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Stereoisomers are molecules that have the same molecular formula, but have a different spatial arrangement. This means that they are arranged differently in space, despite the fact the atoms are bonded in the same order. In order for this to occur, the molecule must contain a carbon that is bonded to four different groups, This is called a chiral centre, or an asymmetric carbon. Sterioisomers can be divided into further groups according to the specific spatial arrangements.



Enantiomers are stereoisomers that are non-superimposable mirror images of each other[1]. They are usually described as having D or L configuration[2]. Often one configuration exists more frequently in nature, for example the L form of most amino acids is more prominant in nature, possibly because they are slightly more soluble than the D form. You can differentiate between the two forms by investigating the direction in which they rotate polarised light.

A racemate mixture contains equal amounts of enantiomers[3], therefore racemate mixtures are optically inactive because the enantiomers rotate a plane of polarised light by the same number of degrees in opposite directions[4]


  1. Naveen Chhabra, Madan L Aseri, and Deepak Padmanabhan1. A review of drug isomerism and its significance.Int J Appl Basic Med Res. 2013 Jan-Jun; 3(1): 16–18.
  2. Sukanya Mitra and Puneet Chopra.Chirality and anaesthetic drugs: A review and an update.Indian J Anaesth. 2011 Nov-Dec; 55(6): 556–562.
  3. Michael J. Owens, Jonathan McConathy. Stereochemistry in Drug Action.Prim Care Companion J Clin Psychiatry. 2003; 5(2): 70–73.
  4. The Editors of Encyclopaedia Britannica.Racemate. 2011. [cited 27/11/18] Available from:


Molecules that have multiple asymmetric carbons are called diastereoisomers. These are not mirror images of each other. A common example is a monosaccharide that has a carbon chain of three or more, such as glucose. The number of possible stereoisomers is equal to 2n, where n is the number of asymmetric carbons in the molecule. There are two further types of diastereoisomers - epimers and anomers. Epimers are two diastereoisomers that differ at only one of the multiple asymmetric carbons, whereas anomers are cyclic molecules that differ at a new asymmetric carbon that is formed as a result of the ring formation. E-Z or cis-trans isomerism is another type of diastereoisomers, resulting from the restricted rotation around a double bond[1].


  1. Biochemistry, 7th Edition, Jeremy M. Berg, John L. Tymoczko, Lubert Stryer, W. H. Freeman and Company, New York, 2012, chapter 11- page 331, chapter 2 - page 27.
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