Carbohydrates Lecture Notes

Carbohydrates

Learning Objectives

• Describe the four major functions of carbohydrates.

• Classify carbohydrates.

• Identify chiral carbon atoms.

• Use Fischer projections.

• Classify monosaccharides.

• Write reactions for monosaccharide oxidation.

• Describe uses for monosaccharides.

• Draw structures, list sources, and uses for disaccharides.

• Write reactions for disaccharide hydrolysis.

• Describe structures, list sources, and uses for polysaccharides.

Organic and Biochemistry

• Biomolecule: Organic compounds essential to life.

• Biochemistry: Study of compounds and processes in living organisms.

Important Functions of Carbohydrates

• Providing energy through oxidation.

• Supplying carbon for cell components.

• Stored form of chemical energy.

• Forming structural elements of cells and tissues.

Carbohydrates Definition

• Polyhydroxy aldehydes or ketones.

• Example: Ribose

$<br><br>\text{ribose: } \begin{cases}<br><br>\text{CHO} \&lt; \text{H-C-OH} \&lt; \text{H-C-OH} \&lt; \text{H-C-OH} \&lt; \text{CH}_2\text{OH} \end{cases}<br><br>$

Classification of Carbohydrates

• Monosaccharide: Simple carbohydrate (3-6 carbon atoms).

• Disaccharide: Two monosaccharide units.

• Polysaccharide: Many monosaccharide units.

Stereochemistry of Carbohydrates

Enantiomers

• Many carbohydrates exist as enantiomers.

• Enantiomers: Mirror image stereoisomers.

$<br><br>\text{D-glyceraldehyde and L-glyceraldehyde are enantiomers}<br><br>$

Chirality

• Chiral: Cannot be superimposed on mirror image.

• Chiral carbon: Carbon with four different groups attached.

• Glyceraldehyde's center carbon is chiral.

Stereoisomers and Chiral Carbons

• Single chiral carbon gives rise to stereoisomerism.

• Carbon attached to four different groups is chiral.

• If any two groups are identical, it is not chiral.

Multiple Chiral Carbons

• Molecules can have more than one chiral carbon.

• Maximum stereoisomers: $2^n$ ($n$ = chiral carbons).

• Two chiral carbons: $2^2 = 4$ stereoisomers.

• Four chiral carbons: $2^4 = 16$ stereoisomers.

Identifying Chiral Carbons

• Example 1: $CH<em>3CH(OH)CH</em>3$: Not chiral (two $CH_3$ groups).

Identifying Chiral Carbons

• Example 2: $CH<em>3CH(OH)CH</em>2CH*3$: Central carbon is chiral (H, OH, $CH_3$, $CH_2CH_3$).

Determining Number of Stereoisomers

• Example: Glucose (four chiral carbons).

• Stereoisomers = $2^n$, where $n = 4$

• $2^4 = 16$

Fischer Projections

• Depict 3D shapes; chiral carbon = intersection of two lines.

• Carbonyl ($C=O$) at or near the top.

• Hydroxyl group on the chiral carbon farthest from the $C=O$ group determines whether the carbohydrate is D (—OH on right) or L (—OH on left).

Properties of Enantiomers

• Physical properties of D and L isomers are generally the same.

• D and L enantiomers rotate polarized light in equal but opposite directions.

Levorotatory

• Rotates plane-polarized light to the left. (–) enantiomer

Dextrorotatory

• Rotates plane-polarized light to the right. (+) enantiomer

• D and L designations indicate spatial relationship.

Optical Activity

• Optical activity: Property of rotating polarized light.

• Optically active molecule: Molecule that rotates polarized light.

• Measurement differentiates enantiomers.

• Only one stereoisomer is found in nature for carbohydrates and amino acids.

• Examples:

  • Humans metabolize D-isomers of monosaccharides.

  • Most animals utilize L-isomers of amino acids.

Drawing Fischer Projections

• Example: Draw Fischer projections for alanine ($CH<em>3 — CH(NH</em>2) — COOH$)

Drawing Fischer Projections

• Solution: Amino acid alanine has a chiral carbon and a carboxyl group.

