OPTICAL ACTIVITY AND CHIRALITY

In 1813,Jean-Baptiste biot(1774-1862),following an earlier observer by his colleague Arago, discovered that when a quartz crystal is cut in a certain way and exposed to polarized light along a particular axis, the angle proportional to the thickness of the plate. Some quartz crystals turn the plane of polarisation to the right, others to the left . During the period 1815-1838 , he examined several organic substances, both pure and in solution , for optical activity . As optical activity can be displayed by compounds in solution , it must be a property of molecules themselves. The dependence of optical activity on concentration is sometimes called Biots law. As, he studied with the naturally occurring compounds he did not observe organic molecules existed in both dextro and leavo forms.

Only those substances whose molecular configuration is not superimposable on its mirror image are Optically active. The compound with its mirror image molecular configuration is optically active(They are called Enantiomers).

Enantiomers : Enantiomers are the two Stereoisomers having

Enantiomers have identical Physical and chemical Properties under Achiral Environment , but different physical and chemical properties under Chiral Environment.ENANTIOMERS

The term Chirality has come from Greek Word Cheir, which means hand. Obviously, Chirality is related with Handedness.Thus, a Chiral molecule is defined as one that its Molecular Configuration is not superimposable on its mirror image molecular configuration.Chiral Environment: Able to distinguish two Enantiomers properly. Ex: Plane polarised lightAchiral Environment: Not able to distinguish two Enantiomers properly. Ex: Normal light(flash light)

When we look at two Enantiomers we can distinguish them by saying that they are mirror image of each other but not superimposable as we have a perspective view . So, undoubtedly our perspective vision is a Chiral Environment.

During the Synthesis if chiral molecules from achiral one we get both the enantiomers in equal amount.Now we know Enantiomers have different physical and chemical properties under Chiral Environment.So, to obtain one enantiomer properly either we have to synthesize it under Chiral environment or we have to separate them in presence of the environment of the Chiral molecule. Now how can we obtain first vital Chiral compound?

NATURE IS CHIRAL : Nature has a left and a right. It can distinguish between them.All human s have stomach on their left and their lever on the right.Some snail spiral to the left, but the vast majority of the marine snail shells spiral to the right. Snails

SPIRAL OF HONEYSUCKLE AND BINDWEEDBindweed (a type of climbing wild plant) spirals to the rightHoneysuckles (a climbing shrub with sweet smelling yellow or pink flowers) climbs by spiralling to the left

(-)LimonenePresent in lemon (+) LimonenePresent in orange Sharp and LemonySmells rounded and orangey

Carvone(+)Carvone(-)CarvoneSource: Spearmint Oil Source: oil of Caraway Use : For flavouring Chewing-gum Use: For flavouring toothpaste

Sugars, a major source of food : Sucrose(table sugar) and Lactose(milk sugar) are examples. These two disaccharides contain Glucose moiety which have a definite handedness.

D-DOPA(MORE EFFECTIVE)L-DOPA(LESS EFFECTIVE)Both are used in

Structure of dexfenfluramineIt is an appetite suppressant-it stimulates the production of hormone Serotonin and makes the body feel satisfied. But only the Enantiomer shown beside is active and the racemic fenfluramine has undesirable side effects. It is used for the treatment of overweight patients

Used in Asthma, Bronchitis, cold, flu. And tropical menthol used to feel free from headaches. StructureTree from which Menthol extracted

25 microgram of japonilure can trap thousands of beetles. Japanese beetlesThe simple lactone is the pheromone released by Japanese beetles as a means of communication. The cis isomer is virtually useless-it retains only about 10% activities. It is not only inactive but acts as a powerful inhibitor of the original pheromone-even 1% other enantiomer in a sample of pheromone destroys the activity. Not used usedJaponilure used to trap Japanese beetels

Even bacteria know their right from their left: Pseudomonas putida is a bacterium that can use a aromatic hydrocarbon as food stuff, degrading them to diols. The diol produced from chlorobenzene is formed as one enantiomer only.

For catalysis to happen, the substrate molecule has to fit momentarily on the surface of the enzyme and form a complex. The reaction takes place while the complex exists . If anything is wrong with the shapes of either the substrate or the enzyme, there can be no fit and, therefore no reaction(Pictured here are two highly schematic ways of representing these ideas about shape and fit:)

Used as an analgesic, given to arthritis suffers.

Marketed as BrufenCures painDoes not cure painOne enantiomer actually cures pain. The compound is administrated as racemateThe body does the rest, racemising the compound by enolising it

There is no clear relationship between molecular chirality and chirality of life forms. Right and left handed people are made from amino acids and sugars of same handedness.

Why one Enantiomer is produced by NATURE?There is no scope to back in EVOLUTION(+)Enantiomer(-) EnantiomerB2,C2,N2,CO2,H O2,NH3,Urea

CAN YOU SOLVE?It is October 1929, in a lonely cottage of Devonshire, George Harrison in middle aged amateur mycologist has died shortly after eating a mushroom stew be prepared from Amanita rubescens. Cause of death: poisoning by Muscarine; an alkaloid found in Amanita muscaria. You have been asked to solve a knotty problem crucial to the investigation; whether a deadly Amanita muscaria found its way accidentally into the mess of closely similar, but harmless Amanita rubescens or a lethal dose of synthetic Muscarine(filched from a London laboratory) was deliberately added to the stew pot- perhaps by the lover of beautiful Mrs. Harrison. You have available a solution of Muscarine that you have isolated from left over stew, a well equipped(for 1929) laboratory and two minutes.