Ketohexose Structure

9 Key excerpts on "Ketohexose Structure"

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  eBook - ePub

  Essentials of Chemical Biology

  Structures and Dynamics of Biological Macromolecules In Vitro and In Vivo

  • Andrew D. Miller, Julian A. Tanner(Authors)
  • 2023(Publication Date)
  • Wiley

    (Publisher)

  Chapter 4 .

  1.3 Carbohydrate structures

  1.3.1 Primary structure

  Carbohydrate polymers are formed from the linear and branched combination of a wide variety of different naturally occurring simple sugars known as monosaccharides. These are a reasonably diverse set of molecular building blocks each capable of linking to other monosaccharides in a variety of different ways, with the result that the exquisite hierarchy of structural elements found in protein structures are not so readily duplicated with carbohydrate polymers. Having said this, those carbohydrate homopolymers that are constructed from only one or two monosaccharide building blocks can possess really impressive 3D structures. Therefore, read on.

  Monosaccharides may be classified into families according to the number of carbon atoms they contain, usually between three and seven. The triose family has the empirical formula C3 H6 O3 , the tetrose family C4 H8 O4 , the pentose family C5 H10 O5 , the hexose family (C6 H12 O6 ) and the heptoses C7 H14 O7 . An alternative classification has been to name monosaccharides as either aldehydo-aldose or ketose sugars, depending upon whether they possess an aldehyde or ketone functional group respectively. By way of illustration, the well-known sugar glucose is both a hexose, with six carbon atoms, and an aldehydo-aldose monosaccharide, owing to the aldehyde functional group at carbon atom 1 (C-1) (Figure 1.33 ). Fructose is also a hexose, but is otherwise known as a ketose sugar because of the ketone functional group positioned at carbon atom 2 (C-2) (Figure 1.34 ). Each exists in two enantiomeric forms (either D or L), as defined by the absolute stereochemistry of the penultimate carbon atom in the chain (in both cases carbon atom 5, C-5) with reference by convention to the C-2 stereochemistry of D-/L-glyceraldehyde (Figure 1.35

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  eBook - ePub

  Chemistry of Biomolecules, Second Edition

  • S. P. Bhutani(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  D (+)-tagatose occur in nature.

  1.13  STRUCTURE OF D(−)-FRUCTOSE

  D (−)-Fructose is present in the free state in some fruits and honey and in the combined form with glucose in the most important disaccharide, sucrose. Fructose is also quite sweet. Naturally occurring fructose is laevorotatory and is, therefore, called as laevulose.

  Open-chain structure of D (−)-fructose is derived on the basis of the following observations:

  1.  D (−)-Fructose has the molecular formula C6 H12 O6 as deduced from elemental analysis and molecular weight determination.

  2.  Fructose forms the oxime when treated with hydroxylamine and a cyanohydrin is formed with hydrogen cyanide. These reactions indicate the presence of a carbonyl group. 3.  Fructose is a reducing sugar. It reduces Tollen’s reagent and Fehling solution.

  4.  Fructose is reduced with sodium-amalgam and water to a mixture of hexahydric alcohols, sorbitol and mannitol. The alcohols on treatment with hydriodic acid and red phosphorus at 100°C give a mixture of n-hexane and 2-iodohexane.

  5.  Fructose forms the penta-acetate when treated with acetic anhydride and anhydrous sodium acetate indicating the presence of five hydroxyl groups.

  6.  Fructose is not oxidised by bromine water. However, oxidation with nitric acid affords a mixture of trihydroxy glutaric acid, tartaric acid and glycollic acid. Since all these acids contain fewer carbon atoms than the fructose molecule, the carbonyl group in the fructose must be present as a keto group.

  Fig. 1.10. Reactions of fructose.

