Enantiomer (optical) isomerism

Enantiomers are mirror images of asymmetrical compounds with one or more chiral centre. Chiral is just a term used to show that the molecule is asymmetric and thus non-superimposable and is most commonly caused by a carbon atom. What this means is that though they are essentially exactly the same chemicals, and mainly work in the same way, they don’t have the exact same spacial arrangement and thus need to be recognised.

To visualise this, it may help to imagine both your hands as optical isomers of one another; with the backs of your hands facing you, try lining up your fingers and thumbs and you'll see your thumbs don't match up. Both hands have four fingers and one thumb and are used just the same but aren't exact copies which is the important difference when these compounds are used in living systems. Enzymes, as I'm sure we're all aware, require specifically shaped compounds. While one enantiomer may be specific, the other will not. Most of the time one enantiomer will be useful and the other simply won’t bind to the enzyme. Other times, like with for example Limonene, both will cause different affects such as one enantiomer having a scent characterised as orange and the other lemon. A more known example of this difference is seen in the drug Thalidomide which in the late 50s was prescribed to pregnant women to reduce morning sickness but unfortunately caused birth defects. One optical isomer causes these malformations whereas the other is the effective sedative. It’s important now that all drugs with a chiral centre have all the possible isomers tested separately for separate side effects and in the interest of saving money and resources, only the useful enantiomer is likely used.

Polarimeters are thus used to distinguish between the different optical isomers. Only chiral molecules will rotate the visible light (after passing plane-polarized light through a polarimeter) and are said to be optically active. Racemic solutions (50/50 opposite enantiomer isomer solutions) or solutions without any chiral molecules have no effect on plane polarized light. Different enantiomers will rotate the light left or right and are said to be +/-, R/S or D/L. 


Resources: http://en.wikipedia.org/wiki/Thalidomide , http://en.wikipedia.org/wiki/Enantiomer , http://www2.vuw.ac.nz/staff/paul_teesdale-spittle/organic/chiral_web/context.htm , http://www.chemistryexplained.com/Ce-Co/Chirality.html#b , http://en.wikipedia.org/wiki/Chirality_(chemistry)#The_identity_of_the_stereogenic_atom and http://www.chemguide.co.uk/basicorg/isomerism/optical.html
Author: Grace Ronnie