Properties of Monosaccharides

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Properties of MonosaccharidesOptical Activity

2

• Monosaccharides are non-superimposable on their mirror

images and are thus chiral and optically active.

• Optical activity is the ability of a compound to rotate the

plane of polarized light.

• The degree of rotation of the plane of polarized light, usually

measured with a polarimeter, is commonly represented as

specific rotation.

Carbohydrate Specific

rotation

Carbohydrate Specific

rotation

D-Glucose +52.7 D-Galactose +80.2

D-Fructose -92.4 D-Mannose +14.2

D-Arabinose -105.0 Lactose +55.4

L-Arabinose +104.5 Sucrose +66.5

Maltose +130.4 Invert sugar -19.8

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Properties of MonosaccharidesMutarotation

3

• The prefix muta- refers to change.

• Mutarotation is the change in specific rotation of a solution

of any one of a pair of pure anomers of a sugar to an

equilibrium or intermediate optical rotation.

O

H

HO

H

HO

H

HOH

HOH

O

H

HO

H

HO

H

OHOH

HH

-D-Glucopyranose -D-Glucopyranose

] +112.2o [] +18.7o

Mutarotation

O

H

HO

H

HO

H

OHOH

HH

O

H

HO

H

HO

H

HOH

HOH

+

-D-Glucopyranose (36%) -D-Glucopyranose (64%)

H2OH2O

] +52.6o Change in specific optical rotationChange in specific optical rotation

Equilibrium mixture of anomersPure anomer Pure anomer

OH OH OH OH

• When one of either the or -anomer of glucose is dissolved

in water, an interesting change in specific rotation is

observed.

Anomerization of Monosaccharides:The Cause of Mutarotation

4

• Anomerization is the interconversion between two anomers

of a sugar in solution as a result of the existence of an

equilibrium between the two anomers (cyclic hemiacetals)

through a common linear aldehyde intermediate.

• Once in the linear aldehyde intermediate, free bond rotation

on C-1, makes it possible for recyclization to the hemiacetal

form to occur to yield any one of two possible anomers.

• This results in the mutarotation of the sugar.

O

H

HO

H

HO

H

HOH

HOH

O

H

HO

H

HO

H

OHOH

HH

-D-Glucopyranose -D-Glucopyranose

HHO

OHH

OHH

CH2OH

CHO

OHH

OH

H

HO

H

HO

H

OOH

HH

OH

H

HO

H

HO

H

HOH

H

O

Fischer projection

H+ H+

Free bond rotation

Anomerization

OHOH

OHOH

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Properties of MonosaccharidesPercentage Composition of Anomers on Mutarotation

5

• The phenomenon of mutarotation (change of optical rotation

of a substrate to an intermediate value) is a consequence of

the existence of a reversible equilibrium through an open

chain intermediate via which the and -anomers of the

hemiacetal interconvert.

Sample Question and Calculation:

• The specific optical rotations of pure and -D-

glucopyranoses are +112.2o and +18.7o, respectively.

Assuming that only the - and -pyranoses are present;

calculate the percentage of each anomer present in an

equilibrium mixture of specific rotation +52.6o.

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Properties of MonosaccharidesPercentage Composition of Anomers during Mutarotation

6

Solution:

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Properties of MonosaccharidesAnomeric Effect

7

• Monosaccharides in the pyranose form exist as mixtures of

anomers ( and -anomers), whose ratio is determined by

two competing and opposing factors:

1. The steric influence of the hydroxyl group at the anomeric

carbon and

2. The electronic effect of the hemi-acetal oxygens.

• The equatorially positioned substituents of a carbohydrate

ring are, for steric reasons, the most energetically favoured,

compared to the axial counterparts, as is the case with every

molecule with a chair conformation.

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Properties of MonosaccharidesAnomeric Effect: The Steric Factor

8

Steric factor

OO

O H

H

H

OH

HOHO

HO

OH

HO

HOHO

HO

-anomer-anomer

Axial substituent

Equatorial substituent

Sterically favourableSterically unfavourablebecause it allows for theunfavourable 1,3-diaxialinteractions

• However, the anomerically bound groups in carbohydrates

do not follow this rule completely: D-pyranoses with the

anomeric group located in an axial position are often more

thermodynamically stable than would be predicted from the

steric interactions they have with adjacent substituents.

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Properties of MonosaccharidesAnomeric Effect: The Electronic Factor

9

Electronic factor

• The anomeric effect is a phenomenon in pyranoses of

monosaccharides that appears to destabilize the -anomers

of monosaccharides over the -anomers.

• This effect arises out of the spatial electronic interaction

(repulsion) between the orbitals of the lone pairs of electrons

on the pyranose ring oxygen and those on the hydroxyl group

at the anomeric carbon of anomeric pyranoses.

O

O

O

Anomer which isfavoured by theanomeric effect

H

H

HO

H

HOHO

HO

OHHO

HOHO

HO

-anomer-anomer

Closer proximity of theelectron pairs leading to a stronger electronrepulsion and destabilization

Lone pair electronorbitals far apart andhence less interaction

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Properties of MonosaccharidesAnomeric Effect: The Electronic factor

10

Electronic factor• Due to the close proximity between the hemi-acetal oxygen

atoms and hence their electron pairs, there is greater

electronic repulsion between the lone-pair electron orbitals

in the -anomer compared to those of the -anomer.

• This phenomena leads to, what amounts to, a preferential

stabilization of the -pyranose (axial OH) over the -

pyranose (equatorial OH).

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Modified Monosaccharide SugarsIntroduction

11

• The most common monosaccharides are the aldoses, which

have oxygen attached to each of the carbons.

• However, the structures of many sugars differ from the

general formula Cn(H2O)n.

• They are modified by:

i. Deoxygenation to generate deoxysugars

ii. Substitution with amino groups to generate amino

sugars.

• Many of such modified sugars are frequently found as

constituents of biologically significant molecules such as

DNA, alkaloids and antibiotics.

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Modified Monosaccharide SugarsDeoxy Sugars

12

• Deoxy sugars lack an oxygen function at one or more of the

carbons of its skeleton due to replacement of the hydroxyl

groups of a monosaccharide via reduction with hydrogen.

• One of the most celebrated examples of deoxy sugars is 2-

deoxy-D-ribose, the sugar component of DNA.

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Deoxygenation

at C-2

OHH

CH2OH

OHH

H OH

D-Ribose

CO H

OHH

CH2OH

OHH

H H

CO H

2-Deoxy-D-ribose

H

OH

HOH OH

H HO

HO

H

OH

HOH H

H HO

HO

Exists in DNA as the

-furanose

Modified MonosaccharidesAminosugars

14

• Another important class of substituted sugars is the amino

sugars in which one of the hydroxyl groups of the

monosaccharide has been replaced by an amino group.

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• Such amino sugars are named as derivatives of the parent

sugar with the stereochemistry at the carbon atom carrying

the amino group being expressed with the amino group

being regarded as equivalent to an OH of the parent sugar.

Modified Monosaccharide SugarsAminosugars

15

• Some of these amino sugars are important biologically. For

example, D-glucosamine is readily produced in the body,

where it is a key building block for making joint cartilage.

• Reduction of glucosamine production with age can lead to

osteoarthritis, a disease in which cartilage in joints become

stiffer and may wear away.

• The pain associated with arthritis can be relieved by

glucosamine supplements leading to improvement in mobility.

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