Carbohydrates classes

Carbohydrates are the most abundant of all organic compounds in the biosphere. Many members of the carbohydrate class have the empirical formula Cx(H20)y, and are literally hydrates of carbon. The fundamental units of the carbohydrate class, the monosaccharides, are polyhydroxy aldehydes or ketones and certain of their derivatives. As with other classes of biologically important compounds, much of the function of the carbohydrates derives from the ability of the monosaccharides to combine, with loss of water, to form polymers oligosaccharides and polysaccharides. The chemistry of carbohydrates is, at its core, the chemistry of carbonyl and hydroxyl functional groups, but these functional groups, when found in the same compound, sometimes exhibit atypical properties. The discussion that follows is designed to review the aspects of carbohydrate chemistry that are especially important for isolation, analysis, and structure determination of biologically important carbohydrates. 

Class 44 Purines and pyrimidines Class 45 Carbohydrates Class 47 Nucleosides and nucleotides Class 48 Amino acids and peptides. 

Henceforth, crystal structure analyses of carbohydrates (class 45), amino acids (class 48), purines and pyrimidines (class 44) and nucleosides and nucleotides (class 47) are referenced by means of their Cambridge Crystallographic Data Base REFCODES. All other crystal structure analyses are referenced in the General Index. 

Saccharum is the Latin word for sugar and the derived term saccharide is the basis of a system of carbohydrate classification. The simplest sugars belong to the carbohydrate class, monosaccharide they include fructose and glucose. Glucose and fructose are structural isomers. The molecular formula is as follows ... 

The carbohydrate class, polysaccharide, represents compounds in which the molecules contain many units of monosaccharides joined together by glycoside links. Upon complete hydrolysis, a polysaccharide delds monosaccharides. Starch is the most valuable polysaccharide. The starch molecules (amylose and anylopectin) are tree-like, containing 250 to 1000 or more glucose units per molecule joined together through alpha linkages. 

The termination ose, is given to all carbohydrate classes and to those individuals which have sugar character, e.g.j mono saccharoses glycerose, glucose, etc. 

It is important to realize that a(2-3), (2-6), and (2-8) linkages occur in all complex carbohydrate classes, and may be combined within one molecule in some cases (see chapter B, Strecker and Montreuil 1979, Corfield et al. 1981 a), and that the specificity outlined above also relates to the different classes of complex... 

The best examples of substances with multiple chirality centers are the carbohy drates (Chapter 25) One class of carbohydrates called hexoses has the constitution... 

The major classes of organic compounds common to living systems are lipids pro terns nucleic acids and carbohydrates Carbohydrates are very familiar to us— we call many of them sugars They make up a substantial portion of the food we eat and provide most of the energy that keeps the human engine running Carbohy drates are structural components of the walls of plant cells and the wood of trees Genetic information is stored and transferred by way of nucleic acids specialized derivatives of carbohydrates which we 11 examine m more detail m Chapter 28... 

The task of relating carbohydrate configurations to names requires either a world class memory or an easily recalled mnemonic A mnemonic that serves us well here was pop ularized by the husband-wife team of Lours F Fieser and Mary Fieser of Harvard Uni versity m their 1956 textbook Organic Chemistry As with many mnemonics it s not clear who actually invented it and references to this particular one appeared m the chem ical education literature before publication of the Fiesers text The mnemonic has two features (1) a system for setting down all the stereoisomeric d aldohexoses m a logical order and (2) a way to assign the correct name to each one... 

Glycosides are a large and very important class of carbohydrate derivatives character ized by the replacement of the anomeric hydroxyl group by some other substituent Gly cosides are termed O glycosides N glycosides S glycosides and so on according to the atom attached to the anomeric carbon... 

The simplest nutritional role of fats in the diet is that of energy supply. There are differences between members of the same class of food materials, but the accepted convention attributes a value of 9 kcal (37.7 kJ) of energy per gram of fat, and 4 kcal (16.7 kJ) of energy per gram of all carbohydrates and proteins. This is a serious consideration in generating weight loss diets. 

Colorimetric Methods. Numerous colorimetric methods exist for the quantitative determination of carbohydrates as a group (8). Among the most popular of these is the phenol—sulfuric acid method of Dubois (9), which rehes on the color formed when a carbohydrate reacts with phenol in the presence of hot sulfuric acid. The test is sensitive for virtually all classes of carbohydrates. Colorimetric methods are usually employed when a very small concentration of carbohydrate is present, and are often used in clinical situations. The Somogyi method, of which there are many variations, rehes on the reduction of cupric sulfate to cuprous oxide and is appHcable to reducing sugars. 

The term sugar describes the chemical class of carbohydrates (qv) of the general formula C (H20) or for monosaccharides. Colloquially, sugar... 

Glycoconjugates. Another class of carbohydrates are the glycoconjugates (14), composed of glycoproteins, proteoglycans, peptidoglycans, and glycohpids. 

Since polysaccharides are the most abundant of the carbohydrates, it is not surprising that they comprise the greatest part of industrial utiliza tion (9,22). Most of the low molecular weight carbohydrates of commerce are produced by depolymerization of starch. Polysaccharide materials of commerce can be thought of as falling into three classes cellulose, a water-insoluble material starches, which are not water-soluble until cooked and water-soluble gums. 

Many classes of natural product possess heterocyclic components (e.g. alkaloids, carbohydrates). However, their structures are often complex, and although structure-based names derived by using the principles outlined in the foregoing sections can be devised, such names tend to be impossibly cumbersome. Furthermore, the properties of complex natural product structures are often closely bound up with their stereochemistry, and for a molecule containing a number of asymmetric elements the specification of a particular stereoisomer by using the fundamental descriptors (R/S, EjZ) is a job few chemists relish. 

Class of colloidal substances prepared from plants. Composed of complex carbohydrates and organic acids that swell in water. Also, a number of natural resins are gums. 

Regarding the total amount of assimilated biomass, carbohydrates/polysaccha-rides represent the dominant class with an estimated amount of 1-4 X 10" tons of dry matter produced by photosynthesis annually ... 

Sections Carbohydrates undergo chemical reactions characteristic of aldehydes and 25.17-25.24 ketones, alcohols, diols, and other classes of compounds, depending on their structure. A review of the reactions described in this chapter is presented in Table 25.2. Although some of the reactions have synthetic value, many of them are used in analysis and structure deter-mination. 

Lipids differ from the other classes of naturally occuning biomolecules (carbohydrates, proteins, and nucleic acids) in that they are more soluble in nonpolar to weakly polar- solvents (diethyl ether, hexane, dichloromethane) than they are in water. They include a variety of str-uctural types, a collection of which is introduced in this chapter. 

Long-chain polyisoprenoid. molecules with a terminal alcohol moiety are called, polyprenols. The dolichols, one class of polyprenols (Figure 8.18), consist of 16 to 22 isoprene units and, in the form of dolichyl phosphates, function to carry carbohydrate units in the biosynthesis of glycoproteins in animals. Polyprenyl groups serve to anchor certain proteins to biological membranes (discussed in Chapter 9). 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

A number of reviews of mass spectra of carbohydrates have been published from which references to the original papers are available (4, 9, 11, 24, 26). The application of mass spectrometry to this field was initially limited by the relatively low volatility of free carbohydrates and by the complex spectra obtained from some derivatives. These limitations have been partially overcome by new inlet techniques and by pioneering studies on classes and derivatives in order to understand the characteristic fragmentations and rearrangements of the molecular ions of a wide range of carbohydrates. 

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