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Carbohydrates

Carbohydrates. Chapter 25. Carbohydrates. Carbohydrate: A polyhydroxy aldehyde , a polyhydroxy ketone , or a compound that gives either of these compounds after hydrolysis. Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.

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Carbohydrates

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  1. Carbohydrates Chapter 25

  2. Carbohydrates • Carbohydrate: A polyhydroxyaldehyde, a polyhydroxyketone, or a compound that gives either of these compounds after hydrolysis. • Monosaccharide:A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate. • They have the general formula CnH2nOn, where n varies from 3 to 8. • Aldose: a monosaccharide containing an aldehyde group. • Ketose: a monosaccharide containing a ketone group.

  3. Importance of Carbohydrates to us….

  4. Monosaccharides • Monosaccharides are classified by their number of carbon atoms:

  5. Monosaccharides • There are only two trioses: • These compounds are referred to simply as trioses, tetroses, and so forth tellling the number of carbon atoms present. Chiral Center R and S stereoisomers

  6. Review Fischer Projections • Fischer projection:A two dimensional representation for showing the configuration of carbohydrates. • Horizontal lines represent bonds projecting forward. • Vertical lines represent bonds projecting to the rear. • The more highly oxidized carbon is shown at the top.

  7. D,L Monosaccharides • In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.

  8. D,L Monosaccharides • According to the conventions proposed by Fischer: • D-monosaccharide: A monosaccharide that has the same configuration at its penultimate (next to bottom) carbon as D-glyceraldehyde; that is, its -OH is on the right when written as a Fischer projection. • L-monosaccharide: A monosaccharide that has the same configuration at its penultimate carbon as L-glyceraldehyde; that is, its -OH is on the left when written as a Fischer projection.

  9. D,L Monosaccharides • D-aldotetroses and the two most abundant D-aldopentoses in the biological world: D-aldotetroses D-aldopentoses Expect four stereoisomer 22 for aldotetroses. Two D and two L. Expect total of 8 (=23) steroisomers. Four D, four L.

  10. D,L Monosaccharides • The most abundant hexoses: ketohexose aldohexoses Expect 16 stereoisomers 24 for aldohexoses. Eight D and eight L. Expect 8 stereoisomers 24 for ketohexoses. Four D and four L.

  11. D Aldohexoses binary display Allaltruists gladly make gum in gallon tanks. L.Fieser There is also the L series, the mirror image structures 0= 000 1=001 2=010 3=011 4=100 5=101 6=110 7=111

  12. Amino Sugars • Amino sugar: A sugar that contains an -NH2 group in place of an -OH group. • Only three amino sugars are common in nature • N-Acetyl-D-glucosamine is a derivative of D-glucosamine.

  13. Physical Properties • Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol. • sweetness relative to sucrose:

  14. Cyclic Structure • Monosaccharides have hydroxyl and carbonyl groups in the same molecule and those with five or more carbons exist almost entirely as five- and six-membered cyclic hemiacetals. • Anomeric carbon: The new stereocenter created as a result of cyclic hemiacetal formation. • Anomers:Carbohydrates that differ in configuration at their anomeric carbons named a and b.

  15. Haworth Projections • Haworth projections • Five- and six-membered hemiacetals are represented as planar pentagons or hexagons viewed through the edge. • They are commonly written with the anomeric carbon on the right and the hemiacetal oxygen to the back right. • The designation - means that the -OH on the anomeric carbon is cis to the terminal -CH2OH; - means that it is trans to the terminal -CH2OH.

  16. Haworth Projections Lay molecule on side. top cis, b trans, a

  17. Haworth Projections • Six-membered hemiacetal rings are shown by the infix -pyran-. • Five-membered hemiacetal rings are shown by the infix -furan-.

  18. Conformational Formulas • Five-membered rings are so close to being planar that Haworth projections are adequate to represent furanoses.

  19. Conformational Formulas • Other monosaccharides also form five-membered cyclic hemiacetals. • Here are the five-membered cyclic hemiacetals of D-fructose, a ketohexose.

  20. Conformational Formulas; b to a conversion • For pyranoses, the six-membered ring is more accurately represented as a chair conformation. Open chain form

  21. Conformational Formulas • The orientations of groups on carbons 1-5 in the Haworth and chair projections of -D-glucopyranose are up-down-up-down-up.

  22. Mutarotation • Mutarotation: The change in specific rotation that occurs when an  or  form of a carbohydrate is converted to an equilibrium mixture of the two.

