Configuration of Monosaccharides

7 Key excerpts on "Configuration of Monosaccharides"

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  eBook - PDF

  Carbohydrate Chemistry

  Monosaccharides and Their Oligomers

  • Hassan El Khadem(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Monosaccharides This page intentionally left blank 2 Structure, Configuration, and Conformation of Monosaccharides Monosaccharides (monomeric sugars) exist as chiral polyhydroxy-alkanals, called aldoses, or chiral polyhydroxyalkanones, called ketoses. These are further classified (see Table I), according to the number of carbon atoms in their chains, into trioses, tetroses, pentoses, hexoses, etc. and, according to the type of ring they form, into furanoses and pyranoses (five- and six-membered rings). D -Glucose is the most abundant monosaccharide and therefore the most extensively studied. In this chapter it will be used as an example to illustrate how the structure, configuration, and conformation of monosac-charides can be determined. Although reference may be made to how this was originally done, the emphasis will be on the methods used today to elucidate the structures of monosaccharides and their derivatives. I. STRUCTURE OF MONOSACCHARIDES The first point to be determined in elucidating the structure of an or-ganic compound is the molecular formula, which is achieved by the com-bined use of elemental (combustion) analysis and determination of molec-ular weight. The most accurate method available for molecular weight 11 10Θ-. 40 60 100 120 140 160 180 35 60-73 20 π h i 145 127 103 115 ' • A • D-GLUCOSE CI (CH 4 ) 163 181 ι ι ι ι I 30 100 120 140 160 180 200 Fig. 1. M a s s spectra of D -glucose, using electron impact (top) and chemical ionization (bottom). I. Structure of Monosaccharides 13 determination is mass spectrometry (MS). For carbohydrates, chemical ionization mass spectrometry (CI-MS) is the preferred method (see page 81). The advantage of this type of MS over the simpler, electron impact mass spectrometry (ΕΙ-MS), is that it requires less energy to ionize the molecule, which increases the chances of observing the molecular peak (M + ) in sensitive molecules.

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  Conformation of Carbohydrates

  • V. S. R. Rao(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  The twentieth century has witnessed the greatest growth in the structure determination of biopolymers and hence increased our understanding of these biomolecules. The versatile properties of carbohydrates are fully commensurate with a diverse array of molecules that can be generated from a limited number of monosaccharides as building blocks by linking them in a variety of ways. Thus, a basic knowledge of the chemical structure of carbohydrates is essential towards the development of conformational analysis and visualization of the geometries of complex carbohydrates.

  1.2    Classification

  Derived from the Latin word saccharum for sugar, which itself has its origin in the Sanskrit word sarkhara, carbohydrates are commonly classified as monosaccharides, disaccharides, trisaccharides, oligosaccharides and polysaccharides. A monosaccharide is defined as that unit which cannot be further hydrolyzed into smaller carbohydrates. A disaccharide, as the name implies, can be hydrolyzed into two monosaccharides. Similarly, tri-, tetra- and penta-saccharides on hydrolysis give 3,4 and 5 monosaccharides, respectively. Generally, the term oligosaccharide (from oligos, Greek for few) is used to describe a molecule that contains 10 to 20 monosaccharides and the term polysaccharide is reserved to describe a large carbohydrate molecule.

  Carbohydrates are often found covalently linked to other biomolecules such as proteins (glycoproteins and proteoglycans) and lipids (glycolipids and lipopolysaccharides). To distinguish these molecules from pure carbohydrates, they are collectively called “glycoconjugates.”

  1.3    Simple Monosaccharides

  Monosaccharides are generally represented by the empirical formula Cn H2n On or Cn .(H2 O)n . The latter representation led to the belief that they are hydrates of carbon, and so this family of compounds came to be called “carbohydrates.” The carbon skeleton of acyclic monosaccharides is unbranched and a hydroxyl group is attached to each carbon except for the one which carries the carbonyl oxygen (Figure 1.1 ). Hence, these molecules are more appropriately known as “polyhydroxyaldoses” or “polyhydroxyketoses” (or simply, aldoses or ketoses) depending on the position of the carbonyl group. In aldoses, the carbonyl group is at one end of the chain, whereas in ketoses it can be in any position except at the end. Ketoses are further classified as 2-ketoses, 3-ketoses, etc

