Biochemistry: Monosaccharides and Chirality

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Questions and Answers

What is the orientation of the –CH2OH group in D sugars when represented in Haworth structure?

Up (correct)

Down

Horizontal

None of the above

Which form of D-glucose is more stable due to the equatorial position of its substituents?

D-fructose

α-D-glucose

D-galactose

β-D-glucose (correct)

What process describes the interconversion between α and β isomers of glucose in solution?

Oxidation

Epimerization

Hydration

Mutarotation (correct)

If a solution starts with 100% of α-D-glucose, what will happen to the specific rotation at equilibrium?

It will decrease to 52˚ Signup and view all the answers

In the Fischer structure of D sugars, which groups are positioned 'down'?

All groups on the right Signup and view all the answers

What configuration does the C-5 hydroxyl group have when α-D-glucose is formed?

It is 'down' Signup and view all the answers

What is the specific rotation of the β form of D-glucose at equilibrium?

+52˚ Signup and view all the answers

Which term refers to the carbon atom that forms the hemiacetal in monosaccharides?

Anomeric carbon Signup and view all the answers

What do carbohydrates that reduce Cu(II) to Cu(I) classify as?

Reducing sugars Signup and view all the answers

Which of the following reagents is a stronger oxidizing agent that converts aldehyde and primary alcohol groups to carboxylate groups?

Aqueous nitric acid Signup and view all the answers

Which of the following statements about osazones is TRUE?

Osazones are crystalline derivatives often used for sugar identification. Signup and view all the answers

What happens to monosaccharides in the presence of an equal amount of alcohol in the hemiacetal form?

They form glycosides. Signup and view all the answers

Which of the following describes the products formed when glycosides are hydrolyzed in aqueous acid?

The monosaccharide and the alcohol are produced. Signup and view all the answers

During the reaction with phenylhydrazine, what structural feature do aldoses destroy?

The stereochemistry at C-2 Signup and view all the answers

What is the color change observed when Benedict’s reagent is used to identify reducing sugars?

Blue to brick-red Signup and view all the answers

What type of sugars cannot reduce Cu(II) or Ag(I) ions?

Non-reducing sugars Signup and view all the answers

What distinguishes trioses from other monosaccharides?

They have three carbon atoms. Signup and view all the answers

What defines an epimer in the context of monosaccharides?

Monosaccharides that differ only at one carbon configuration. Signup and view all the answers

How are D and L sugars classified?

According to the structural similarity with glyceraldehyde. Signup and view all the answers

What structural feature is common to cyclic forms of monosaccharides?

Formation of a hemiacetal. Signup and view all the answers

What is the role of glyceraldehyde in classifying monosaccharides?

It serves as a reference for stereochemistry. Signup and view all the answers

Which of the following statements about monosaccharides is true?

Aldoses are monosaccharides with a carbonyl group as an aldehyde. Signup and view all the answers

What occurs when an additional carbon is added to a monosaccharide?

It introduces a new chiral carbon and two new isomers. Signup and view all the answers

In Haworth formulas for monosaccharides, what does the placement of the methylol group indicate?

Whether the sugar is a D or L type. Signup and view all the answers

What type of sugars are glycosides classified as?

Non-reducing sugars Signup and view all the answers

Which enzyme specifically hydrolyzes β glycosidic linkages?

Emulsin Signup and view all the answers

What is the composition of maltose?

Two units of D-glucose Signup and view all the answers

Which disaccharide has a β–1,4 linkage and can be hydrolyzed by emulsin?

Cellobiose Signup and view all the answers

What happens to lactose during hydrolysis?

It produces one unit of D-galactose and one of D-glucose Signup and view all the answers

Why is sucrose classified as a non-reducing sugar?

It has no free anomeric carbons Signup and view all the answers

What is the bond type connecting glucose and fructose in sucrose?

α–1,2–glycoside bond Signup and view all the answers

Maltase specifically targets which type of glycosidic linkage?

α-1,4 Signup and view all the answers

What are the two main polysaccharides that make up starch?

