Physical properties of enantiomers
Optical isomers possess identical physical properties (except in some cases they form asymmetric mirror image crystals), with one important exception; a pure optical isomer, or solution of a pure optical isomer rotates the plane of plane polarised light through an angle. The angle of rotation depends on the nature of the optical isomer, the solution strength and path length.
Each optical isomer rotates the plane of polarised light by the same amount, but in the opposite sense, i.e. if one enantiomer rotates polarised light clockwise, then the other enantiomer will rotate plane polarised light through the same angle anticlockwise.
Equal concentrations of two enantiomers do not rotate plane polarised light, as the opposite effects of each optical isomer cancel each other out. This is known as external compensation. Such an externally compensated solution, containing equimolar amounts of two enantiomers, is known as a racemic mixture, or racemate.
Although optical isomerism may seem at first sight to be rather a trivial form of isomerism, it is extremely important in biological systems. The chemical properties of optical isomers are identical EXCEPT when reacting with other optical isomers. This is the basis of the ‘key and lock’ activation of organic compounds by enzymes.
Biological systems contain many proteins and complex aminoacids, which are optically active. They may be stimulated to react in certain ways by optically active enzymes (biological catalysts).
Chemical properties of enantiomers
The relative positions of all of the component atoms in a pair of optical isomers are identical. This means that in almost every situation the optical isomers have identical chemical properties. However, there is one situation in which two optical isomers can have different reactivity, including one being totally unreactive while the other is reactive, and this is when they are reacting with another molecule at a site which itself is asymmetric and has optical isomers.
The simplest way to appreciate this is to consider the two optical isomers to be a pair of hands, one left hand and one right hand. The reagent which they wish to react with is a left glove. Clearly, the left hand can fit into the left glove, while the right hand cannot.
This principle occurs regularly in biological systems. Many enzymes are chiral and many of the active sites that they bind to are also chiral. The type of optical isomer, + or -, becomes very important. This is sometimes referred to as the lock and key principle. The enzyme is the key and the protein with which it is going to react is the keyhole of the lock. If the key is incorrect it cannot open the lock.
Measuring optical rotation
The ‘optical rotation’ means the angle through which plane polarised light is turned by the optical isomer under investigation. It is measured using a polarimeter.
An optical isomer may rotate plane polarised light in a clockwise manner (+) or in an anticlockwise manner (-). The isomers are said to be dextrorotatory (rotate to the right) and Laevorotatory (cause rotation to the left). There is no way of knowing how a specific optical isomer will affect plane polarised light just by looking at the structure.