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:



a-carbon:

-NH3+ group:

-COO- group:

Hydrogen:

Side chain (-CH3):

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.


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