G. Goethals

Substitution effects on the liquid crystalline properties of D,L-xylitol amphiphiles

In this article we describe the self-assembling properties of alkyl substituted xylitols in relation to both thermotropic and lyotropic liquid crystalline mesophases. Three series of substituted xylitols were prepared where aliphatic chains of varying length were attached to a xylitol moiety via ether, thioether and ester linking groups. The thermotropic properties were investigated by thermal polarized light microscopy and differential scanning calorimetry, and evaluated as a function of chain length and linking group. The lyotropic phase behaviour was investigated via the addition of water to each material at room temperature. The efficiency for forming thermotropic phases was found to be reversed for the lyotropic phases in respect of the three series, i.e. as a function of the linking unit.

The dependence of mesomorphic behaviour on the extent of hydrogen-bonding in sugar derived polyols

The thermotropic liquid crystalline properties of a variety of chiral and racemic dodecyloxy substituted polyols derived from carbohydrates were investigated as a function of the number of hydroxyl groups associated with the polyol unit. It was found that all of the materials exhibited smectic A* phases, and that the clearing points increased monotonically with the number of hydroxyl groups. The linear increase was found to be independent of stereochemical structure and the degree of optical purity. A model suggesting that the smectic A* phase has an internal microphase separated structure can be used to account for these observations.

An homologous series of 6-O-n-alkyl-alpha-D-galactopyranoses: synthesis and thermotropic mesomorphic properties

An homologous series of 6- O - n -alkyl- alpha -D-galactopyranoses has been prepared. The length of the terminal chains has been varied systematically and the effect on the liquid crystal transition temperatures studied. Most homologues of the series exhibit enantiotropic smectic A* phases. X-ray analysis indicates a lamellar structure for the smectic A* phase with hydrogen-bonded carbohydrate cores at the layer centre, either with no interdigitation of the tilted terminal alkyl chains but with a high degree of chain melting, or with some degree of chain intercalation. The 6- O - n -alkyl- alpha -D-galactopyranoses possess clearing points at higher temperatures than those of the corresponding n -alkyl alpha -D-galactopyranosides. The introduction of a higher degree of hydrogen bonding by the replacement of the oxygen atom in the ether linkage between the chain and the carbohydrate ring by an amide linkage leads to higher transition temperatures. The dependence of the liquid crystalline behaviour on the po...

NMR on-line monitoring of esterification catalyzed by cutinase.

A nuclear magnetic resonance (NMR) method has been developed to monitor on-line lipase-catalyzed esterification reactions without the need to sample the reaction medium. The technique, through (1)H NMR, measures the concentrations of alcohol, ester, hydroxylic hydrogens in the organic phase, and hydroxylic hydrogens in the aqueous phase, if any. Also, the chemical shift evolution of the two types of hydroxylic hydrogens has been followed, providing information on water content of the organic phase and on the appearance of a distinct aqueous phase. As far as (13)C NMR is concerned, it has been possible to measure, first the acid and the ester concentrations in the carbonyl region, and second, the alcohol and the ester concentrations in the methylene region. All (1)H and (13)C results are in agreement with one another. Furthermore, NMR allows for the choice of detection zone. Preliminary studies on the solid phase proved the presence of much more water in the solid phase than in the organic phase, and also gave evidence of the existence of two types of esters, one in the organic phase, mainly associated with the acid, and the other one not associated with the acid, most probably entrapped within the solid enzyme.

Novel reversed cyclonucleoside analogues with a d-ribofuranose glycone

Abstract Two novel ribofuranose cyclonucleoside analogues have been synthesised by a route using 5-azido-5-deoxy-1,2- O -isopropylidene-α- d -ribofuranose as the starting material. This derivative was converted into two azole-reversed nucleosides, which were cyclised regiospecifically and stereospecifically by formation of a pentofuranosylamine. An alternative route, starting from a methyl β- d -ribofuranoside, was much less efficient, reflecting the need for the correct anomeric configuration in the cyclisation step.

Synthesis of peracetylated chacotriose

Steroidal glycoalkaloids of many Solanum species have recognized biological activities, especially those containing the glycosyl moiety alpha-L-rhamnopyranosyl-(1-->2)-[alpha-L-rhamnopyranosyl-(1-->4)]-D-glucopyranose (chacotriose) whose peracetate is here synthesized and characterized by complete 1H and 13C NMR assignment.

Spacer arm influence on glucidoamphiphile compound properties

Abstract We prepared glucidoamphiphile derivatives from d -glucose, d -galactose and xylitol, in which the glucidic moiety and the hydrophobic alkyl chain are separated by spacer arm E (E=glyceryl, (OEt) 2 -α-polypropyleneglycyl and butyloxy). Their amphiphile characteristics are compared to those of the corresponding analogs 3- O -alkyl- d -glucopyranoses, 6- O -alkyl- d -galactopyranoses and 1- O -alkyl- d , l -xylitols. We discussed the spacer arm influence on hydrophobic lipophilic balance (HLB), critical micellar concentration (CMC), water solubility ( S w ) and phase transition temperatures of thermotropic and lyotropic mesophases.

Liquid crystalline derivatives of galactose and galactitol: dependence of thermotropic mesomorphism on carbohydrate form

An almost complete homologous series of the acyclic, open-chain 6- O - n -alkyl-D-galactitols has been prepared. Most homologues in this series exhibit an enantiotropic smectic A* phase. The liquid crystal transition temperatures of these open-chain carbohydrates are compared with those of the corresponding homologues in the cyclic form, i.e., with those of the 6- O - n alkyl-alpha-D-galactopyranoses. In general the carbohydrates in the open-chain form exhibit higher clearing points than those of the analogous cyclic pyranoses. However, the melting point of the galactitols is generally significantly lower than that of the corresponding galactopyranoses. This leads to a much wider temperature range of the smectic A* phase for the carbohydrates in the pyranose form. The lowest clearing point was found for related galactoses in the furanose form. Alternation was found for the clearing point of the 6-O-n alkyl-alpha-D-galactopyranoses and the 6- O - n -alkyl-D-galactitols. Powder X-ray diffraction studies ind...

13C NMR measurement of hydrogen bonding effects for hydroxylic compounds: relationships between the OH stretching frequencies, the chemical shifts of proton donor group and electron reorganization

Abstract The carbon chemical shifts induced by hydrogen bonding have been measured for complexes between phenol, methanol, benzylic alcohol and π or n bases. Linear relationships are obtained between corrected induced chemical shifts and the IR frequency shifts ΔνOH but reverse slopes result for C-1 sp2 and sp3 carbons.

The Effect of Molecular Shape and Microphase Segregation on the Formation of Liquid Crystal Phases in Poly-Ols

Abstract In this article we examine the effect of molecular shape on the formation of thermotropic phases of alkyl-substituted poly-ols. The relationship between the cross-sectional area of the hydrophilic head group with respect to the that of the hydrophilic tails determines the type of mesophase formed. The results obtained are similar to those found for the formation of lyotropic phases.