Monosaccharides

MONOSACCHARIDESSimple sugars.Cannot be further hydrolyzed into simple

carbohydrates catagorized by the number of carbon

(typically 3-8) and whether an aldehyde or ketone.

MonosaccharidesAldoses (e.g., glucose) have an aldehyde group at one end.

Ketoses (e.g., fructose) have a keto group, usually at C2.

MONOSACCHARIDE STRUCTURESFISHER PROJECTIONSHAWORTH PERSPECTIVE FORMULAS

The asymmetric(chiral) carbon atom and D and L monosaccharidesAny carbon atom which is connected to four different groups will be chiral, and will have

twononsuperimposable mirror images; it is a

chiralcarbon or a center of chirality.

•When a molecule has more than one chiral carbon,each carbon can possibly be arranged in either theright-hand or left-hand form, thus if there are nchiral carbons, there are 2n possible stereoisomers.

Maximum number of possible stereoisomers = 2n

Isomerism in monosaccharidesCompounds that possess the same

chemical formula are called isomers.Glucose,fructose,mannose and

galactose all have the same formula,i.e C6H12O6 and therefore isomers of each other.

1.STERIOISOMERISMGeneral term,means two or more

monosaccharides having the same structure but differing from each other in that the configuration,i.e the arrangement of their atoms in space differ from each other.

2.ENANTIOMERS.Pair of stereoisomerism that are mirror

images of each other in regard to asymetric carbon atoms present in their molecules.

For example glucose occurs as D and L glucose isomers.

3.OPTICAL ISOMERISMThe enantiomeric monosaccharides by virtue of their

content of asymmetric carbon atom can rotate the plane-polarized light either to right or left.

The monosaccharides that rotate this light to right are called dextrorotatory(+type)

Those rotating this light to left are called levorotatory(-type).

Glucose is dextrorotatory(+),while fructose is levorotatory(-)

4.EPIMERSMonosaccharides(isomers) which differ in

configuration around one specific carbon atom other than the carbon atom of carbonyl group.

Glucose Galactose

5.ANOMERSIsomers that differ in configuration around

the anomeric carbon i.e the carbon atom of the carbonyl group which is carbon no.1 in aldosed and carbon no.2 in ketoses.

These representations of the cyclic sugars are called Haworth projections.

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, & .

(OH below the ring) (OH above the ring).

H O

OH

H

OHH

OH

CH2OH

H

-D-glucose

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose

23

4

5

6

1 1

6

5

4

3 2

6.PYRANOSE-FURANOSE ISOMERISMThe ring structures of monosaccharides may

be similar to either pyran or furan and accordingly the monosaccharide is said to occur in pyranose or furanose form.

RECEMIC MIXTUREEqual amounts of dextro- and levorotatory

isomers in solution results no net rotation of plane of polarized light.Such a mixture is called racemic mixture.

MESOCOMPOUNDSDonot rotate the light inspite of asymmetric

carbon due to internal compensation.

MUTAROTATIONProcess by which anomers are interconverted.