• Direction of the —NH2 group determines D and L notations.

$<br><br>\text{L-alanine and D-alanine are mirror images}<br><br>$

Classification of Monosaccharides

• Is it an aldehyde (aldose) or ketone (ketose)?

• How many carbon atoms are present?

Aldoses vs. Ketoses

• Most monosaccharides are aldoses.

• Almost all natural monosaccharides belong to the D series.

Family of D Aldoses

• D-glyceraldehyde (Aldotriose)

• D-erythrose and D-threose (Aldotetroses)

• D-ribose, D-arabinose, D-xylose, and D-lyxose (Aldopentoses)

• D-allose, D-altrose, D-glucose, D-mannose, D-gulose, D-idose, D-galactose, and D-talose (Aldohexoses)

Physical Properties of Monosaccharides

• Most monosaccharides and disaccharides are called sugars because they taste sweet.

• All carbohydrates are solids at room temperature.

• Monosaccharides are water-soluble.

Monosaccharide Reactions

• Monosaccharides with at least five carbon atoms exist as cyclic hemiacetals and hemiketals.

• Haworth structure: Depicts 3D carbohydrate structures.

Cyclization of Glucose

• Formation of $\alpha$-D-glucose and $\beta$-D-glucose.

$<br><br>\text{Cyclization of Glucose}<br><br>$

Cyclization of Monosaccharides

• Open-chain structure numbered closest to carbonyl carbon.

• Alcohol group on the carbon next to the last carbon atom adds to the carbonyl group.

• Glucose: alcohol group on carbon 5 adds to aldehyde group on carbon 1 -> pyranose ring forms.

• Pyranose ring: Six-membered ring containing an oxygen atom.

• Fructose: alcohol group on carbon 5 adds to ketone group on carbon 2 -> furanose forms.

• Furanose ring: Five-membered ring containing an oxygen atom.

• Former carbonyl carbon atom is now chiral (anomeric carbon atom).

Anomers

• Anomeric carbon: Acetal, ketal, hemiacetal, or hemiketal carbon atom giving rise to two stereoisomers.

• Stereoisomers:

  • $\alpha$ anomer: —OH on the anomeric carbon pointing down.

  • $\beta$ anomer: —OH on the anomeric carbon pointing up.

• Anomers: Stereoisomers differing in 3-D arrangement.

Cyclization of Fructose

• Formation of $\alpha$-D-fructose and $\beta$-D-fructose.

Rules for Drawing Haworth Structures

• Draw the ring with its oxygen to the back.

• Put the anomeric carbon on the right side of the ring.

• Envision the ring as planar with groups pointing up or down.

• The terminal —$CH_2OH$ group is always shown above the ring for D-monosaccharides.

Oxidation of Monosaccharide

• All monosaccharides are reducing sugars.

• Reducing sugar: Sugar oxidized by weak oxidizing agents.

• Benedict's reagent tests for reducing sugars:

$\text{Reducing sugar} + Cu^{2+} \rightarrow \text{oxidized compound} + Cu_2O$

• A red-orange precipitate forms from a deep blue solution.

Formation of Phosphate Esters

• The —OH groups of monosaccharides react with acids (especially phosphoric acid) to form esters.

Glycoside Formation

• Glycoside: Carbohydrate containing an acetal or ketal group.

• Formed when cyclic monosaccharides react with alcohols in the presence of acid.

• Glycosidic linkage: New carbon–oxygen–carbon linkage.

• Glycosides are not reducing sugars.

Drawing Haworth Structures

• Example: Aldohexose D-galactose exists in cyclic forms.

• Draw the Haworth structure for the $\alpha$ anomer.

• Label the new compound as $\alpha$ or $\beta$.

Drawing Haworth Structures

• Solution: Draw the pyranose ring with the oxygen atom to the back.

• Number the ring starting at the right side.

• Position number 1 will be the anomeric carbon.

• Place the —OH group at position 1 in the up direction ($\beta$ form).

• Place groups at the other positions exactly as they are in the given compound.

• Remember that anomers differ only in the position of the OH attached to the anomeric carbon.

