Carbohydrates pentoses

Many natural products are derived from tetrahydrofuran. Of great biological importance are those carbohydrates (pentoses, and to a lesser extent hexoses and glycosides derived therefrom), which contain a tetrahydrofuran ring and which are therefore known as furanoses . 

Phosphodiester Bond—covalent bonds between a phosphate group and two 5-carbon ring carbohydrates (pentoses) over two ester bonds. Phosphodiester bonds are central to most life on Earth, because they make up the backbone of the strands of DNA. In DNA and RNA, the phosphodiester bond is the linkage between the 3 carbon atom of one sugar molecule and the 5 carbon of another, deox nibose in DNA and ribose in RNA. 

Nucleic acids were discovered by Miescher in 1869 he found them in pus corpuscles, in fish sperm, and other biologic material. They are high molecular weight polymeric substances whieh are hydrolyzable into heterocyclic organic bases, carbohydrate (pentose), and phosphoric acid. 

Pentose (Section 25 4) A carbohydrate with five carbon atoms... 

Monosaccharides are those carbohydrates that cannot be hydrolyzed into simpler carbohydrates They may be classified as trioses, tetroses, pentoses, hex-oses, or heptoses, depending upon the number of carbon atoms and as aldoses or ketoses depending upon whether they have an aldehyde or ketone group. Examples are listed in Table 13-1. 

There is known one more catabolic route for carbohydrates commonly referred to as the pentose phosphate cycle (also called hexose mono phosphate shunt, or phosphogluconate pathway). 

R. Goldberg and J. Tewari, Thermodynamic and transport properties of carbohydrates and their monophosphates The pentoses and hexoses, J. Phys. Chem. Ref. Data, 18 (1989) 809-822. 

Fig. 7.—The Pentose Phosphate Cycle for the Incorporation of Carbon Dioxide into Carbohydrates During Photosynthesis.86-, 38...

Plant metabolism can be separated into primary pathways that are found in all cells and deal with manipulating a uniform group of basic compounds, and secondary pathways that occur in specialized cells and produce a wide variety of unique compounds. The primary pathways deal with the metabolism of carbohydrates, lipids, proteins, and nucleic acids and act through the many-step reactions of glycolysis, the tricarboxylic acid cycle, the pentose phosphate shunt, and lipid, protein, and nucleic acid biosynthesis. In contrast, the secondary metabolites (e.g., terpenes, alkaloids, phenylpropanoids, lignin, flavonoids, coumarins, and related compounds) are produced by the shikimic, malonic, and mevalonic acid pathways, and the methylerythritol phosphate pathway (Fig. 3.1). This chapter concentrates on the synthesis and metabolism of phenolic compounds and on how the activities of these pathways and the compounds produced affect product quality. 

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. 

M5. Marks, P. A., A newer pathway of carbohydrate metabolism the pentose phosphate pathway. Diabetes 5, 276-283 (1956). 

In the pentose series, attack has also been observed to occur mainly from the less-hindered side of the double bond, as in the addition of trifluoro(fluoroxy)methane to di-O-acetyl-D-arabinal. Trifluoromethyl 3,4-di-0-acetyl-2-deoxy-2-fluoro-/3-D-arabinopyranoside and 3,4-di-0-acetyl-2-deoxy-2-fluoro-/J-D-arabinopyranosyl fluoride were obtained,72,74 together with a very small proportion of 3,4-di-0-acetyl-2-deoxy-2-fluoro-a-D-ribopyranosyl fluoride.74 With di-O-acetyl-D-xylal, however, the lyxose derivatives were mainly obtained.72,73 The suggestion has been made73 that the greater conformational mobility of pentopyranoses results in a preponderance of the C d) conformer of the D-lyxose derivatives,72,73 such that F-l and CFsO-l are axially disposed. For further reactions of unsaturated carbohydrates with tri-fluoro(fluoroxy)methane, see Section 11,9. 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

By careful choice of both the reaction conditions and the phenolic compound used, it may be possible to produce a colour that is characteristic of a particular carbohydrate or related group, so giving some degree of specificity to the method. Thus, Seliwanoff s test uses hydrochloric acid and either resorcinol or 3-indolylacetic acid to measure fructose with minimal interference from glucose. The colour produced by pentoses with orcinol (Bial s reagent) or p-bromoaniline is sufficiently different from that produced by hexoses to permit their quantitation in the presence of hexoses. However, none of the methods based on the formation of furfural or its derivatives can be considered to be entirely specific. 

Several monophasic solvent systems are useful for the separation of carbohydrate mixtures, and in all those listed in Table 9.3 the smallest solute molecules have the fastest mobility. Thus pentoses have higher RF values than hexoses, followed by disaccharides and oligosaccharides. 

Very useful for many different carbohydrates. Very sensitive for pentoses. Colours (red/brown) can be preserved by coating with vinyl from aerosol... 

The nucleotides of RNA and DNA consist of three components a carbohydrate, a phosphate group and an organic nitrogenous base. There are two types of carbohydrate molecule in nucleic acids, both of which are D-pentoses, i.e. contain five carbon atoms. The carbohydrate in RNA is ribose, while DNA contains deoxyribose, which has a hydrogen atom instead of a hydroxyl group attached to the carbon in the 2 position (Figure 13.1). 

Carbohydrates Ceiiuiose Starch 1 Hemiceiiuiose Lignin J ( monosaccharides "j ( hexoses "j Cx(H20)y < oligosaccharides > pentoses > [ chitin J ( glucosamine J (C2H20)4 unsaturated aromatic alcohols —> polyhydroxy carboxylic aoids HPOy, GO2, CH4, glucose, fructose, galactose, arabinose, ribose, xylose... 

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