Amylose and Amylopectin Signup and view all the answers

Which type of bond does amylose use to link its D-glucose monomers?

α–1,4–glycoside bonds Signup and view all the answers

Why can't humans use cellulose as a food source?

Humans lack β-glycosidases for breaking down β-glycosidic bonds. Signup and view all the answers

What is the primary function of glycogen in animals?

To serve as an energy reserve Signup and view all the answers

How does excess glucose in the body primarily get stored?

As glycogen in the liver Signup and view all the answers

What characteristic of cellulose contributes to its high strength?

Hydrogen bonding between its chains Signup and view all the answers

Which statement correctly describes amylopectin?

It has a highly branched structure. Signup and view all the answers

What type of sugar is kanosamine classified as?

An amino sugar Signup and view all the answers

Study Notes

Monosaccharides

Classified by number of carbons (triose, tetrose, pentose, hexose) and carbonyl group type (aldose or ketose)
Two trioses exist, each with two hydroxyl groups and one carbonyl group
Glyceraldehyde, the simplest D-aldose, has one chiral carbon and exists in two enantiomeric forms

Chirality in Monosaccharides

Glyceraldehyde is used as a reference for describing carbohydrate structure
D-monosaccharides have the same conformation around the chiral carbon farthest from the aldehyde or ketone group as D-glyceraldehyde
D and L symbols indicate structural similarity to D-glyceraldehyde, not optical rotation
D and L refer to the stereochemistry at the highest numbered chiral carbon

Epimers

Pairs of monosaccharides that differ only in configuration at one carbon
D-ribulose and D-xylulose are epimers, differing at C-3

L-Sugars

Mirror images of corresponding D sugars
Conformation at all chiral carbons is reversed

Cyclic Hemiacetal Structures of Monosaccharides

Monosaccharides exist in cyclic hemiacetal form where the carbonyl carbon forms an ether group and a hydroxyl group
Hemiacetal formation involves the reaction of an alcohol with an aldehyde or ketone
For aldohexoses, C-5 hydroxyl group reacts with the aldehyde carbon
This is an equilibrium reaction favoring the ring form

Conventions for Writing Monosaccharide Structures

Fischer structures can be adapted to show cyclic structures
Haworth formulas use planar hexagons to represent cyclic structures
Ring oxygen is on the top right
Methylol group (CH2OH) is up for D sugars and down for L sugars
For D sugars, groups on the left of the Fischer structure are "up" in Haworth, and groups on the right are "down"
Ring is numbered clockwise, starting from the carbon adjacent to the oxygen
Hydrogen atoms are usually not shown
For L sugars, the methylol group is "down"
To convert a Haworth structure to a chair structure, "up" groups are above the equatorial plane, and "down" groups are below
The CH2OH group is generally equatorial

Monosaccharide Anomers

Stereochemistry at carbons 2, 3, 4, and 5 is fixed in a given aldohexose
Two possible conformations of the newly formed hydroxyl group when C-5 hydroxyl reacts with carbonyl carbon:
- α: hydroxyl group opposite to the CH2OH group ("down")
- β: hydroxyl group in the same orientation as the CH2OH group ("up")
β form of D-glucose is most stable due to equatorial positions of CH2OH and OH groups
These two diastereomers are called anomers
The carbonyl carbon that reacts to form the hemiacetal is the anomeric carbon
Anomers can be isolated and have different chemical and physical properties
Mutarotation is the interconversion between α and β isomers in solution
α-D-glucose has a specific rotation of +112°, while the β form has +19°
A freshly prepared solution of α-D-glucose has a specific rotation of 112°, but decreases to 52° at equilibrium
For β-D-glucose, the specific rotation increases from 19° to 52° at equilibrium

