Lecture 11: Monosacchardies and Disaccharides

Chapter 7.1

What is an aldose?

a molecule where the end of the carbonyl chain is an aldehyde

What is a ketose?

a molecule where the carbonyl group is in any other position and is a ketone

What is the defining formula for sugars?

(CH₂O)n

All monosaccharides contain at least 1 chiral carbon atom. What is the exception?

dihydroxyacetone

What are enantiomers?

two different optical isomers that are mirror images; non-superimposable

True or False: In general, a molecule with n chiral centers can have 2ⁿ stereoisomers.

true

What is the reference carbon?

the chiral center most distant from the carbonyl carbon

What are the 2 groups of stereoisomers?

D- and L-isomers

How do you distinguish between L- and D-isomers?

the hydroxyl group (relative to the reference group) is on the left side for L-isomers and on the right side for D-isomers

Are most hexoses L- or D-isomers?

D-isomers

Do you have the structures of D-glucose and D-ribose memorized?

yes, of course!

What are epimers?

two sugars that differ only in the configuration around one carbon atom

True or False: Aldehydes and ketones react with water and alcohols.

true

When aldehydes and ketones react with alcohols, what can be formed?

hemi-acetals or acetals

True or False: Hemi-acetals are highly reactive and unstable while acetals are very stable molecules.

true

Do you know the formation of hemiacetals and hemiketals?

yes, of course!

Are you familiar with the intramolecular hemiacetal formation?

yes, of course I am!

True or False: In aqueous solutions, aldotetroses and all monosaccharides with 5+ backbone carbon atoms occur as cyclic structures.

true

When two cyclic forms of D-glucose react, what is formed?

both alpha and Beta stereoisomers

What are anomers?

isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom

How do you distinguish between alpha and Beta stereoisomers?

the hydroxyl group on the anomeric carbon is below for alpha-isomers and above for Beta-isomers

How do translate a fischer projection to a haworth projection?

• If -OH is on the right side of the fischer projection, then it will point down in the haworth projection

• If -OH is on the left side of the fischer projection, then it will point up in the haworth projection

• The CH₂OH group projects upward for D-enantiomers and down for L-enantiomers

In aqueous solutions with D-glucose, what is the percentage of a-D-glycose and B-D-glucose?

36% a-D-glucose and 64% beta-D-glucose

Why is there a higher percentage of beta-D-glucose?

the beta stereoisomers is thermodynamically favored due to the atoms being in the equitorial position

What are pyranoses?

six-membered ring compounds

What are furanoses?

five-membered ring compounds

True or False: In aqueous solution, 99% of glucose exists in the pyranose form, and only a miniscule amount in the furanose form.

true

What are reducing sugars?

aldoses and sugars that form aldehydes; sugars that can undergo a redox reaction where free aldehyde groups convert Cu²⁺ to Cu⁺ (forms red precipitate)

Can ketoses form aldehydes, and are they reducing sugars?

yes, they can tautomerize and be considered a reducing sugar

In tautomerization, there’s an equilibrium between what?

the aldehyde and ketone version

What is the O-glycosidic bond?

a covalent linkage joining two monosaccharides

True or False: The formation of a glycosidic bond can render a sugar nonreducing.

true

What is the reducing end of a monosaccharide?

the end of a disaccharide or polysaccharide chain with a free anomeric carbon

Where is maltose found?

dairy, beer, and whiskey

How do you name reducing oligosaccharides?

1. With the nonreducing end on the left, give the configuration alpha or beta at the anomeric carbon joining the first unit to the second

2. Name the nonreducing residue using “furano” or “pyrano”

3. Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers

4. Name the second residue and repeat for additional residue

What are some basic characteristics of lactose?

• reducing sugar

• disaccharide

• more reactive

What are some characteristics of sucrose and trehalose?

• nonreducing sugar

• more stable

• longer shelf0life

• use to make medications

What is trehalose found in?

seaweed and mushrooms