stereochemistry and carbohydrates

Question 1

describe the positions of isomers

Answer 1

constitutional and stereoisomers
1. constitutional isomers: have differ IUPAC names, different or the same functional groups, differ physical and chemical properties.
2. stereoisomers: differ only in the way atoms are oriented in space. Have the same IUPAC names- except for cis/trans and have the same functional groups.
A particular 3 dimensional arrangement is called a CONFIGURATION. stereoisomers DIFFER in configuration

Question 2

Describe chirality, enantiomers, diastereomers, and racemic mixtures.

Answer 2

Chiral molecule= a molecule that is NOT superimposable. Cannot be reflected on a mirror.
Enantiomer= are stereoisomers whose molecules are non-superimposable mirror images of each other.
same mp, bp, solubility, spectra and refractive index, and react with achiral molecules in the same way. ONLY differ in the way they reflect plan polarized light. IN OPPOSITE direction.

racemic mixture= an equal amount of two enantiomers. optically inactive

diastereomers= stereoisomers that are not mirror images of each other. differ in mp and bp

Question 3

what is the importance of stereochemistry in binding to chiral enzymes and receptors.

Answer 3

Biomolecules can discriminate between enantiomers (isomers). ex. drugs. Enzymes are capable of distinguishing stereoisomers.

Question 4

Describe the importance of enantiomeric drugs and the dangers.

Answer 4

Enantiomeric drugs can randomly switch from and R configuration to an S configuration where one can be beneficial and the other dangerous. ex. thalidomide
Must distinguish appropriate stereoisomer

Question 5

what is an aldose, ketose, triose, pentose and hexose?

Answer 5

aldose= sugar with an aldehyde 
ketose= sugar with a ketone
triose= a monosaccharide with 3 carbon atoms. 
pentose= a mono. with 5 carbons 
hexonse= a mono. with 6 carbons

Question 6

what is a hemi-acetal and hemi-ketal?

Answer 6

Hemiacetals are molecules formed from aldehyde and ketones and is formed from an alcohol and a carbonyl group. In a hemi-acetal a Carbon is connected to a H, R, OR, and OH.
IN a hemi-ketal a Carbon is connected to a R, R, OR, and OH.
Acetal: Carbon with a H, R, OR, OR
Ketal: Carbon with a R, R, OR, OR

Question 7

what is a glycosidic bond?

Answer 7

is a covalent bond that joins a carbohydrate (sugar) to another group.
C-O in disaccharide’s
C-N in nucleosides
glycosidic bonds only involves the anomeric carbon

Question 8

what are epimers and anomers?

Answer 8

1. Epimers: one of two optical isomers that differ from each other only in the configuration about one chiral atom. ex. D-mannose versus D-glucose
2. Anomers: cyclization of monosaccharides changes the former carbonyl C into a new chiral center. Newly formed chiral center forms two stereoisomers called anomers.
   non-hydrogen substituent above plane= B, non-hydrogen substituent below plane= A

Question 9

what is mutorotation?

Answer 9

mutorotation is the interconversion of the anomeric forms of a monosaccharide.
In solution, an equilibrium exists b/w an open chain form and two anomeric forms of a monosaccharide.
Mutorotation is the change in optical rotation.

Question 10

what is a reducing sugar?

Answer 10

Any sugar that in basic solution forms some aldehyde or ketone. Allows the sugar to act as a reducing agent–gets oxidized.
Tollen’s and Benedicts reagents are used to test for reducing agents. Tollen gives a silver mirror when Ag+ is reduced to Ag. Benedicts reagent gives a brick red precipitate of Cu2O when reduced.

Question 11

Describe the structure of mannose

Answer 11

mannose is the C-2 epimer of glucose
mannose C2 -OH and C3 left and C4 and C5 right.
glucose C2 -OH right, C3 left, C4 and C5 right.

Question 12

Describe the structure of sucrose

Answer 12

table sugar
D-glucopyranose attached to D-fructofuranose via an alpha 1, beta 2 glycosidic linkage.
It is a NON-REDUCING sugar because it has an ACETAL group.

Question 13

Describe the structure of lactose

Answer 13

milk sugar
D-galactopyranose attached to D-glucopyranose by a beta-(1,4) glycosidic linkage. Lactose is a REDUCING sugar because the glucose moiety contains a hemi-acetal functionality group.

Question 14

Describe the structure of cellobiose

Answer 14

results from partial hydrolysis of cellulose.
two D-glucopyranoses bonded by a beta-(1,4) glycosydic linkage. It is a REDUCING sugar because the glucose moiety contains a hemi-acetal functionality

Question 15

Describe the structure of amylose

Answer 15

A form of starch

consists typically of more than 1000 D-glucopyranose units connected by alpha (1,4) glycosidic linkages)

Question 16

describe the structure of amylopectin

Answer 16

a form of starch
similar to amylose, but has branching points every 24-30 units. branches are connected by alpha-(1-6) glycosidic linkages.

Question 17

describe the structure of glycogen

Answer 17

similar to amylopectin except has far more branching. branching occurs every 8-12 glucose units. alpha-(1,4) backbones with alpha-(1,6) branching. high branching allows for hydrolyzing enzymes to have many chain ends for in which glucose molecules can be hydrolyzed.

• *glycogen phosphorylase breaks alpha-(1,4) linkages
• *de-branching enzymes break down alpha-(1,6) linkages.

Question 18

describe the structure of cellulose

Answer 18

In cellulose, glucose molecules are connected by BETA-glycosidic linkages. cellulose chains are relatively straight. H-bonding with each other to form sheets that make strong and insoluble fibers.
Humans lack enzyme to cleave the beta-linkages in cellulose and so cannot use cellulose as a form of energy.

Question 19

describe the structure of maltose

Answer 19

malt sugar
two glucopyranose monomers attached by alpha-(1,4) glycosidic linkages. Is a REDUCING sugar b/c the 2nd glucopyranose moiety contains a hemi-acetal functionality.

Question 20

Describe some features of carbohydrates

Answer 20

1. highly polar therefore soluble
2. parent monosacchride’s do not have ionizable groups.
3. several different “connect” or reactive points.
4. can be derivatized or oxidized easily
   - condensation for polymers
   - oxidations for extraction of energy
5. biochemical energy content is substantial