General Biochemistry Enantiomer: Chiral Carbon

DON MARIANO MARCOS MEMORIAL STATE UNIVERSITY
MID LA UNION CAMPUS

CITY OF AN FERNANDO, LA UNION
GENERAL BIOCHEMISTRY
Enantiomer
The designation of the configuration as L or D depends on the arrangement at the chiral carbon with
highest number. In the case of both glucose and fructose, it is C-5.
In the Fisher projection of the D configuration, the hydroxyl group is on the right of the highest
numbered chiral carbon.
In the Fisher projection of the L configuration, the hydroxyl group is on the left of the highest numbered
chiral carbon.
Stereoisomers
Ex.1. What are the possible stereoisomers for an aldotreose (4-carbon monosaccharide)?
The aldotreose has 2 chiral carbon, C-2 and C-3, and there are 22 or four, possible stereoisomers.
Chiral carbon- carbon atom that is attached to four (4)

different types of atoms or group of atoms.
22 means two chiral carbon
22 = 2X2 = 4 possible stereoisomers
Two of the isomers have the D configuration and two have the L configuration.
D- Erythrose
The two D isomers have the same configuration at C-3, but they differ in configuration (arrangement of the
-OH group) at the other chiral carbon, C-2. These two D isomers are called D-erythrose and D-threose.
They are not superimposable on each other, but neither they are mirror images of each other. These are
called diastereomers.
Superimposable - The ability for an object to be placed over another object, usually in such a way that both
will be visible.
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****Some molecules are like hands. Left and right hands are mirror images, but they are not identical
or superimposable.
D-ISOMERS
D- Erythrose
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The two L isomers are L-erythrose and L-threose.

L-ISOMERS
L-erythrose is the enantiomer (mirror image) of D-erythrose, and L-threose is the enantiomer of D-threose.
ENANTIOMER
L-
threose is a diastereomer of both D-and L-
erythrose, and L-erythrose, and L-erythrose is a
diastereomer of both D - and L-threose.
DIASTOMER
Diasteromers that differ from each other in the

configuration at only one chiral carbon are called
epimers. D-erythrose and D-threose are epimers.
DIFFERENCE BETWEEN ENANTIOMER AND DIASTOMER

ENANTIOMER DIASTOMER
Stereoisomers whose molecules are Stereoisomers whose molecules are not mirror
nonsuperimposable mirror images images of each other
Opposite configuration at all chirality centers Opposite configuration at some chirality centers
Cyclic Structure: Anomers

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Sugars, especially those with five and six carbon atoms, normally exist as cyclic molecules rather than as the
open-chain forms. The cyclization takes place as a result of interaction between the functional groups on
distant carbons such as C-1 and C-5 to form cyclic hemiacetal (aldohexose). Another possibility is the
interaction between C-2 and C-5 to form a cyclic hemiketal (ketohexose). In either case, the carbonyl
carbon becomes a new chiral center called the anomeric carbon. The cyclic sugar can take either of two
different forms, designated α and β, and are called anomers of each other.
Anomeric hydroxyl group- a new hydroxyl group formed on C-1 as the ring
closes.
Anomeric carbon atom – the carbon in which the anomeric hydroxyl group
is attached.
Alpha anomer (α-anomer) – anomeric hydroxyl group is on the opposite face

of the ring to carbon -6 D-series sugars--- anomeric hydroxyl group points down.
Beta anomer (β-anomer) – anomeric hydroxyl group is on the same face of the ring as carbon-6 D-series
sugars----anomeric hydroxyl group points up.
The Fisher projection of the α-anomer of a D sugar has the anomeric hydroxyl group to the right of the
anomeric carbon (C-OH), and the β-anomeric carbon of a D-sugar has the anomeric hydroxyl group to the
left of the anomeric carbon.
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Fisher projection formulas are useful for describing the stereochemistry of sugars, but their long bonds and
right-angle bends do not give a realistic picture of the bonding situation in the cyclic forms, nor they
accurately represent the overall shape of the molecules.
Haworth projection- useful shorthand in for the structures of reactants and products in many reactions. The
Haworth projection represent the stereochemistry of sugars more realistically than do the Fisher projection.
- the cyclic structure sugars are shown in perspective drawings as planar five- or six-
membered rings viewed nearly edge on.
Furanose- A five-membered ring Pyranose- A six-membered ring
For a D sugar, any group that is written to the right of the carbon in a Fisher projection has a downward
direction in Haworth projection; any group that is written to the left in a Fisher projection has an upward
direction in a Haworth projection.
Remember when converting sugar from Fischer to Haworth projection: “Downright uplefting"
Right in Fischer = Down in Haworth
Left in Fischer = Up in Haworth
The terminal -CH2OH group, which contains the carbon atom with the highest number in the numbering
scheme, is shown in an upward direction.
Although the ring structures are drawn with flat faces in the Haworth formulae, in reality they are not planar
rings but are bent and could be visualized more as a boat or a chair configuration.
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D- GLUCOSE IN FISCHER PROJECTION, HAWORTH PROJECTION AND CHAIR

CONFORMATION
REACTIONS OF MONOSACCHARIDES
Oxidation-Reduction Reactions
- Oxidation of sugars provides energy for organisms to carry out their life processes; the highest yield of
energy from carbohydrates occurs when sugars are completely oxidized to CO2 and H2O in aerobic
processes. The reverse of complete oxidation of sugars is the reduction of CO 2 and H2O to form sugars, a
process that takes place in photosynthesis.

General Biochemistry Enantiomer: Chiral Carbon

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DON MARIANO MARCOS MEMORIAL STATE UNIVERSITY

MID LA UNION CAMPUS

Chiral carbon- carbon atom that is attached to four (4)

22 means two chiral carbon

22 = 2X2 = 4 possible stereoisomers

The two L isomers are L-erythrose and L-threose.

Diasteromers that differ from each other in the

DIFFERENCE BETWEEN ENANTIOMER AND DIASTOMER

Cyclic Structure: Anomers

Alpha anomer (α-anomer) – anomeric hydroxyl group is on the opposite face

Furanose- A five-membered ring Pyranose- A six-membered ring

D- GLUCOSE IN FISCHER PROJECTION, HAWORTH PROJECTION AND CHAIR

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