches - page 324

H-bonding than the cyclic system. It is not so clear, however, why the
odd-even effect is so strong for the galactitols.
In the systems discussed so far the head group has been either
flexible or somewhat rigid in structure and the aliphatic portion of the
molecular architecture has been flexible. However, by adding in steroidal
units it is possible to confer rigidity to the non-polar portion of the
architecture. In doing so it would be expected that the clearing
temperatures would rise significantly in relation to flexible non-polar
moieties. For example, the steroidal-based materials 10-13 which have
differing points of attachment to the glucose unit, were found to
decompose at high temperatures (W230 1C), with the cholestanyl de-
rivatives, 10 and 11, shown to melt to smectic A* phases at temperatures
of 202 and 181 1C respectively. However, the cholesteryl derivatives
decomposed at temperatures above 230 1C, without exhibiting any
mesophases. 19 Thus, for a small change in the architecture of the
aliphatic unit mesophase formation was suppressed by the simple
inclusion of an unsaturated unit. Simulations of the structures of the
cholestanyl compounds allowed for speculative modelling of the lamellar
phase, where the steroid units pack tightly together, but because of
their bulky nature the sugar units cannot pack closely together. This
potentially leads to interdigitation of the sugar units between the layers.
The molecular architecture of one-tail one head-group for glycolipids
invariably supports the formation of lamellar phases on change in
temperature. Therefore putting two such structures back-to-back would
be expected also to favour lamellar phase formation. In the following
we use derivatives of sucrose to emphasize this point. For example
the disubstituted sucrose ester, 6,6 0 -di-O-palmitoylsucrose, exhibited a
smectic A phase over a 50 1C temperature range, 20,21 see Fig. 14,
compound 14. Furthermore, Bottle and Jenkins 22 previously reported on
its biological activity, which showed it possessed immunostimulant
properties, however they did not investigate the melting behaviour of the