General Index
Isomerism
With the name of Isomers we know those compounds having the same chemical composition but different 3D structure. As the tridimensional structure has a capital importance in Biochemistry, it is necessary to have a clear concept of the different types of isomers.
First we have Skeletal Isomers, when two comounds having the same composition differ in their carbon skeleton. Thus, Butane and Methylpropane:
Two compounds having the same composition may differ in the position of a functional group. They are called Positional Isomers. Such is the case of 1-Propanol and 2-Propanol:
Relative positions of groups around a double bond give Geometrical Isomers, like cis-2-butene and trans-2-butene:
The tetrahedrical shape of orbitals around a sp3 carbon allows another type of isomerism, called Optical Isomerism. When four different groups substitute a saturated carbon atom, they can do that in two (and only two) different ways, one of them being the mirror image of the other. Such is the case of Glyceraldehyde, that can present two optical isomers, called D- and L-:
These isomers are called Optical because compounds with asymmetric carbons deviate the plane of polarization of light. Enantiomers like D- and L-Glyceraldehyde deviate this plane with the same magnitude but opposite sign. In this case, D-Glyceraldehyde is Dextrorotatory (+), that is, the plane is deviated to the right, and L-Glyceraldehyde is Levorotatory (-), meaning that the plane is deviated to the left.
In the biomolecules there usually is a preference for one of the two opitcal isomers. Natural monosaccharides are generally of the D-series; aminoacids are usually of the L-series.
Let's see the case of the aminoacid Alanine. In this case the asymmetric carbon is 2. We proceed to load in the left panel L-Alanine and in the right D-Alanine:
The two molecules are Enantiomers, that is, mirror images of each other. All natural proteins are formed by L-series aminoacids.
A particular case of optical isomerism takes place in the fusion of two alicyclic rings. Such is the case of Decalin, formed by the fusion of two cyclohexane rings in the chair conformation. Note that these two rings are not aromatic (like in the case of naphtalene). The fusion can take place in two different ways, cis-Decalin and trans-Decalin:
Note that the hydrogens substituting C1 and C6 are directed to the same side in the cis- isomer and to opposite sides in the trans- isomer:
We'll see that this kind of isomerism is seen in Steroids.