Lecture 7: Hemiacetals and Acetals in carbohydrates: Furanoses, Pyranoses, and Glycosidic Linkages

Question 1

How does a linear glucose molecule create a hemiacetal?

Answer 1

Through a nucleophilic addition rxn: the O attached to the second to last C (which becomes the anomeric C), attacks the carbonyl C, creating a ring with the hydroxy O becoming a member of that ring. The carbonyl O becomes a hydroxyl group.

Question 2

How many chiral centers does linear glucose have? In cyclic form?

Answer 2

Has 4 chiral centers in linear form, but 5 in cyclic form

Question 3

As the linear monosaccharide cyclizes, the carbonyl C becomes a hemiacetal, which is now called the _____ C

Answer 3

anomeric

Question 4

the anomeric C is C# ___ for aldoses, and C# ___ for ketoses

Answer 4

C1 for aldoses and C2 for ketoses

Question 5

What are anomers?

Answer 5

epimers in which the molecules differ only in configuration at anomeric C1

Question 6

alpha-D-glucose and ß-D-glucose are ____ of each other (what type of isomer?)

Answer 6

anomers (a type of epimer)

Question 7

In cyclic form, the alpha form of a monosaccharide has the anomeric OH ____(up/down); whereas the ß form of a monosaccharide has the anomeric OH ____(up/down)

Answer 7

alpha = down if drawn as a flat molecule, and axial if drawn in the chair conformation

beta = up if drawn as a flat molecule, and equitorial if drawn in the chair conformation

Question 8

Why is there more ß-D-glucopyranose found in a solution compared to alpha-D-glycopyranose when that solution is at equillibrium?

Answer 8

Because the ß-D-glucopyranose has a more stable chair conformation compared to the alpha. The ß confirmation has all the OH gorups and the CH₂OH group in an equitorial position, whereas the alpha confirmation has the anomeric OH in the axial position.

Question 9

What is a 5 membered and 6 membered ring called?

Answer 9

5 membered ring = furanose

6 membered ring = pyranose

Question 10

aldo hexoses form ___ membered rings when C5 attacks C1

Answer 10

6

Question 11

ketohexoses form ___ membered rings when C5 attacks C2

Answer 11

5

Question 12

Aldopentoses form ___ membered rings when C4 attacks C1

Answer 12

5

Question 13

What is mutarotation?

Answer 13

Refers to the spontaneous interconversion of anomers (via linear form) in aqueous solution via rapid ring opening and cyclization

Question 14

Why does D-glucose not exist in a 50/50 mixture of beta and alpha confirmations?

Answer 14

Because of steric hinderance. ß-D-glucose has less steric hinderance than alpha

Question 15

In linear fischer projection form for glucose, which OH groups on the chiral carbons are positioned to the R? to the L?

Answer 15

R, L, R, R

C1 = R

C2 = L

C3 = R

C4 = R

Question 16

In cyclic form, which positions (up/down) are each of the OH/CH₂OH groups on the cyclic C?

Answer 16

up, down, up, down, up

C1 OH = up

C2 OH = down

C3 OH = up

C4 OH = down

C5 CH₂OH = up

Question 17

1) In oxidation, you ___ O, ____ e^-, and/or _____ H
2) In reduction, you ___ O, ____ e^-​, and/or _____ H

Answer 17

1) oxidation: gain O, lose e^-, or lose H. (oxidation state increases)
2) reduction: lose O, gain e^-, or gain H. (oxidation state decreases)

Question 18

Glucose and other aldoses are called _____(oxidation or reducing) sugars because their free aldehyde can undergo _____

Answer 18

called reducing sugars because their free aldehyde can undergo oxidation

Question 19

Oxidation of an aldose at C1 converts it to the corresponding ___ acid

Answer 19

aldonic acid

i.e. ß-D-glucose that undergoes oxidation becomes ß-D-gluconic acid

Question 20

What is the structure of D-glyceraldehyde?

Answer 20

Question 21

What is the structure of L-glyceraldehyde?

Answer 21

Question 22

What is the fischer projection structure of D-ribose?

Answer 22

Question 23

What is the fischer projection of 2’-deoxyribose?

