Biochemistry: Sugars and Isomerism

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Questions and Answers

What type of isomers have the same molecular formula but differ in the grouping of their functional groups?

Functional group isomers (correct)

Positional isomers

Cis-trans isomers

Structural isomers

Which of the following best describes a racemic mixture?

A mixture containing equal concentrations of d- and l- isomers (correct)

A mixture with one optical isomer

A mixture of glyceraldehyde and dihydroxyacetone

A mixture of two structural isomers

What property is exhibited by compounds with an asymmetric carbon atom?

Thermal conductivity

Increased melting point

Chemical stability

Optical activity (correct)

Which isomer has substituent groups positioned differently on carbon atoms?

Positional isomers Signup and view all the answers

What is the term used for isomers that have the same structural formula but differ in spatial arrangement?

Stereoisomers Signup and view all the answers

What type of isomerism occurs when there is a distinction in orientation around double bonds?

Cis-trans isomerism Signup and view all the answers

Which of the following is true about dextrorotatory and levorotatory compounds?

They rotate plane polarized light in opposite directions Signup and view all the answers

What occurs when a recemic mixture is formed?

The optical activities cancel each other out Signup and view all the answers

What type of stereoisomers are the two aldotetroses described?

Enantiomers Signup and view all the answers

In the context of aldohexoses, how do C-4 epimers differ?

They differ only in the position of the hydroxyl group on carbon 4. Signup and view all the answers

Which representation is primarily used for depicting straight chains of sugars?

Fisher projection Signup and view all the answers

In the Haworth projection, if a substituent is on the right in the Fisher projection, where will it be drawn?

Down Signup and view all the answers

For D-sugars, where is the hydroxyl group at the anomeric position drawn in the Haworth projection?

Down Signup and view all the answers

How are the configurations of D-sugars and L-sugars differentiated in their representation?

By the orientation of the highest numbered carbon Signup and view all the answers

Which of the following best describes the representation of aldohexoses?

Most are depicted as six-membered rings. Signup and view all the answers

What type of cyclic representation includes oxygen as one of its atoms?

Haworth projection Signup and view all the answers

What characterizes an enantiomeric pair?

They have the same molecular formula but differ in spatial arrangement. Signup and view all the answers

What is the effect of a dextrorotatory compound on polarized light?

It rotates light in a clockwise direction. Signup and view all the answers

If a compound has 4 asymmetric carbon atoms, how many different optical isomers can it have?

16 Signup and view all the answers

Which of the following statements is true about aldotetroses?

D-erythrose and L-erythrose are enantiomers. Signup and view all the answers

Which of the following compounds is levorotatory?

D-arabinose Signup and view all the answers

What is the label for a compound that rotates polarized light counterclockwise?

Levorotatory Signup and view all the answers

How many asymmetric carbon atoms does a 5-carbon aldose have?

3 Signup and view all the answers

Which of the following compounds has a specific rotation of +66.5 degrees?

Sucrose Signup and view all the answers

What is the number of optical isomers for a ketose with 1 asymmetric carbon atom?

2 Signup and view all the answers

Which factor determines the number of optical isomers a compound can have?

The number of asymmetric carbon atoms in the molecule. Signup and view all the answers

What is the primary form of D-glucose in aqueous solution?

Pyranose form Signup and view all the answers

What characterizes the pyranose ring structure?

It consists of a six-membered cyclic ether. Signup and view all the answers

What type of reaction results in the formation of a pyranose structure from D-glucose?

Intramolecular nucleophilic addition Signup and view all the answers

Which of the following statements about anomers is true?

They are an example of stereoisomers. Signup and view all the answers

What happens during mutarotation?

Conversion of one anomer to another. Signup and view all the answers

Which conformation is not typically associated with pyranose sugars?

Plane conformation Signup and view all the answers

How is the anomeric center defined?

As the former carbonyl carbon in cyclic form. Signup and view all the answers

Which statement accurately describes D-ribose?

It exists as a pentose sugar. Signup and view all the answers

In B-D-glucose, what type of substituents are present in the chair conformation?

Both axial and equatorial Signup and view all the answers

What is the equilibrium ratio of α-anomer to β-anomer of D-glucose?

36% α-anomer, 63% β-anomer Signup and view all the answers

Which of the following best describes a chair conformation?

Allows for minimal steric hindrance. Signup and view all the answers

What distinguishes an aldohexose from a ketohexose?

Presence of a carbonyl group on C1. Signup and view all the answers

Which sugar structure has a higher propensity for glycosidic bond formation?

α-anomer Signup and view all the answers

Which of the following is NOT typically a link found in polysaccharides?

γ(1→4) Signup and view all the answers

What impact does water have on the structure of glucose?

It promotes the formation of a six-membered ring. Signup and view all the answers

Study Notes

Differences in Sugars

Variations in sugar structures lead to differences in physical properties, chemical reactions, and physiological effects.
Key physical properties influenced by structure include crystalline form, solubility, and rotatory power.
Chemical reactions can include oxidation, reduction, and condensation processes.
Physiological differences can affect nutritive value for humans and bacteria, sweetness perception, and absorption rates.

Isomerism

Structural isomers have the same molecular formula but differ in structure.
- Functional group isomers have different functional groups (e.g., glyceraldehyde vs. dihydroxyacetone).
- Positional isomers have substituent groups on different carbon atoms.
Stereoisomers share the same structural formula but differ in spatial arrangement.
- Cis-trans isomers have different configurations around double bonds (e.g., fumaric acid and maleic acid).
- Optical isomers display optical activity due to asymmetric carbon atoms, rotating polarized light either to the right (dextrorotatory) or left (levorotatory).
Racemic mixtures contain equal concentrations of d- and l- isomers, showing no net optical activity due to canceling effects.

Optical Isomers

A compound with 'n' asymmetric carbon atoms can produce 2^n optical isomers.
D-glucose, for example, has specific optical activity values:
- D-glucose +52.7, D-fructose -92.4, and D-galactose +80.2, among others.

Types of Isomers

Aldoses and ketoses are types of sugars with varying degrees of asymmetric carbon atoms, impacting the number of optical isomers possible.
D- and L-sugars differ in orientation of the highest numbered carbon, influencing their chemical behavior and interaction.

Fisher Projection and Haworth Projection

Different structural representations exist for sugars: Fischer (straight chain) and Haworth (ring structure).
The Down-Right Rule dictates the drawing orientation of substituents in Haworth projections based on Fischer projections.

Sugar Cyclization

D-glucose can form cyclic structures (pyranose and furanose) through intramolecular nucleophilic addition.
Pyranose form is six-membered, while furanose form is five-membered, influenced by the positioning of hydroxyl groups.

Conformational Isomers

Pyranose rings exhibit chair and boat conformations.
Mutarotation is the process where anomeric forms (α and β) of sugars interconvert over time, establishing an equilibrium.

Anomers and Anomeric Center

Anomers are diastereomers that differ at the anomeric carbon, which is derived from the carbonyl carbon during ring formation.
The terms D- and L- refer to configuration based on the highest numbered chiral center's orientation.

Summary of Sugar Structures

Most aldohexoses tend to form six-membered rings, while smaller sugars may form five-membered rings.
Anomalous linkages exist in polysaccharides, affecting their structure and flexibility.

Conclusion

Understanding sugar isomerism is crucial in biochemistry, influencing nutrition, metabolism, and cellular functions.
The diverse structural forms of sugars lead to significant biochemical implications.

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Description

Explore the fascinating differences in sugar structures and their impact on physical properties, chemical reactions, and physiological effects. Dive into isomerism, understanding both structural and stereoisomers, along with their significance in biochemistry.