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Questions and Answers
Which form of D-glucose predominates in solution due to reduced steric hindrances?
What is the ratio of β to α anomer conformations for D-glucose in an equilibrium solution?
Which of the following best describes why D-glucose is an important fuel for most organisms?
Which statement about reducing sugars is correct?
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What happens during mutarotation of cyclic sugars?
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Which form of glucose does not react with oxidizing agents?
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What is true about the chair and boat forms of β-D-glucose?
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Which of the following sugars is a known reducing sugar?
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What is the empirical formula for carbohydrates?
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Which of the following describes the structural classification of monosaccharides?
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What is a constitutional isomer?
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How many carbon atoms can monosaccharides contain?
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Which type of monosaccharide is classified as a pentose?
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What term is used to describe sugars that differ in configuration only at a single asymmetric center?
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What happens to an aldehyde when it reacts with an alcohol?
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What is the role of the anomeric carbon in glucose?
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Which of the following terms refers to diastereoisomers that are not mirror images of each other?
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What type of cyclic structure is formed by sugars that resemble furan?
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Study Notes
Carbohydrates and Glycoproteins
- Carbohydrates are carbon-based molecules with a high proportion of hydroxyl groups.
- Their empirical formula is (CH2O)n, but they can have additional groups or modifications.
- They are better described as polyhydroxy aldehydes and ketones.
- Monosaccharides are the simplest carbohydrates and consist of three to seven carbons.
- They are also called simple sugars.
Monosaccharide Nomenclature
- Nomenclature is based on carbon chain length:
- Three carbons: trioses
- Four carbons: tetroses
- Five carbons: pentoses
- Six carbons: hexoses
- Seven carbons: heptoses
- Nomenclature is also based on the identity of the most oxidized group:
- Keto group: ketose
- Aldehyde group: aldose
Isomers
- Constitutional Isomers: Molecules with identical molecular formulas but differ in the order of their atoms.
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Stereoisomers: Molecules that differ in spatial arrangement but not bonding order.
- They have either D or L configuration.
- Can be enantiomers (mirror images of each other) or diastereoisomers (not mirror images of each other).
- The number of possible stereoisomers is 2n, where n is the number of asymmetric carbon atoms.
Common Monosaccharides
- Epimers: Sugars that are diastereoisomers differing in configuration only at a single asymmetric center.
Cyclic Forms of Monosaccharides
- Most monosaccharides exist primarily in their ring forms.
- An aldehyde can react with an alcohol to form a hemiacetal.
- A ketone can react with an alcohol to form a hemiketal.
Pyranose Formation
- Occurs when a six-membered ring is formed.
- Named after the compound pyran.
Furanose Formation
- Occurs when a five-membered ring is formed.
- Named after the compound furan.
Anomers of Glucose
- Anomer: A diastereoisomeric form of sugars that forms when a cyclic hemiacetal forms and an additional asymmetric center is created.
- In glucose, C-1 (the anomeric carbon atom) becomes an asymmetric center, forming two ring structures:
- α-D-glucopyranose: Hydroxyl group attached to C-1 is on the opposite side of the ring as C-6.
- β-D-glucopyranose: Hydroxyl group attached to C-1 is on the same side of the ring as C-6.
D-Fructose Ring Structures
- C-2 is the anomeric carbon atom.
- The pyranose form predominates in solution due to reduced steric hindrances.
- The furanose form predominates in fructose derivatives.
Chair and Boat Forms of β-D-Glucose
- The chair form predominates because all axial positions are occupied by hydrogens.
- The boat form is disfavored because it is sterically hindered.
D-Glucose as a Fuel
- Blood Sugar: D-glucose circulating in the blood.
- Used as the sole fuel by the brain (in non-starvation conditions) and red blood cells.
- Potential reasons why D-glucose is an important fuel:
- Glucose is formed from formaldehyde under prebiotic conditions and may have been available as a fuel source for primitive biochemical systems.
- It is relatively inert.
- The most stable ring structure is β-D-glucopyranose.
Solutions of Glucose
- Two anomeric forms (α and β) are in equilibrium that passes through the open-chain form.
- The β anomer is more stable in solution.
Reducing Sugars
-
Reducing Sugars: Sugars that react with oxidizing agents.
- All monosaccharides can adopt linear structures in solution.
- Non-Reducing Sugars: Sugars that do not react with oxidizing agents.
- Fehling's solution: Solutions of Cu2+ used to test for the presence of reducing sugars.
Mutarotation
- The change in the specific rotation of a cyclic sugar as it reaches an equilibrium between its alpha and beta anomeric forms.
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