  The above reactions (given in Fig. 1.10) show that fructose has a six carbon straight chain structure as given above:

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  Organic Chemistry

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

    (Publisher)

  As a result, there are only four d ketohexoses, rather than eight. The most common naturally occurring ketose is d-fructose. Each compound in Figure 24.6 has a corresponding l enantiomer that is not shown. FIGURE 24.6 A family tree of the D ketoses that have between three and six carbon atoms. CH 2 OH CH 2 OH C O CH 2 OH OH H CH 2 OH C O OH H OH H OH H CH 2 OH CH 2 OH C O D-Psicose H HO OH H OH H CH 2 OH CH 2 OH C O D-Fructose OH H H HO OH H CH 2 OH CH 2 OH C O D-Sorbose C O H HO H HO OH H CH 2 OH CH 2 OH D-Tagatose CH 2 OH OH H OH H CH 2 OH C O D-Ribulose CH 2 OH H HO OH H CH 2 OH C O D-Xylulose D-Erythrulose Dihydroxyacetone CONCEPTUAL CHECKPOINT 24.7 Draw and name the enantiomer of D-fructose. 24.8 Which of the following terms best describes the relationship between D-fructose and D-glucose? Explain your choice. (a) Enantiomers (b) Diastereomers (c) Constitutional isomers 1160 CHAPTER 24 Carbohydrates 24.5 Cyclic Structures of Monosaccharides Cyclization of Hydroxyaldehydes Recall from Section 19.5 (Mechanism 19.5) that an aldehyde can react with an alcohol in the pres- ence of an acid catalyst to produce a hemiacetal. O H ROH H HO OR Hemiacetal [H + ] + We saw that the equilibrium does not favor formation of the hemiacetal. However, when the alde- hyde group and the hydroxyl group are contained in the same molecule, an intramolecular process can occur to form a cyclic hemiacetal with a more favorable equilibrium constant. O Cyclic hemiacetal H HO OH O H O OH H [H + ] Six-membered rings are relatively strain free, and the equilibrium favors formation of the cyclic hemiacetal. This type of reaction is characteristic of bifunctional compounds containing both a hydroxyl group and a carbonyl group (an aldehyde or ketone group). When drawing the hemiacetal of a hydroxyaldehyde or hydroxyketone, it is crucial to keep track of all carbon atoms. It is a com- mon mistake to draw the hemiacetal with either too many or too few carbon atoms.

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  Handbook of Biochemistry and Molecular Biology

  • Roger L. Lundblad, Fiona Macdonald, Roger L. Lundblad, Fiona Macdonald(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  glycero-. In practice this is often omitted, and such compounds can often be named equally well as aliphatics.

  The consecutive asymmetric carbon atoms need not be contiguous. Thus, the following four arrangements are all L - erythro- (X is attached to the lowest-numbered carbon atoms).

  Fundamental Ketoses

  The most important ketoses are the hexos-2-uloses HOCH2 (CH2 )3 COCH2 OH such as fructose. They have one less chiral centre than the aldoses of the same chain length – that is, there are only four diastereomerically different hexos-2-uloses.

  The trivial names for the 2-hexuloses and their formulae are: Modified Aldoses and Ketoses

  Suffixes are employed to denote modification of functional groups in an aldose or ketose, for example, by oxidation of an OH group (Table 2 ).

  TABLE 2 Suffixes Used in Carbohydrate Nomenclature

  -ose	aldose	X = CHO, Y = CH2 OH
  -odialdose	dialdose	X = Y = CHO
  -onic acid	aldonic acid	X = COOH, Y = CH2 OH
  -uronic acid	uronic acid	X = CHO, Y = COOH
  -aric acid	aldaric acid	X = Y = COOH
  -itol	alditol	X = Y = CH2 OH
  -ulose	ketose	X = Y = CH2 OH
  -osulose	ketoaldose	X = CHO, Y = CH2 OH
  -ulosonic acid	ulosonic acid	X = COOH, Y = CH2 OH
  -ulosuronic acid	ulosuronic acid	X = CHO, Y = COOH
  -ulosaric acid	ulosaric acid	X = Y = COOH
  -odiulose	diketose

  Higher Sugars

  Sugars having more than six carbon atoms are named using two prefixes, one defining the configuration at C-2 to C-5 as in a hexose and the other, which appears first in the name, defining the configuration at the remaining chiral centres.

  Examples of the use of configurational prefixes are: Cyclic Forms: Anomers

  When a monosaccharide exists in the heterocyclic intramolecular hemiacetal form, the size of the ring is indicated by the suffixes - furanose, - pyranose and - septanose for five-, six- and seven-membered rings, respectively.

