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Optically Active Compounds and Their Effect & Importance in Biological Systems

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Sabrina Lee

on 25 May 2015

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Transcript of Optically Active Compounds and Their Effect & Importance in Biological Systems

Optically Active Compounds and Their Effect & Importance in Biological Systems
What Are Optically Active Compounds?
Compounds that are optically active contain molecules that are chiral. Chirality is a property of a molecule that results from its structure.
stereoisomers
Stereoisomers are isomers which:
What Makes A Compound Chiral?
In order for a compound to be chiral, it must:
It is a necessary condition for the existence of enantiomers
Have 4 different groups attached to the chiral carbon
Not be super imposable on it's mirror image
Be sp3 hybridised (single bonded)
Example
:
Have the same molecular formula
Have the same functional groups
Differ in the way atoms are oriented in 3 dimensions
There are 2 types of stereoisomers
Geometric isomers
Optical isomers
Geometric Isomers
Cis (same side)
Trans (opposite sides)
Cis But-2-ene
Molecular formula c4h8
structural formula
ch3chchch3
Trans But-2-ene
Molecular formula c4h8
structural formula
ch3chchch3
OPTICAL Isomers
Because geometric isomers have different chemical and physical properties, they behave differently in our bodies

i.e. cis-platin - used for chemotherapy as it can enter cancer cells and interact with DNA. Trans-platin is not an active isomer.
Enantiomers
Enantiomers are chiral molecules that are mirror images of one another. Furthermore, the molecules are non-super imposable on one another. This means that the molecules cannot be placed on top of one another and give the same molecule.
They are labelled either R or S enantiomers
Labelling enantiomers
The first stage in labelling enantiomers is to assign priority to the groups attached to the chiral center
The group with the highest atomic number is assigned the highest priority
Priority:
1: Br - Atomic no. 35
2: Cl - Atomic no. 17
3: F - Atomic no. 9
4: H - Atomic no. 1
Rotation
Once priority has been assigned, it can then be established what enantiomer each molecule is
E.g.
R Enantiomer
S Enantiomer
** Lowest priority group is positioned behind the plane of polarised light **
racemic mixtures
A racemic mixture is a mixture of 50% left rotating and 50% right roting optical isomers.
e.g. Citalopram is sold as a racemic mixture.
Asymmetric Synthesis
Asymmetric synthesis is the synthesis of a compound by a method that favors the formation of a specific enantiomer or diastereoisomer
Enantioselective synthesis is a key process in modern chemistry and is particularly important in the field of pharmaceuticals, as the different enantiomers or diastereomers of a molecule often have different biological activity.
Why is stereoisomerism important?
Right and left handed molecules interact with living systems in very different ways and give different results.
e.g.
Different smells
Why is stereoisomerism important?
Biomolecules (sugars, amino acids, proteins and steroids) are chiral
Proteins are built from L-amino acids, which implies enzymes are chiral
Receptors are chiral and the natural ligand to a receptor is often a specific enantiomer
This is why R and S enantiomers are important. They can have radically different activities in the body (i.e. effectivity, toxicity, taste etc)
Reactions of optically active compounds
Reactions which produce optically active compounds often yield racemic mixtures. The recognition of this and the subsequent separation of these mixtures are important in synthetic chemistry.
E.g. Thalidomide
One optical isomer of thalidomide is useful in treating depression
The other isomer causes fetal mutations and death
Reactions of optically active compounds
Stereoselective reactions:
Stereoselective Reaction: A reaction in which there is a choice of pathway, but the product stereoisomer is formed due to its reaction pathway being more favourable than the others available.
Stereospecific reactions:
A reaction in which the stereochemistry of the reactant completely determines the stereochemistry of the product without any other option.

How Does optical activity affect the taste of food and drink?
The occurance of amino acids in a specific optical configuration is essential for sensory quality of foodstuffs.
Amino acids, depending upon their configuration, are neutral, sweet or bitter in taste.
E.g. Asparagine, Tryptophan, Tyrosine and Isoleucine
L-form
(Anti-clockwise
rotation)
D-form
(Clockwise rotation)
Bitter
Sweet
How Does optical activity affect the taste of food and drink?
Source: Taste Chemistry By R.S. Shallenberger
reactions at chiral centers & walden inversion
Steric Hindrance
Steric hindrance occurs when the large size of groups within a molecule prevents chemical reactions that are observed in related molecules with smaller groups. Although steric hindrance is sometimes a problem (it prevents SN2 reactions with tertiary substrates from taking place), it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection) or by leading to a preference for one stereochemical reaction
i.e. stereoselective reactions
walden inversion
Walden inversion is the inversion of a chiral center in a molecule in a chemical reaction. Since a molecule can form two enantiomers around a chiral center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to the other
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