Monosaccharides chemical methods

Enzymic methods for the quantitation of monosaccharides are employed when a higher degree of specificity is required than can be achieved by the majority of the chemical methods. They often enable the quantitation of one stereoisomer in the presence of others and can often differentiate between the a and 0 anomeric forms. 

The above example demonstrates the potentialities of mass spectrometry as a method for elucidating the position of isopropylidene groupings. This problem has often to be solved in synthetic monosaccharide chemistry, especially when the compound may form several isomeric isopropylidene derivatives and the chemical methods are tedious and, sometimes, unreliable. 

Structural studies of glycosphingolipids involves determination of the structure of the oligosaccharide chain and of the lipid moiety. For the oligosaccharide chain, it is necessary to determine the composition, molar ratio, and sequence of the monosaccharides, their pyranose or furanose nature, and the position of glycosidic bonds and their configuration for the lipid moiety, the composition of the fatty acids and sphingosine bases must be determined. Used for these purposes are the classical, chemical methods, conventionally accepted in the chemistry of carbohydrates and lipids and based on the degradation of compounds, enzymic, and physicochemical methods, primarily mass spectrometry and n.m.r. spectroscopy. 

ABSTRACT This article describes recent developments in the chemistry of an important family of complex monosaccharides which have diverse structures and participate in a wide range of biological processes. For example 3-deoxy-D-/n nno-2-octulosonic acid (KDO) is a key component of the lipopolysaccharides (LPS) of Grammnegative bacteria, 3-deoxy-D-araftmo-2-heptulosonic acid (DAH) is a key intermediate in the biosynthesis of aromatic amino acids in bacteria and plants. A number of their syntheses that were achieved by homologation reactions of the natural carbohydrate units using enzymatic or chemical methods, as well as by total synthetic approaches are here included. Special emphasis is placed on new methodologies and their correlation with the biosynthetic pathway of the corresponding ulosonic acids. 

C A Browne and F W Zerban, Physical and Chemical Methods of Sugar Analysts, 3rd edition, John Wiley and Sons, New York, 1941, J Stanek, M Carny, J Kocourek and J Pacak, The Monosaccharides, Academic Press, New York, 1963, pp 865 955, G R Pigman in The Carbohydrates, (Ed W Pigman), Academic Press, New York, 1957, pp 602 640... 

As before, this Chapter deals with specific trl- and higher saccharides most references related to their syntheses by specific chemical methods. It does not cover higher, non-specific compounds made by the oligomerisation of monosaccharide derivatives nor does it deal with the cyclodextrins. The synthesis of, e.g.. pentasaccharides is dealt with under that heading and the required preparations of constituent parts are assumed and are not covered in their sections. 

The Englyst procedure uses enzymatic and chemical methods to measure NSP. (For detailed working protocols, see Further Reading.) All starch is hydrolyzed enzymatically and NSP are measured as the sum of the constituent sugars released by acid hydrolysis. The sugars may be measured by GC or by HPLC to obtain values for individual monosaccharides, or a single value for total sugars may be obtained by colorimetry. Values may be obtained for total, soluble, and insoluble NSP, and a small modification allows cellulose to be measured separately. 

Starch and cellulose are polysaccharides. Sugar is easily solvable in water, where it forms hydrogen bonds with the water molecules. In Figure 11.4, the structures have been optimized by quantum chemical methods (PM3). Water molecules are added to all possible cavities to simulate the solvated structure. Hydrogen bonds are formed between the saccharide and the water molecules. If optimization is done in open space, on the other hand, hydrogen bonds are formed between and in the monosaccharides. This structure appears in a water-free crystal. In the aqueous solution simulated in Figure 11.4, the final strnctme is relevant for aqueous solution. 

Aspects of chemical methods used in the structural elucidation of polysaccharides and complex carbohydrates have been reviewed. In a critical examination of the use of g.l.c.-m.s. in the identification of TMS ethers of monosaccharides, a standardized method, which uses a medium resolution mass spectrometer and short chromatographic columns, has been proposed. TMS Ethers of monosaccharides have been characterized by g.l.c.-chemical ionization m.s. with ammonia as reagent gas. Molecular weights were determined, and fragment ions were produced in a quantity high enough to differentiate between stereoisomers (epimers and anomers). Disaccharides have been determined by permethylation followed by g.l.c. The method has been used in the detection of carbohydrate intolerance secondary to intestinal disaccharidase deficiency. 

As in previous volumes, this Chapter deals with specific tri- and higher oligosaccharides most, but not all papers cited relate to their synthesis by chemical methods. It does not cover compounds made by oligomerisation of monosaccharide derivatives but, for the first time, a separate section on cyclodextrins is included which deals mainly with their chemical syntheses and modifications. Their use, for example as catalysts, and other aspects are not covered. 

In many areas of chemistry, chemical methods of structure proof have given way to spectroscopic methods (Chapter 14) and analysis by mass spectrometry. In the field of carbohydrate chemistry, however, chemical reactions play an important part in the elucidation of the structures of unknown carbohydrates. The reason for this is that many proteins and lipids contain carbohydrates that have very complex structures, and these do not yield easily to spectroscopic methods. Also, since it is notoriously difficult to crystallize many carbohydrates, proof of structure by X-ray crystallography is often not possible. In this section we will discuss methods for determining determine the structure of a monosaccharide by chemical methods. Several synthetic reactions can be used to convert one monosaccharide into another. If the structure of a related monosaccharide is known, and the mechanism of the reaction that interconverts them is well understood, then the structure of the unknown monosaccharide can be established. 

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