  Two configurations, known as anomers, may result from the formation of the ring. These are distinguished by the anomeric prefixes α- and β-, which relate the configuration of the anomeric carbon atom to the configuration at a reference chiral carbon atom (normally the highest-numbered chiral carbon atom). For example, consider the glucopyranoses:

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  Principles of Organic Chemistry

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  When the hydroxyl group and the carbonyl group are part of the same molecule, a cyclic hemiacetal is formed in an intramolecular reaction. Cyclic hemiacetals containing five or six atoms in the ring form readily because of the proximity of the two functional groups.

  The cyclic hemiacetal or hemiketal forms of aldo-and ketohexoses and pentoses are the predominant forms of monosaccharides rather than the open-chain structures we have discussed to this point. Cyclic hemiacetals and hemiketals of carbohydrates that contain five-membered rings are called furanoses ; cyclic hemiacetals and hemiketals that contain six-membered rings are called pyranoses . These structures are usually represented by planar structures called Haworth projection formulas.

  Haworth Projection Formulas

  In a Haworth projection formula, a cyclic hemiacetal or hemiketal is represented as a planar structure and viewed edge-on. Bond lines representing atoms toward the viewer are written as heavy wedges; bond lines away from the viewer are written as ordinary bond lines. The carbon atoms are arranged clockwise with C-1 of the pyranose of an aldohexose on the right. For the furanose of the ketohexose shown, C-2 is placed on the right side of the structure.

  We can convert the Fischer projection formula of D -glucose into a hemiacetal written as a Haworth projection formula in the following way: The open-chain form of D -glucose is shaped like a “C” that is arranged vertically on the page. Turn this curved chain to the right so that it is horizontal. Groups on the right in the Fischer projection are then directed downward, whereas groups on the left are directed upward.

  In this conformation, the C-5 –OH group is not near enough to the carbonyl carbon atom to form a ring. To bring the C-5 —OH group near the carbonyl carbon atom, rotate the structure about the bond between C-4 and C-5. The —CH2 OH group is now above the plane of the curved carbon chain, and the C-5 hydrogen atom is below the plane.

  Now add the oxygen atom of the C-5 –OH group to the carbonyl carbon atom, and add a hydrogen atom to the carbonyl oxygen atom. A six-membered ring that contains five carbon atoms and one oxygen atom results. There are now four different groups bonded to C-1 in this cyclic hemiacetal. Thus, a new stereogenic center is formed at the original carbonyl carbon atom, and two configurations are possible at C-1. If the hydroxyl group of the hemiacetal is directed below the plane, the compound is α-D -glucopyranose; if it is above the plane, the compound is β-D

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  Carbohydrate-Modifying Biocatalysts

  • Peter Grunwald(Author)
  • 2011(Publication Date)
  • Jenny Stanford Publishing

    (Publisher)

  However, it has to be noted that the absolute structure of the molecule cannot be directly inferred from the name; it can only be indirectly obtained on the background of additional knowledge of the glucose structure. Diastereoisomers that exhibit the opposite configuration at only one of two or more tetrahedral stereogenic centres present in the respective molecular entities are called epimers . This holds, for example, for the diastereomeric pair d -glucose and d -mannose (see Fig. 1.2). Alternatively, the R-/ S- notation that is based on the Cahn– Ingold–Prelog (CIP) rules can be applied to carbohydrates with several stereogenic centres. For example, strict assignment of each stereogenic centre in d-glucose to the R-/ S- notation results in the exact IUPAC name (2 R ,3 S ,4 R ,5 R ,6)-pentahydroxyhexanal. In contrast to the d / l notation, this name allows an exact assignment of each stereogenic centre to its absolute structure. However, as this name is very inconvenient in publications, the name d-glucose as well as the respective names of other sugars according to the d/l notation are still dominating. 1.2.1.2 Ring structures of carbohydrates As early as 1883 Tollens had recognized that simple aldohexoses such as glucose do not show all characteristics of aldehydes. In Classification of Carbohydrates 6 Basics in Carbohydrate Chemistry particular, no addition of sodium hydrogensulfate or ammoniak were observed, and no reaction with Schiff’s reagent takes place. Tollens inferred from these observations that simple sugars do not prefer the aldehyde or keto form arrangement. Instead, an intramolecular hemiacetal formation may lead to cyclic structures, as shown in Fig. 1.4. These Tollens formulae suggest that basically five- or six-membered rings can be formed by reaction of the carbonyl function with the hydroxy group at C4 or C5. Furthermore, it is evident that an additional stereogenic centre is formed at the C1 position.

