(from Greek, isos: equal, meros: part) Certain chemicals can exist in two different forms, called isomers. This property arises when there is an asymmetry in the structure of a MOLECULE. Because of this asymmetry, any other components of the molecule attached to the asymmetric component will be biased in one direction or another. The bias may be to the right (the D-isomer) or to the left (the L-isomer): ‘D’ stands for dextrorotatory (rotating to the right, or clockwise), ‘L’ stands for levorotatory (rotating to the left, or anticlockwise). It is important to note that isomeric forms of a molecule have essentially the same properties, though there may be important differences. For example, AMINO ACIDS exist in isomeric forms.
In this case, the isomerism comes from the fact that one of the carbon atoms, to which a side chain bonds, is asymmetric. Physiologically, L-amino acids are more important than the D- form. For instance, only L-amino acids are incorporated into PROTEINS. AMPHETAMINE is another example of an isomerized chemical. D-amphetamine is more potent that L-amphetamine, and a RACEMATE form also exists.
Reference
Feldman R., Meyer J.S. & Quenzer L.F. (1997) Principles of Neuropsychopharmacology, Sinauer Associates: Sunderland MA.
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