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  eBook - ePub

  Food Carbohydrate Chemistry

  • Ronald E. Wrolstad(Author)
  • 2011(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  1 Classifying, Identifying, Naming, and Drawing Sugars and Sugar Derivatives Structure and Nomenclature of Monosaccharides Aldoses and Ketoses Configurations of Aldose Sugars D- vs. L-Sugars Different Ways of Depicting Sugar Structures Fischer, Haworth, Mills, and Conformational Structures Classifying Sugars by Compound Class—Hemiacetals, Hemiketals, Acetals, and Ketals Structure and Nomenclature of Disacchaarides Structure and Optical Activity A Systematic Procedure for Determining Conformation (C-1 or 1-C), Chiral Family (D or L), and Anomeric Form (α or β) of Sugar Pyranoid Ring Structures Structure and Nomenclature of Sugar Derivatives with Relevance to Food Chemistry Glycols (Alditols) Glyconic, Glycuronic, and Glycaric Acids Deoxy Sugars Amino Sugars and Glycosyl Amines Glycosides Sugar Ethers and Sugar Esters Vocabulary References Structure and Nomenclature of Monosaccharides

  Sugars are polyhydroxycarbonyls that occur in single or multiple units as monosaccharides, disaccharides, trisaccharides, tetrasaccharides, or oligosacharides (typically three to ten sugar units). Monosaccharides (also known as simple sugars) exist as aldoses or ketoses, with glucose and fructose being the most common examples. Glycose is a generic term for sugars. Sugars are also classified according to the number of carbon atoms in the molecule (e.g., trioses, tetroses, pentoses, hexoses, heptoses, etc.).

  Figure 1.1 Structure and nomenclature of glucose, fructose, and arabinose.

  Aldoses and Ketoses

  Aldoses contain an aldehyde functional group at carbon-1 (C-1), whereas ketoses contain a carbonyl group that is almost always located at carbon-2 (C-2). C-1 for aldoses and C-2 for ketoses are the reactive centers for these molecules and are known as the anomeric carbon atoms . Figure 1.1 shows the structure for D-glucose, D-fructose, and, in addition, D-arabinose. Sugars have common or trivial names with historical origins from chemistry, medicine, and industry. There is also a systematic procedure for naming sugars (some examples are shown in Table 1.1 ). Glucose is also commonly known as dextrose. In systematic nomenclature, its suffix is hexose, indicating a 6-carbon aldose sugar, and the prefix is gluco- , which shows the orientation of the hydroxyl groups around carbons 2–5. The symbol D refers to the orientation of the hydroxyl group on C-5, the highest numbered asymmetric carbon atom, also known as the reference carbon atom . Since fructose (also known as levulose) has just three asymmetric carbon atoms, its configurational prefix is the same as that for the pentose sugar arabinose. Thus, the systematic name for glucose is D-gluco -hexose and fructose is D-arabino

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  Organic Chemistry

  • David R. Klein(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  Complex sugars, such as disaccharides and polysaccharides, are made by joining monosac- charides together, and they will be discussed in Sections 24.7 and 24.8. Monosaccharides generally contain multiple chiral centers, and Fischer projections are used to indicate the configuration at each chiral center. Glucose and fructose are two examples of simple sugars. C OH H H HO OH H OH H CH 2 OH O H Glucose CH 2 OH C O H HO OH H OH H CH 2 OH Fructose LOOKING BACK For a review of Fischer projections and the skills necessary to interpret these drawings refer to Section 5.7. 24.2 Classification of Monosaccharides 1155 The suffix -ose is used to signify a carbohydrate. Hundreds of different monosaccharides are known, each of which can generally be classified as either an aldose or a ketose. Aldoses contain an aldehyde group, while ketoses contain a ketone group. According to this classification scheme, glucose is an aldose and fructose is a ketose. Aldoses and ketoses can be further classified based on the number of carbon atoms they con- tain. This is accomplished by inserting a term (tri-, tetr-, pent-, hex-, or hept-) immediately before the suffix -ose. An aldopentose C OH H OH H OH H CH 2 OH H O 1 2 3 4 5 A ketohexose CH 2 OH C O OH H OH H OH H CH 2 OH 6 1 2 3 4 5 The first compound is an aldose with five carbon atoms, and it is therefore called an aldopentose. The second compound is a ketose with six carbon atoms, and it is therefore called a ketohexose. In this way, carbohydrates are classified using three descriptors: 1. aldo or keto, indicating whether the compound is an aldehyde or a ketone 2. tri-, tetr-, pent-, hex-, or hept-, indicating the number of carbon atoms 3. -ose, indicating a carbohydrate D and L Sugars Glyceraldehyde is one of the smallest compounds considered to be a carbohydrate. It has only one chiral center and therefore can exist as a pair of enantiomers.

