General Index
Aminoacids: Stereoisomery
All protein aminoacids have a common structure. Let's see the case of L-Alanine:
A saturated carbon atom:
known ase a-carbon, substituted by: (1) An amino group, -NH2, that appears in its protonated form -NH3+:
(2) A carboxyl group, -COOH, in its dissociated form -COO-
(3) A Hydrogen atom -H
and (4) A side chain, that is different for each aminoacid. In this case is a methyl group -CH3, the side chain of L-Alanine:
Note that both the amino and the carboxyl groups appear ionized: amino as -NH3+ and carboxyl as -COO-.
In one case, the amino group appears substituted by the side chain, forming a secondary amine; is the case of L-Proline:
With only one exception, the a-carbon appears substituted by four different groups. Then it is an asymmetric or chiral carbon, and can present optical isomerism. By convention, isomers around a-carbon are called L- and D-, by analogy with Glyceraldehyde. Let's compare the structures:
Carbon 2:
-OH group:
Aldehyde group:
Hydrogen:
CH2OH group:
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a-carbon:
-NH3+ group:
-COO- group:
Hydrogen:
Side chain (-CH3):
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Let's compare now the structures of L-Alanine (left) with that of D-Alanine (right):
The only exception is the aminoacid Glycine, the simplest of all, in which the side chain is a hydrogen atom and thus the a-carbon is not chiral.
All the protein aminoacids belong to the L-series. We can find D-aminoacids in the cell wall of Bacteria and in some peptide antibiotics.
From now on we'll understand that all the aminoacids seen in this demo belong to the L-series.