Pentoses, separation from hexoses

In the determination of carbohydrates, sensitivity can often be increased by using fluorescence rather than absorbance for the final determination. With compounds that are not normally fluorescent, it becomes necessary to find fluorescent derivatives. Hirayama [160] concentrated the carbohydrates in coastal water samples, using electrodialysis and evaporation, and made fluorescent derivatives using anthrone and 5-hydroxyl-1-tetralone, determining pentoses separately from hexoses in the process. While this method does seem to have the extra sensitivity expected from fluorescent methods, the extra manipulations render it unsatisfactory for routine use. 

The most important pentose is xylose which can be produced from hardwoods by mild acid hydrolysis. Reduction of xylose gives xylitol, an interesting sweetener because of its ability to prevent dental caries. Industrial production of xylitol from birch wood hydrolyzates started in the 1970s in Finland. An interesting process was developed in which an ion exclusion separation technique is applied for purification and separation of xylose and xylitol from other impurities. Among the corresponding reduction products of hexoses, mannitol, which is also a natural product, has found some use. It can be separated from other alditols by crystallization. Under more drastic... 

Cblurim Chromatography. - The separation of hexoses, pentoses, and polyols on polystyrene-based cation-exchange resins with water as eluant has been examined as a function of the bound cations (Ca, Sr, Ba, Pb, Y and Pr ). The separations depend upon the extent of complexation. Aldoses form weak bidentate complexes, while polyols form tridentate complexes. Pure xylitol can be separated from its mixture with L-arabinitol on a La -form cation-exchange resin, due to its stronger complexation. The mixture of epimeric 1-deoxy-l-nitroalditols and parent aldoses formed from addition of nitromethane to aldoses can in some instances be separated on such La -form resin columns. ... 

It has been demonstrated that the chromatographic separation, on filter paper, of the corresponding sugar from the products of condensation of ethyl acetoacetate with hexoses and pentoses is possible. The solvent mixture used (butanol-acetic acid-water) displaced all the products of condensation at the same rate.17... 

Chang, C.H. (1987) Process for separating arabinose from a pentose/ hexose mixture. U.S. Patent 4,664,718. 

If organisms could be found or metabolically engineered that efficiently ferment both the pentoses and hexoses under practical conditions at high yields and short residence times, fermentation ethanol technology would then have reached another plateau with low-cost lignocellulosic feedstocks. Simultaneous saccharification and fermentation or separate saccharification and fermentation of essentially all the sugars that make up the polysaccharides would each be able to approach the theoretical limit of fermentation ethanol production from the polysaccharides in low-cost lignocellulosic biomass. 

There are many publications and comprehensive handbooks on the thin-layer chromatography (TLC) of carbohydrates (e.g., Refs. 1 and 2). The reason is their great importance in life science and the great diversity of cases monosaccharide, disaccharide, trisaccharide, oligosaccharide, polysaccharide, aldose, ke-tose, triose, tetrose, pentose, hexose, as well as reducing and nonreducing sugars. In addition, when extracted from natural products or produced by fermentation, carbohydrates are accompanied by many impurities. That is why separation methods are used predominantly for their analysis. 

Dideoxy-DL-hexoses were prepared from 137 in the following way the carbon atom chain of the substrate was extended to a seven-carbon atom chain by a low temperature Grignard reaction with methylmagnesium bromide. The product obtained, 145, was separately cis-hydroxylated (to stereoisomeric tetraols 14<) or epoxidized (to epoxides 147). Products 146 and 147 were hydrolyzed and subsequently subjected to Ruff degradation. Pairs of stereoisomeric 2,6-dideoxy-DL-hexoses could be separated by column chromatography. Essentially the same approach served for a synthesis of 2-deoxy-DL-eo f/ ra-pentose. In this case compound 138 (X = OH) was used as a substrate for epoxidation. 

The number of bacterial strains that use peutoses beside hexoses is rather limited therefore, a multistep hydrolysis might be needed, followed by separation of the different sugars. In the case of lignoceUulose hydrolysate, pentoses are mainly represented by arabinose and xylose deriving from the hemicellulose fraction, whereas the hexose glucose is generated from the cellulose fraction. 

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