Carbohydrates 1

Question 1

What are the most smallest monosaccharides composed of ?

Answer 1

Three carbons: note that glyceraldehyde and dihydroxyacetone are constitutional isomers of each other (same atoms but in different order)

Question 2

Examples of six monosaccharides ?

Answer 2

Especially common: ribose and deoxyribose (in RNA and DNA), glucose, mannose, galactose and fructose

Question 3

Examples of aldoses and ketoses ?

Answer 3

Aldoses = glucose
Ketoses = fructose

Question 4

What is Stereochemistry the study of ?

Answer 4

study of arrangement of atoms in 3D space (remember D and L amino acids!)

Question 5

Stereoisomers are ?

Answer 5

Molecules that have the same bonds connecting the same atoms but different relative orientations of the bonds

Question 6

What is meant by chiral?

Answer 6

Identical molecules can be made to superimpose by rotating and flipping, but different compounds cannot be made to superimpose

Question 7

What is meant by asymmetric ?

Answer 7

Not the same as their mirror image: hand-like property

Question 8

So what would asymmetric molecules be termed as ?

Answer 8

Chiral

Question 9

Carbohydrates can have more than one ?

Answer 9

Chiral carbon: in general a molecule with n chiral centers can have 2n stereoisomers

Question 10

All monosaccharides (except dihydroxyacetone) contain ?

Answer 10

One or more assymetric (chiral) carbons

Question 11

Most hexoses in living organisms are ?

Answer 11

D stereoisomers

Question 12

What are epimers

Answer 12

• Sugars that differ only in configuration around one of many chiral carbons. (Carbohydrates which vary in one position for the placement of the -OH group)

Question 13

What is an enantiomer ?

Answer 13

A chiral compound and its mirror image

Question 14

What is the stereochemical configuration of a sugar defined by ?

Answer 14

By the chiral carbon at the greatest distance from the carbonyl carbon

Question 15

If hydroxyl (OH) on chiral carbon at the greatest distance from carbonyl carbon:

Answer 15

to the right – D-glyceraldehyde

to the left – L-glyceraldehyde

• Dexter = right in Latin
• Laevus = left in Latin

Question 16

How are chiral compounds usually drawn and how are chiral carbohydrates usually represented ?

Answer 16

• Chiral compounds can be drawn using perspective formulas (wide end of wedge projects towards you, dashed line projects away)
• However, chiral carbohydrates are usually represented by Fischer projections

Question 17

In a Fischer Projection what do the bonds mean?

Answer 17

Horizontal bonds are pointing toward you; vertical bonds are projecting away from you

Question 18

What type of amino acid do proteins contain in nature ?

Answer 18

L amino acids

Question 19

Enzymes evolved to fit ?

Answer 19

Mostly D stereoisomers

Question 20

In aqueous solution many monosaccharides have ?

Answer 20

Cyclic structures

Question 21

What is the chemical basis for ring formation ?

Answer 21

• It is the reaction of an aldehyde with an alcohol to form a hemiacetal
• Whereas a ketone can react with an alcohol to form a hemiketal

Question 22

What would be the anomeric carbon ?

Answer 22

The hemiacetal or hemiketone carbon

Question 23

What is Haworth Projections ?

Answer 23

Cyclic structures depicted by 3D drawing adhering to a set of rules

Question 24

Hexoses commonly form two types of ring structure. What are these?

Answer 24

• Most common 5 membered ring is furanose

- Most common 6 membered ring is pyranose

Question 25

What is Furan derived from ?

Answer 25

The furan ring: furan 4 carbon ring with 1 oxygen

Question 26

What is Pyranose derived from ?

Answer 26

Pyran ring: pyran ring is 5 carbon ring with 1 oxygen

Question 27

The open-chain form of glucose cyclizes when ? and forms ?

Answer 27

- The C-5 hydroxyl group attacks the oxygen atom of the C-1 aldehyde group to form an intramolecular hemiacetal - Two anomeric forms, designated α and β, can result

Question 28

The open-chain form of fructose cyclarises to a five-membered ring when? and forms ?

Answer 28

The C-5 hydroxyl group attacks the C-2 ketone to form an intramolecular hemiketal. Two anomers are possible, but only the α anomer is shown.

Question 29

Why is pyranose ring not planar ?

Answer 29

Due to geometry of saturated C atoms

Question 30

What kind of conformations do cyclohexane rings have ?

Answer 30

"chair" and "boat" conformations

Question 31

What ratio anomer does a solution of glucose contain ?

Answer 31

A solution of glucose contains one-third α anomer, two-thirds β anomer, and about 1% open chain

Question 32

As the two anomeric forms are in equilibrium that passes through an open-chain form, the free open-chain form reacts with ?

Answer 32

Oxidising agents, therefore glucose is a reducing sugar

Question 33

What can aldehyde reduce ?

Answer 33

Cu2+ to Cu+ (Fehling’s/Benedict’s test) or can reduce Ag+ to Ag0 (Tollens’ test)

Question 34

What do modern detection techniques use?

Answer 34

Colorimetric and electrochemical tests

Question 35

Phosphorylation is a common modification of carbohydrates. What does it make ?

Answer 35

It makes the sugars anionic and prevents them from leaving the cell

Question 36

What does Phosphorylation facilitate?

Answer 36

The metabolism of sugars

Question 37

Phosphorylated sugars are key intermediates ?

Answer 37

In energy generation and biosynthesis: the three sugars are all glycolytic intermediates - G-6P - DHAP - GAP

Question 38

Modified carbohydrates are often expressed on ?

Answer 38

Cell surfaces as parts of glycoproteins and glycolipids

Question 39

A bond formed between the anomeric carbon atom and an oxygen atom of an alcohol is called ?

Answer 39

An O-glycosidic bond

Question 40

A bond formed between the anomeric carbon atom and an amine is called ?

Answer 40

An N-glycosidic bond