Carbohydrates Note For Nurses Part II
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General Properties in Reference to Glucose
Asymmetric carbon: A carbon atom
to which four different atoms or groups of atoms are attached is said to be
asymmetric
Van’t Hoff’s rule of ‘n’: The number of
possible isomers of any given compound depends upon the number of asymmetric
carbon atoms the molecule possesses.
An asymmetric
carbon atom is a carbon atom that is attached to four different
types of atoms or groups of atoms.
According to Van’t Hoff’s rule of ‘n’; 2n equals the possible
isomers of that compound, where, n = represents the number of asymmetric carbon
atoms in a compound.
Stereoisomerism:
The presence of asymmetric carbon atoms in a compound gives rise to
the formation of isomers of that compound. Such compounds which are identical
in composition and differs only in spatial configuration are called
stereoisomers.
Two such isomers of glucose—D-Glucose and L-Glucose are mirror image of each other.D-Series and L-Series: The orientation of the H and OH groups around the carbon atom just adjacent to the terminal primary alcohol carbon, e.g. C-atom 5 in glucose determines the series.
When the – OH group on
this carbon is on the right, it belongs to D-series, when the – OH group is on
the left, it is a member of L-series.
Most of the monosaccharides occurring in mammals are D-sugars, and
the enzymes responsible for their metabolisms are specific for this
configuration.
Optical activity:
Presence of asymmetric carbon atoms also confers optical activity
on the compound. When a beam of plane-polarized light is passed through a
solution exhibiting optical activity, it will be rotated to the right or left
in accordance with the type of compound, i.e. the optical isomers or enantiomorphs;
when it is rotated to right, the compound is called
• Dextrorotatory (D or + sign),
when rotated to left, the compound is called
• Levorotatory (I or – sign).
Racemic:
When equal amounts of dextrorotatory and levorotatory isomers are
present, the resulting mixture has no optical activity, since the activities of
each isomer cancels each other. Such a mixture is said to be Racemic.
Resolution: The separation
of optically active isomers from a racemic mixture is called resolution.
Cyclic Structures
As the two reacting groups aldehyde and alcoholic group belong to
the same molecule, a cyclic structure takes place.
If the open-chain form of D-Glucose, which may be called as
Aldehydo-D-Glucose is taken, and condense the aldehyde group on carbon-1, with
the alcoholic-OH group on carbon-5, two different forms of glucose are formed.
When the OH group extends to right, it is α-D-Glucose and it extends to left,
it is β-D-Glucose (Fig. 3.4).
Anomers and anomeric carbon: Carbon-1, after cyclization has four different groups attached to it and thus it becomes now asymmetric. The two cyclic compounds, α and β have different optical rotations, but they will not be same because the compounds as a whole are not mirror-images of each other.
Compounds related in this way are called anomers
and carbon-1, after cyclisation becomes asymmetric is called now anomeric
carbon atom.
Muta-Rotation
Definition: When an aldohexose is first dissolved in water and the solution is put in optical path so that plane polarized light is passed, the initial optical rotation shown by the sugar gradually changes until a constant fixed rotation characteristic of the sugar is reached.
This phenomenon of change of rotation is called as
mutarotation.
Explanation: Ordinary crystalline glucose happens to be in the α-form. The above change in optical rotation represents a conversion from α-Glucose to an equilibrium mixture of α and β-forms.
The mechanism of mutarotation probably involves opening of the
hemiacetal ring to form traces of the aldehyde form, and then recondensation to
the cyclic forms. The aldehyde form is extremely unstable and exists only as a
transient intermediate.
Haworth Projection
(a) Pyranoses: Haworth in
1929 suggested that the six-membered ring forms of the sugars be called
Pyranoses, because Pyran possesses the same ring of 5 carbons and oxygen.
(b) Furanoses: Similarly, Haworth designated sugar containing 5-membered rings as the furanoses, because furan contains the same ring.
The Pyranose forms of the sugars are internal hemiacetals formed by combination of the aldehyde or ketone group of the sugar with the OH group on the 5th carbon from the aldehyde or ketone group.
Similarly, the furanose forms of the sugars are formed by reaction between the
aldehyde or ketone group with the OH group on the 4th carbon from the aldehyde
or ketone group.
Epimers and epimerization: Two sugars
which differ from one another only in configuration around a single carbon atom
are termed Epimers.
Epimerization: Process by which one epimer is converted to other is called epimerization and it requires the enzyme epimerase, e.g. conversion of galactose to glucose in liver.
Reference:
Notes Made By The Help of "The Text Book of Medical Biochemistry By MN. Chatterjea 8th Edition"
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