Monosaccharides are the simplest carbohydrates; they conform to the general chemical formula (CH2O)x and are termed simple sugars. The most commonly occurring monosaccharides contain three to six carbon atoms in an unbranched single-bonded chain. Monosaccharides are signified by the suffix -ose.

1. SIDRA MAHMOOD
BS chemistry
MS biochemistry

2. Asymmetric carbon atom
&
D-&L-Monosaccharide

3.  As already mentioned, monosaccharide's are
ployhydroxy compounds.
 In other words they posses alcoholic
group(hydroxyl group).
 Depending upon the number of these groups
monosaccharide's contain one or more
centers of asymmetry.
 This affect their optical and biological activity.
 The ketotriose dihyroxyacetone is the only

4.  Exception of a monosaccharide that does not
posses any asymmetric carbon atom.
 The structure of the smallest sugar containing
three carbon atoms name glycerose usually
called glyceraldehyde can be represent in the
following form called fischer’s projection or
straight chain formula.
 In this case the carbon chain is written
vertically with carbon No.1 at the top and the
–H and –OH are written to the lift or right.

5.  The middle carbon atom in both cases is an
asymmetric carbon atom which is define as
that carbon atom to which four different
atoms or group of atoms are attached.
 In the given structure no 1 –OH group
attached to the asymmetric carbon atom no 2
on the right and this is called D-glycerose.
 In the structure no 2 –OH group attached to
carbon no 2 is on the left and this is called L-
glycerose.

7. All monosaccharide's having more then
three carbon atoms have to or more
asymmetric carbon atom.
In these cases the designation D & L is
given by seeing the configuration of H &
OH around the penultimate* carbon atom.
i.e. the asymmetric carbon atom situated
farthest from the potential aldehyde or
ketone i.e. carbonyl group.

8. If the penultimate carbon atom has the
same configuration of H & OH around it as
the asymmetric carbon atom of D –
glycerose.
The monosaccharide is of D type.
On the other hand a monosaccharide
whose penultimate carbon atom has the
same configuration as the asymmetric
carbon atom of L-glycerose is called L-
monosaccharide.

9.  The D & L form of a monosaccharide are
stereoisomers of each other & are also the mirror
image of each other in regard to asymmetric
carbon atom.
 Such a monosaccharide have been given a special
name ENANTIOMERS.
 Note
 It should be understand that D & L do not mean
dextro & levo but these terms only signify the
configuration around the carbon atom.
 Glycerose is called the reference sugar.
 Because all monosaccharide are compare with the
D-& L-glycerose for this propose.

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