Answer 23

Question 24

Distinguish between the cyclic structure of ribose and 2’-deoxyribose

Answer 24

Question 25

Distinguish between the fischer projection of D-arabinose and L-arabinose

Answer 25

Question 26

D-fructose fischer projection structure

Answer 26

Question 27

ß-D-fructofuranose cyclic structure

Answer 27

Question 28

D-glucosamine cyclic structure

Answer 28

Question 29

D-glucosamine fischer projection structure

Answer 29

Question 30

Fischer projection structure of N-acetyl-D-glucosamine

Answer 30

Question 31

N-acetyl-D-glucosamine cyclic structure

Answer 31

Question 32

Glycosidic linkages are stable in aqueous solution under __1___ or __2___ conditions, but can be hydrolyzed under __3___ conditions. Fill in the blanks, with acidic, basic, and neutral.

Answer 32

1) neutral
2) basic
3) acidic

Question 33

a glycosidic linkage is a monosaccharide ______(hemiacetal or acetal)

Answer 33

acetal

Question 34

monosaccharide acetal = ______ linkage

Answer 34

glycosidic

Question 35

Glycosidic bond between 2 monosaccharides always connecs __1___ C on one monosaccharide to an __2__ on the other monosaccharide

Answer 35

1) anomeric C
3) OH

Question 36

Maltose is made of what 2 monosaccharides?

Answer 36

2 glucose monosaccharides

Question 37

Maltose is a disaccharide made of 2 glucoses. Specify the type of linkage between these 2 glucoses: alpha or beta, and carbon numbers

Answer 37

the linkage is an alpha 1 → 4 linkage

Question 38

What is the systemic name for maltose?

Answer 38

alpha-D-glucopyranosyl1(1→4)-D-glucopyranose

or

Glc(alpha1→4)Glc

Note that the first glucose suffix = “syl” because it is considered a substituent to the other molecule

Question 39

1) Is a sugar that has a hemiacetal anomeric C capable of acting as a reducing sugar?
2) Is a sugar that has an acetal anomeric C capable of acting as a reducing sugar?

Answer 39

1) yes
2) no

Question 40

What must the anomeric C have in order to be a reducing sugar?

Answer 40

must be a hemiacetal (cannot be an acetal)

Question 41

What type glycosidic linkage constitutes lactose? alpha/beta and carbon numbers

Answer 41

ß1→4 linkage between C1 of galactose and C4 of glucose

(galactose is the nonreducing end and glucose is the reducing end)

Question 42

What is the systemic name for lactose?

Answer 42

ß-D-galactosyl1-(1→4)-D-glucopyranose

or

Gal(ß1→4)Glc

Question 43

How are glycosidic linkage hydrolyzed?

Answer 43

via a strong acid i.e. 6M HCl

Question 44

Can lactase that hydrolyzes a ß1→4 linkage between galactose and glucose also break down a ß1→4 linkage between 2 glucoses

Answer 44

NO!

Question 45

What is the type of glycosidic linkage in sucrose? alpha/beta and carbon #

Answer 45

ß-D-fructofuranosyl2-(2→1)-alpha-D-glucopyranoside

(can also be written vice-versa)

or

Fru(2ß→alpha1)Glc

(can also be written vice-versa: Glc(alpha1→2ß)Fru

Question 46

Why is sucrose not a reducing sugar?

Answer 46

because the anomeric C of glucose (C1) is linked to the anomeric C of fructose (C2)- hence both are acetals; there are no hemiacetals and, therefore, no reducing end.

Question 47

What is a key to analyzing whether a cyclic ß-D-fructose drawing is right side up or upside down?

Answer 47

Look at the anomeric C of fructose (which has the OH and OR attachment)

If the anomeric C has it’s OH facing up, it is right side up (the anomeric C will be on the R side of the drawn structure)

If the anomeric C has it’s OH facing down, its upside down (the anomeric C will be on the L side of the drawn structure)

Question 48

How many anomeric C are there in a monosaccharide?

Answer 48

one

Question 49

Why do some sugars taste sweeter than others?

Answer 49

because the proteins on our taste buds bind with much higher affinity with some sugars compared to others. The closer the binding, the sweeter the sugar will taste

Question 50

What 2 monosaccharides make up trehalose? Is this a reducing or non-reducing sugar?

Answer 50

2 glucose molecules; trehalose is a non-reducing sugar (because the 2 anomeric C of each glucose bond with eachother, forming an acetal and leaving no hemiacetal to undergo mutarotation or oxidation)

Trehalose: Glc(alpha1→1alpha)Glc

Question 51

A reducing end of a sugar is a free __1__ carbon, which can undergo __2__ to interconvert between anomers and become __3__ while in the aldehyde form

Answer 51

A reducing end of a sugar is a free anomeric carbon, which can undergo mutarotation to interconvert between anomers and become oxidized while in the aldehyde form