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  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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  Functional and Molecular Glycobiology

  • Dr Susan Brooks, Dr M Dwek, Udo Schumacher(Authors)
  • 2023(Publication Date)
  • Garland Science

    (Publisher)

  1.10 ).

  In the open chain form, one carbon has a double bond to an oxygen atom forming a carbonyl (C == O) group, and each of the other carbon atoms carries a hydroxyl (OH) group. The carbon atoms carrying the hydroxyl groups are chiral centres (see 1.4 ), which give rise to the many stereoisomers of monosaccharides found in nature. If the carbonyl group is at the end of the chain (i.e. in an aldehyde group), the monosaccharide is an aldose; if the carbonyl group is at any other position (i.e. in a ketone group), the monosaccharide is a ketose. The two main families of monosaccharides are therefore aldoses and ketoses. From this, carbohydrates usually have trivial names that end in ‘-ose’, for example, glucose, sucrose and cellulose.

  A monosaccharide is an aldose if the carbonyl (C = O) group is at the end of the chain and a ketose if it is at any other position.

  The simplest monosaccharides are the three carbon trioses, glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose), illustrated in Figure 1.1 . Monosaccharides with four carbon atoms are tetroses, those with five carbons are pentoses, those with six carbons are hexoses, and those with seven carbon atoms are heptoses. There are aldoses and ketoses with all of these different chain lengths, and some are listed in Table 1.1 . The aldohexose glucose is the commonest naturally occurring aldose and the ketohexose fructose is the commonest naturally occurring ketose.

  1.4 A carbon atom may be asymmetric – the concept of chirality

  Central to the understanding of carbohydrate chemistry is the idea that a carbon atom can be asymmetric. This is illustrated in Figure 1.2 . The two molecules (a) and (b) in Figure 1.2 are said to have different configurations. The central C in the middle of the pyramid carries four different

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  Carbohydrate Chemistry for Food Scientists

  • James N. BeMiller(Author)
  • 2018(Publication Date)
  • Woodhead Publishing and AACC International Press

    (Publisher)

  D -fructose.

  D -Fructose is the principal commercial ketose and the only one of importance in foods. (In the past, D -fructose was called both levulose and fruit sugar, but these designations are rarely used today.) D -Fructose has only three chiral carbon atoms (C3, C4, and C5). Thus, there are only 23 or 8 ketohexoses. The various ketotetroses, -pentoses, and -hexoses are related to nonchiral dihydroxyacetone. The suffix designating a ketose in systematic carbohydrate nomenclature is –ulose (Table 1.1 ). In systematic nomenclature, D -fructose is D -arabino -hexulose because its three chiral carbon atoms have the same configuration as those in D -arabinose. The Rosanoff projection of a ketopentose (pentulose) with the D-threo configuration (that is the configuration of the two chiral carbon atoms in D -threose) is given in Fig. 1.3 as another demonstration of the nomenclature principle.

  Figure 1.3  Rosanoff projection of a ketopentose (D-threo -pentulose, “D -xylulose”) showing the configurations of the two chiral carbon atoms.

  Isomerization

  Simple aldoses and ketoses containing the same number of carbon atoms are isomers of each other (that is, a hexose and a hexulose both have the empirical formula C6 H12 O6 ). Isomerization5 of monosaccharides involves both the carbonyl group and the adjacent hydroxyl group. By isomerization, an aldose is converted into another aldose (with the opposite configuration of C2) and the corresponding ketose, and a ketose is converted into the corresponding two aldoses (Fig. 1.4 ). Therefore, by isomerization, D -glucose, D -mannose, and D -fructose can be interconverted (Fig. 1.5 ). Isomerization can be catalyzed by either a base or an enzyme (Chapter 7

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