a

Optical Isomers, Enantiomers and Chiral Molecules

hi and welcome to another episode of

wonders of chemistry with Nikki G in

this episode I will be discussing that

the concept of enantiomers also known as

optical isomers such molecules are

non-superimposable and are often said to

exhibit a property known as handedness

after are non-superimposable mirror

images of our left and right hands here

is a simple example of such a molecule

it is clear to see that the central

carbon atom depicted in grey is

surrounded by four different

substituents colored in red green blue

and yellow based on this it is

designated a chiral carbon

such molecules possess a mirror image

and exist as mirror images of one

another these isomers are labeled l and

d based on their ability to rotate plane

polarized light either to the left in an

anti-clockwise direction or to the right

in a clockwise direction well it may be

obvious to some of you that the L stands

for left in fact left turning which in

Latin stands for levo rotary the D on

the other hand may not be so obvious in

fact the D stands for

dextra rotary which in Latin means right

turning in fact this is the reason why

glucose is often labeled as dextrose as

it represents the optical isomer which

rotates plane polarized light to the

right and is therefore designated as D

glucose now if we look at these two

molecules that are illustrated here they

look pretty much exactly the same

however there is a major difference if I

was to rotate this molecule to the right

and attempt to superimpose the first

molecule over the second molecule you

can see that they're actually non

superimposable

so what's the big thing with these

molecules and why are they important

well within our body we have receptors

that generally only recognize only one

of these isomers just like our left hand

can only fit into a left-handed glove a

left-handed optical isomer requires a

matching receptor that complements its

geometry let's take an example of a

receptor that only recognizes the L

isomer its geometrical orientation

allows it to set perfectly within its

receptor site now let's take the same

receptor and attempt the same scenario

with the D isomer you will quickly see

that the geometrical orientation of the

D isomer does not match the geometrical

orientation of the receptor binding site

well the blue region of the molecule is

in perfect alignment with the blue

binding site the yellow and green

regions are misaligned this prevents the

molecule from binding it the receptor

binding site and it's subsequently

rejected now you can rotate this

molecule as much as you like but what

you'll find is that it's physically

impossible to get each of its regions to

bind perfectly in alignment with the

receptor binding site in fact this is

the reason why the L isomer of amino

acids are preferred over the D isomer in

the human metabolism as there exists an

overabundance of its corresponding L

receptors when it comes to

monosaccharides however the opposite is

true with the D isomer being preferred

over the L version once again this is

due to an overabundance of its

corresponding and D receptor the key

word here being preferred while it would

be nice to think that only the one

receptor type exists within the body

there are instances when both can exist

and have led to catastrophic

consequences

a classic example of such a scenario

occurred during the 50s and early 60s

with the drug thalidomide which was

marketed as a safe and mild sedative to

reduce morning sickness in pregnant

women however it caused thousands of

babies worldwide to be born with

malformed limbs scientists speculated

that one of the optical isomers of the

drug was acting as a teratogen

preventing the proper growth of limbs

while living organisms have the

necessary machinery to create only one

optical isomer the pharmaceutical

industry tends to produce a mixture of

both L and D based versions of a drug

and here lies the problem

while the right-handed version of the

drug thalidomide was relatively safe its

mirror image the left-handed version was

not since then drug companies have

invested billions of dollars in research

to develop ways to enhance the purity of

the intended optical isomer of a drug

limiting any potential side effects from

its mirror image unfortunately about

more than half of the drugs currently in

use are chiral compounds that is to say

they are a mixture of both the L and D

isomers of the drug with almost 90%

consisting of equal concentrations of

both enantiomers