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  Chemistry, 5th Edition

  • Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Some contain too few oxygen atoms, and some contain too many. Some also contain nitrogen. But the term ‘carbohydrate’ has become firmly rooted in chemical nomenclature and, although not completely accurate, it persists as the name for this class of compounds. At the molecular level, most carbohydrates are polyhydroxyaldehydes, polyhydroxyketones or com- pounds that yield them after hydrolysis. Therefore, the chemistry of carbohydrates is essentially the chemistry of hydroxyl and carbonyl groups, and of the acetal bonds (described in the chapter on aldehydes and ketones) formed between these two functional groups. Almost all carbohydrates are chiral. This means that they interact with plane-polarised light (see the chapter on chirality). Chirality is significant for their biological activity as many cellular interactions are controlled by specific carbohydrate stereoisomers. This is also why blood transfusions require specific matching blood types, the different blood types arise from different carbohydrates present on the surface of cells. Another important factor governed by the different structures of carbohydrates is their perceived sweetness. One carbohydrate may be very sweet while another with only a minor structural difference may 1152 Chemistry have little apparent sweetness. Notably, all of the various carbohydrates — from the simplest through to the most complex stereoisomers important in biological activity — have ultimately arisen from the simple nonchiral CO 2 molecule via plant photosynthesis. 22.2 Monosaccharides LEARNING OBJECTIVE 22.2 Describe monosaccharides using aldose/ketose terminology. The word ‘saccharide’ comes from the ancient Latin word for ‘sweet’ and monosaccharides are the simplest forms of carbohydrate, unable to be hydrolysed to anything smaller. Monosaccharides have the general formula C n H 2n O n , with one of the carbon atoms being part of a carbonyl group of either an aldehyde or a ketone.

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  Organic and Biological Chemistry

  • H. Stephen Stoker(Author)
  • 2015(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 268 CHAPTER 7 Carbohydrates The following structural diagram highlights the hemiacetal functional group that is present in cyclic forms of a monosaccharide. A A A The hemiacetal functional group A O C O O O C O OH The location of the hemiacetal group in glucose CH 2 OH OH OH O C H H HO C H H H OH Special Terminology for Cyclic Monosaccharide Structures Special terminology exists for the hemiacetal carbon atom present in a cyclic monosac-charide structure. This carbon atom is called the anomeric carbon atom. An anomeric carbon atom is the hemiacetal carbon atom present in a cyclic monosaccharide structure. It is the carbon atom that is bonded to an ! OH group and to the oxygen atom in the heterocyclic ring. Cyclic monosaccharide formation always produces two stereoisomers—an alpha form and a beta form. These two isomers are called anomers. Anomers are cyclic mon-osaccharides that differ only in the positions of the substituents on the anomeric (hemi-acetal) carbon atom. The a -stereoisomer has the ! OH group on the opposite side of the ring from the ! CH 2 OH group, and the b -stereoisomer has the ! OH group on the same side of the ring as the ! CH 2 OH group. OH O CH 2 OH a anomer b anomer OH O anomeric carbon atom anomeric carbon atom anomeric carbon atom CH 2 OH O OH Chirality considerations give further insights into the relationship between the alpha and beta forms of a cyclic monosaccharide. The anomeric carbon atom pre-sent in both structures is the carbonyl carbon atom in the open-chain form of glu-cose. This carbon atom, carbon 1, is achiral in the open-chain form but is chiral in the cyclic forms.

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  General, Organic, and Biological Chemistry

  An Integrated Approach

  • Kenneth W. Raymond(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  371 After completing this chapter, you should be able to: 10 C arbohydrates CHAPTER 10 OBJECTIVES ABOUT THIS CHAPTER Early studies on the sugar named glucose showed that it has the molecular formula C 6 H 12 O 6 . Rearranging this formula into C 6 (H 2 O) 6 made it appear that glucose is a hydrate of carbon—six carbon atoms attached to six water molecules—so the name carbohydrate was adopted for glucose and related compounds. Although we now know that carbohydrates are not hydrates of carbon, the name has been retained. 1 Describe the difference between mono-, oligo-, and polysaccharides and explain the classification sys- tem used to categorize monosaccharides. 2 Explain the terms chiral molecule and chiral carbon atom, distinguish between enantiomers and diastereomers, and define the terms dextrorotatory and levorotatory. Describe what distinguishes a D-monosaccharide from an L-monosaccharide. 3 Identify common monosaccharides and describe the structure of the four common types of monosac- charide derivatives. 4 Explain what happens when a monosaccharide is reacted with H 2 and Pt or with Benedict’s reagent. 5 Define the term anomer and describe the difference between pyranoses and furanoses and the differ- ence between a and b anomers. Explain what is meant by the term mutarotation. 6 Identify four important disaccharides and describe how the monosaccharide residues in them are joined to one another. 7 Distinguish homopolysaccharides from heteropolysaccharides and give examples of each. 372 CHAPTER 10 Carbohydrates 6CO 2 + 6H 2 O ¡ C 6 H 12 O 6 + 6O 2 © Karen Hermann/iStockphoto On the earth, more than half of the carbon atoms tied up in organic compounds are found in carbohydrates. Plants produce most of these carbohydrates, doing so through photosynthesis. In this process energy from the sun is converted into chemical energy that is, in turn, used to produce carbohydrates from atmospheric CO 2 (Figure 10.1).

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