Important Monosaccharides

Ribose and Deoxyribose

• Are pentoses.

• Used in nucleic acids (DNA and RNA).

• Ribose forms RNA chains.

• Deoxyribose forms DNA chains.

Glucose

• Hexose, nutritionally important.

• Dextrose or blood sugar.

• Converted from other sugars in the liver.

• Sweetener.

Galactose

• Hexose, similar to glucose.

• Component of lactose (milk sugar).

• Component of nerve tissue.

Fructose

• Ketohexose.

• Sweetest monosaccharide.

• Levulose or fruit sugar.

• Present in honey and corn syrup.

Disaccharides

• Two monosaccharide units linked by glycosidic linkages.

• Glycosidic linkage:

  • Numbers of carbon atoms joined.

  • Configuration of anomeric carbon.

Maltose

• Malt sugar.

• Two glucose units joined by an $\alpha$(1→4) glycosidic linkage.

• Found in germinating grain.

• Formed during starch digestion.

• Reducing sugar.

• Forms two D-glucose molecules on hydrolysis.

Lactose

• Milk sugar (5% cow milk, 7% human milk).

• Galactose and glucose units joined by $\beta$(1→4) glycosidic linkage.

• Reducing sugar.

Sucrose

• Common household sugar.

• Fructose and glucose units joined by an $\alpha$-1$\rightarrow$\beta$-2 glycosidic linkage.</p></li><li><p>Found in plant juices.</p></li><li><p>Not a reducing sugar.</p></li><li><p>Hydrolyzed to invert sugar.</p></li><li><p><strong>Invert sugar:</strong> Glucose and fructose mixture.</p></li></ul><h6 collapsed="false" seolevelmigrated="true">Benedict's Test on Disaccharides</h6><ul><li><p>Maltose and lactose react with Benedict's reagent.</p></li><li><p>Sucrose does not react.</p></li></ul><h6 collapsed="false" seolevelmigrated="true">Comparing Important Disaccharides</h6><table style="min-width: 100px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><th colspan="1" rowspan="1"><p>Name</p></th><th colspan="1" rowspan="1"><p>Monosaccharide Constituents</p></th><th colspan="1" rowspan="1"><p>Glycosidic Linkage</p></th><th colspan="1" rowspan="1"><p>Source</p></th></tr><tr><td colspan="1" rowspan="1"><p>Maltose</p></td><td colspan="1" rowspan="1"><p>Two glucose units</p></td><td colspan="1" rowspan="1"><p>$\alpha$(1→4)</p></td><td colspan="1" rowspan="1"><p>Hydrolysis of starch</p></td></tr><tr><td colspan="1" rowspan="1"><p>Lactose</p></td><td colspan="1" rowspan="1"><p>Galactose and glucose</p></td><td colspan="1" rowspan="1"><p>$\beta$(1→4)</p></td><td colspan="1" rowspan="1"><p>Mammalian milk</p></td></tr><tr><td colspan="1" rowspan="1"><p>Sucrose</p></td><td colspan="1" rowspan="1"><p>Glucose and fructose</p></td><td colspan="1" rowspan="1"><p>$\alpha$-1$\rightarrow$\beta$-2

  Sugar cane and beet

  Polysaccharides

  • Condensation polymers.

  • Not water-soluble, form colloidal dispersions.

  • Example: Starch.

  Properties Compared

  Property	Monosaccharides and Disaccharides	Polysaccharides
  Molecular weight	Low	Very high
  Taste	Sweet	Tasteless
  Solubility in water	Soluble	Insoluble or colloidal dispersions
  Size of particles	Pass through a membrane	Do not pass through a membrane
  Test with $Cu^{2+}$	Positive (except sucrose)	Negative

  Important Polysaccharides

  Starch

  • Polymer of D-glucose units.

  • Storage form of D-glucose in plants.

  • Amylose and amylopectin can be isolated.

  Amylose

  • Unbranched chain (10-20%), 1000–2000 glucose units, $\alpha$(1→4) glycosidic linkages.

  Amylopectin

  • Branched chain (80-90%), $\alpha$(