Oxidation Reactions of Monosaccharides

Strong Oxidising Agents: convert aldehyde group and terminal alcohol group to carboxylate groups
Diacids: formed from hexoses, called aldaric acids
- Examples:
  - Bromine Water (Br2 / H2O) - colorless solution
  - Tollens Reagent (Ag+ as Ag[NH3]2+) - colorless solution, forms silver mirror
  - Benedict’s or Fehling’s Reagent (Cu2+) - blue solution, forms brick-red precipitate
Reducing Sugars: carbohydrates that reduce Cu(II) to Cu(I) or Ag(I) to Ag metal
Non-reducing Sugars: do not reduce these reagents
Reducing sugars contain an aldehyde group or an α-hydroxyketone
Under basic conditions, α-hydroxyketones are in equilibrium with the aldehyde form

Reduction Reactions of Monosaccharides

The carbonyl group of a monosaccharide can be reduced to an alcohol using reducing agents like NaBH4 and hydrogen with a catalyst

Formation of Osazones

Aldoses react with phenylhydrazine (Ph–NH–NH2) to form phenylhydrazones
With excess phenylhydrazine, osazones are formed
Osazones are crystalline derivatives with sharp melting points, used for identifying unknown sugars
Osazone formation destroys stereochemistry at C-2
Compounds differing only at C-2 give the same osazone
C-2 ketoses also give osazones
D-glucose, D-mannose, and D-fructose all give the same osazone

Formation of Glycosides (Acetals)

Monosaccharides in the hemiacetal form react with an alcohol to form acetals
Only the anomeric -OH group is replaced by the -OR group
Glycosides are stable in water and aqueous base, but hydrolysed in aqueous acid
Glycosides are widespread in nature
Glycosides can be hydrolysed enzymatically by emulsin (β linkages) and maltase (α linkages)

Disaccharides

Carbohydrates containing two monosaccharide units joined by a glycoside bond
Examples:
- Maltose: found in germinating grains, corn syrup, and obtained from starch hydrolysis
  - Two D-glucose units joined by α-1,4-glycoside bond
  - Reducing sugar, can be hydrolysed by maltase
- Cellobiose: obtained from cellulose hydrolysis
  - Identical to maltose but with β-1,4-linkage
  - Can be hydrolysed by emulsin
- Lactose: found in mammalian milk
  - D-galactose and D-glucose units joined by β-1,4-linkage
  - Reducing sugar due to free hemiacetal system
- Sucrose: table sugar from sugar cane
  - Glucose and fructose units joined by α-1,2-glycoside bond
  - Non-reducing sugar due to absence of free anomeric carbons
  - Hydrolysis forms 50:50 mixture of glucose and fructose called invert sugar

Polysaccharides

Polymers of monosaccharides
Examples:
- Starch: reserve carbohydrate for plants
  - Two main types: amylose and amylopectin
  - Both are polymers of α-glucose
  - Amylose: linear, unbranched chains of up to 4000 D-glucose monomers joined by α-1,4-glycoside bonds
  - Amylopectin: highly branched structure with 24-30 monomer units joined by α-1,4- and α-1,6-glycoside bonds
- Glycogen: reserve carbohydrate for animals
  - Highly branched chains of D-glucose joined by α-1,4- and α-1,6-glycoside bonds
  - Lower molecular weight and more highly branched than amylopectin
- Cellulose: major component of plant cell walls, also in cotton
  - Linear polymer of D-glucose joined by β-1,4-glycoside bonds
  - Humans lack β-glycosidases and cannot digest cellulose
  - Strong material due to hydrogen bonding between chains

Modified Sugars

Deoxy sugars and amino sugars found in natural and synthetic compounds

Kanosamine

3-amino-3-deoxy-D-glucose
Produced by Bacillus cereus UW85, a Gram-positive bacterium

Neuraminic acid

5-amino-3,5-dideoxy-D-glycero-D-galacto-non-2-ulosonic acid
Nine-carbon monosaccharide, amino derivative of a ketononose (nine-carbon keto sugar)

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Description

This quiz explores the structure and classification of monosaccharides, focusing on their carbon composition and the roles of chirality. Participants will learn about key concepts such as D and L sugars, epimers, and the significance of glyceraldehyde in carbohydrate structure. Test your knowledge on these fundamental biochemical components!