  23. Hemiacetals and Acetals, carbonyls and alcohols Addition reaction. (Unstable in Acid; Unstable in base) (Unstable in Acid; Stable in base) Substitution reaction

  24. Glycosides, anomeric OH becomes OR, acetal formation. • Glycoside:A carbohydrate in which the -OH of the anomeric carbon is replaced by -OR. • methyl-D-glucopyranoside (methyl -D-glucoside)

  25. Glycosides, acetals • Glycosidic bond:The bond from the anomeric carbon of the glycoside to an -OR group. • Glycosides are named by listing the name of the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate with the ending -e replaced by -ide. • methyl -D-glucopyranoside • methyl -D-ribofuranoside

  26. N-Glycosides • The anomeric carbon of a cyclic hemiacetal also undergoes reaction with the N-H group of an amine to form an N-glycoside. • N-glycosides of the following purine and pyrimidine bases are structural units of nucleic acids.

  27. N-Glycosides • The b-N-glycoside formed between D-ribofuranose and cytosine.

  28. Reactions

  29. Reduction to Alditols, aldehyde  alcohol • The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2/M. An alditol

  30. Other alditols • Other alditols common in the biological world are:

  31. Oxidations Oxidation can be done in several ways. Tollens reagent (Ag+(NH3)2 or Benedict’s solution (Cu2+ tartrate complex). Not synthetically useful due to side reactions. Bromine water oxidizes aldoses (not ketoses) to monocarboxylic acids (Aldonic Acids). Nitric Acid oxidizes aldoses to dicarboxylic acids (Aldaric acids). Enzyme catalyzed oxidation of terminal OH to carboxylic acid (Uronic Acid) Periodic Acid oxidizes and breaks C C bonds. Later for that.

  32. Reducing Sugars • Sugars with aldehyde (or ketone group) in solution. The group can be oxidized and is detected with Tollens or Benedicts solution. Ketone groups converted to aldehyde via tautomeric shifts (later).

  33. Problem with Tollens • 2-Ketoses are also oxidized to aldonic acids in basic solution (Tollens). Ketose to aldose conversion via keto enol tautomerism Oxidation Reducing sugar

  34. Oxidation to carboxylic acids

  35. Oxidation to Uronic Acids • Enzyme-catalyzed oxidation of the terminal -OH group gives a -COOH group.

  36. Oxidation by periodic acid, HIO4 or H5IO6 • Periodic acid cleaves the C-C bond of a glycol.

  37. Oxidation by HIO4 • It also cleaves -hydroxyketones • and -hydroxyaldehydes.

  38. Examples. Identify each of the glucose derivatives. Analysis of A: 4 moles of periodic acid used. 4 bonds broken. Products: Formic acid from –CHOH- or CHO-. Formaldehyde from CH2OH- OHC-CO2H from –CHOH-CO2H

  39. Another example • Oxidation of methyl -D-glucoside consumes two moles of HIO4 and produces one mole of formic acid, which indicates three adjacent C-OH groups. 2 HIO4

  40. Osazones, Epimers aldose

  41. Use of osazone in structure determination Fischer found that (+) glucose and (+) mannose yielded the same osazone indicating that they differed only at the C2 configuration. Hence, if we know the configuration of (+) glucose we immediately have that of (+) mannose. Stereoisomers that differ in configuration at only one stereogenic center are called epimers. D-glucose and D-mannose are epimers.

  42. Glucose Assay: diabetes (background) • The analytical procedure most often performed in the clinical chemistry laboratory is the determination of glucose in blood, urine, or other biological fluid.

  43. Glucose Assay • The glucose oxidase method is completely specific for D-glucose.

  44. Glucose Assay • The enzyme glucose oxidase is specific for -D-glucose. • Molecular oxygen, O2, used in this reaction is reduced to hydrogen peroxide H2O2. • The concentration of H2O2 is determined experimentally, and is proportional to the concentration of glucose in the sample. • In one procedure, the hydrogen peroxide oxidizes o-toluidine to a colored product, whose concentration is determined spectrophotometrically.

  45. Killani-Fischer lengthening of chain As lactones Get both epimers.

  46. Ruff Degradation shortening of chain

  47. Fischer proof of structure of glucose • Emil Fischer received the 1902 Nobel prize for determining the structure of glucose. • What was available to him in 1888? • Theory of stereoisomerism • Ruff degradation • Oxidation to aldonic and aldaric acids • Killani-Fischer synthesis • Various aldohexoses and aldopentoses

  48. Fischer proof of structure of glucose

  49. Fischer started with the aldopentoses

  50. Experiments on (-) arabinose Must be an OH here

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