Liquid crystals carbohydrate

To date, the crystal structures of more than 200 mesogenic compounds are known. In this review, we wish to present a general overview of the crystal structures of mesogenic compounds up to the end of 1997. Unfortunately, it is not possible to consider the crystal structure determinations of carbohydrate liquid crystals [13, 14], metallomesogens [15-18], phasmid and biforked mesogens [19-22], perfluorinated mesogenic compounds [23-27], benzoic acids [6, 28-31], cinnamic acids [7, 32, 33], dicarboxylic acids [34, 35], cinnamate compounds [8, 36-40], and discotic liquid crystals [41-43] due to the lack of space. 

Discotic liquid crystals based on carbohydrates are intrinsically chiral due to the chiral nature of their core.49,50 Numerous discotic liquid crystals derived from these mesogens have been reported however, the expression of chirality in the subsequent mesophases has only rarely been seen. An example of the latter is the a-anomer of penta- O -decanoylglucopyranosc (47), which forms a discotic mesophase in which the molecules pack helically in the columns... 

The mesogenic structures of glycolipids are due to the occurrence, on the same molecule, of a hydrophilic and a hydrophobic moiety often referred to as head and tail respectively. As a result, glycolipids are able to self-organize into a large variety of mesophases also called liquid crystals (Fig. 2) [ 10]. Supramolecular assemblies of mesogenic compounds can be caused by a rise in temperature (thermotropic liquid crystals) or by the addition of water (lyotropic liquid crystals) they result from different responses of the carbohydrate and the alkyl chain to temperature or solvent (water), respectively. 

Acyclic carbohydrates with one alkyl chain are either derived from cyclitols or aldonic acids. Acyclic cyclitol derivatives 3 (X=0, S, NH, N(CH3), NHCO, N(CH3)C0 R=Q-Ci6 sug=Glc-ol, Man-ol) form thermotropic liquid crystals... 

Alkyl chain(s) with carbohydrates at both termini (bolaamphiphiles) have also been reported since they are potential building blocks for the construction of membrane mimetics with a single monolayer [78]. Bisgluconamide and lactobionamides 6b (sug=Glc-A or Lac-A, X=NH) were studied for their crystalline properties and their arrangements in water [39, 40]. Alkyl-a,cc)-dimannitol 6b (sug=Man-ol, n=16-22) [66] or bolaamphiphiles with identical or different carbohydrates at both ends of the alkyl chain 6a (sug=D-Glc/, D-Galp, DL-Xyl-oI) were found to form micelles and lyotropic liquid crystals as well [41]. 

Due to the biological roles of glycolipids, many papers have been devoted to their syntheses over the last ten years. The coupling of a fully protected carbohydrate donor to a lipid acceptor requires efficient and highly stereoselective glycosylation methods because lipid derivatives often have low reactivity. A few examples of glycosphingolipids syntheses will be discussed below as well as multistep preparations of other amphiphilic carbohydrates designed as biochemical mimetics, surfactants or liquid crystals. 

New glycolipids have to be synthesized to get further insights into liquid crystal properties (mainly lyotropic liquid crystals), surfactant properties (useful in the extraction of membrane proteins), and factors that govern vesicle formation, stability and tightness. New techniques have to be perfected in order to allow to make precise measurements of thermodynamic and kinetic parameters of binding in 3D-systems and to refine those already avalaible with 2D-arrays. Furthermore, molecular mechanics calculations should also be improved to afford a better modeling of the conformations of carbohydrates at interfaces, in relation with physical measurements such as NMR. 

J. M. Garcia Fernandez, A. Gadelle, and J. Defaye, Difructose dianhydrides from sucrose and fructooligosaccharides and their use as building blocks for the preparation of amphiphiles, liquid crystals and polymers, Carbohydr. Res., 265 (1994) 249-269. 

In Fig. 3 one finds a variety of functional groups that may be subdivided as follows haloalkyl groups [14-16], saturated esters (acetate [17], and benzoate [18,19]) unsaturated esters (methacrylate [19,20], acrylate [21], cinnamate [20], and others [21]) malonate and related esters [22-24] ethers and oligo(oxyethylenes) [25-28] imides [29,30] alkylsilyloxy groups [31] perfluoroalkyls [32-34] and protected carbohydrate groups [35]. Vinyl ethers with mesogenic (liquid-crystal forming) substituents will be treated separately in Section II. B.2. 

Qualitative analysis of chiral compounds, including drugs, pesticides, carbohydrates, amino acids, liquid crystals, and other biochemicals Determination of enantiomeric purity of chiral compounds... 

The unique character of liquid crystals allows the sensitive change of molecular orientation with both temperature and chemical environment. Steroidal crown compounds entrapped in a mixture of cholesteryl nonaoate and cholesterol thus form a liquid crystalline phase whose helical pitch can be modified by different complexed metal salts (Fig. 3.5.4). Potassium R-mandelate, for example, induces a green assembly color, potassium S-mandelate a blue one. For the detection of carbohydrates by steroidal borates, see Figure 4.5.1 (Shinkai et al., 1991). 

There has been little development in the theory of MEM in recent years, however the diversity of applications has increased greatly. Examples include the measurement of rotameric distribution in carbohydrates, peptides,or more generally - in isotropic environments. Other applications include studies of the internal order in liquid crystals, a new attempt at the in vivo application of phase-modulated rotating-frame imaging (PMRFI),- - and a study of amphiphilic molecules in ternary systems. 

---