CHEM 130A: monosaccharides and disaccharides

If a monosaccharide contains an aldehyde or a ketone, what type of sugar is it, respectively?

aldose or ketose

What does the Fischer projection, name of the sugar, and D- or L- assignment tell us about the stereochemistry of monosaccharides?

Fischer projection helps determine stereochemistry

Name of the sugar tells us the relative stereochemistry through the stereoisomer

D- or L- assignment of the sugar tells us the absolute stereochemistry through the enantiomer

What is the different between D- and L- stereochemistry assignments?

D- is dextro so the OH group is on the right, and is assigned to sugars

L- is levo so the OH group is on the left , and is assigned to amino acids

What is the chiral center in monosaccharides?

the most distant chiral C from the carbonyl C

What are epimers?

two sugars that differ only at a single stereocenter

How do you draw Fischer projections of monosaccharides?

the highest priority functional group is an aldehyde (#1) or ketone (#2), followed by the C-chain with the appropriate chiral center and/or epimers

In acyclic to cyclic equilibria, why are intramolecular reactions more probable than intermolecular reactions?

In intramolecular reactions, the sugar is reacting with itself (vs. intermolecular reactions where separate sugars are interacting).

1) carbonyl C and OH group is already in close proximity via the same C-chain

2) no significant loss in translational entropy/freedom → lower ΔS → favorable

When glucose is in solution, which OH group in the C-chain is the best Nuc and why?

The best OH Nuc in the C-chain is C₅ because it forms stable 5 or 6 membered rings, which is effective in intramolecular reactions for C₁-C₄ or C₂-C₅ collisions.

What is a 5 membered and 6 membered glucose ring called, respectively?

furanose and pyranose

When do you assign α or β to a glucose ring?

α: when the OH group at C₁ is trans to the substituent at C₅

β: when the OH group at C₁ is cis to the substituent at C₅

What is an anomer?

monosaccharide isomers where only the hemiacteal or hemiketal C has a different configuration

What is an anomeric C?

the hemiacetal or hemiketal C stereocenter after cyclization (aka the C directly bonded to two oyxgens)

When are sugars reducing agents?

sugars are reducing agents when it has a free unbonded aldehyde or ketone group

What are the two methods to quantify sugars? Describe each method and why they are useful to quantify sugars?

DNS assay: DNS + aldehyde group of reducing sugar → ANS + H₂O

• this reaction occurs under heated conditions

• DNS is reduced to ANS

• the aldehyde is oxidixed to carboxylic acid

• the solution turns from yellow to red-brown

Fehling’s test: R-CHO + 2 Cu²⁺ + 5 OH^- → R-COO^- + Cu₂O + 3 H₂O

• the aldose reducing sugar is oxidized to carboxylate

• the copper is reduced to cuprous oxide

• from the oxidation of copper, the solution turns from blue to red-brown

DNS assay and Fehling’s test are useful to quantify sugars

• utilize aldehyde to act as a reducing agent

• the intensity of color change is directly proportional to the [reducing agent sugar]

• can measure absorbance of resulting solution to quantify amount of [reducing agent sugar]

What happens in a glycoside formation?

• hemiacetal → acetal, forming an opportunity to form a glycoside linkage

• the free OH group attached to the anomeric C → an OR group attached to the anomeric C

  • the OR group is the glycosidic linkage

  • the R group is a Ser or Thr amino acid, not really Tyr

  • the R group provides the OH group for O-linked glycosylation

What is a disaccharide?

2 monosaccharides joined by a glycosidic linkage and is always a sugar

How to name the glycosidic linkage of disaccharides?

1) indicate the two carbons involved in the glycosidic linkage

2) left sugar: give the α (trans) or β (cis) configuration of the anomeric C. Compare that anomeric C with the hydroxymethyl OH group

3) right sugar: the anomeric C is C₁. The C attached to the anomeric C is C₂. Continue numbering Cs around the ring until you reached the C apart of the glycosidic linkage

4) Name the glycosidic linkage of disaccharide (ex: β1→2)