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4.2.2. Properties of amino acids

Stereoisomerism or spatial isomerism

All amino acids, except glycine, have an asymmetric carbon, the α-carbon, attached to four different radicals: an amino group, a carboxyl group, an R radical, and a hydrogen. As a consequence, amino acids present isomerism.

Each amino acid can have two stereoisomers:

  • With configuration D if when arranged in space, so that the carboxyl group is on top, the group -NHIt is located at the right.
  • With L configuration, if the -NH2 group is on the lef .

Both stereoisomers are mirror images and do not overlap each other, so they are enantiomers.

All protein amino acids have the L configuration.

Optical isomerism

The amino acids have optical activity by the existence of the asymmetric carbonbeing able to deflect the plane of polarized light passing through a solution of amino acids.

Depending on where the plane of polarized light deviates, they can be:

  • Right-handed or (+), if the amino acid deflects the plane of polarized light to the right.
  • Levo-gyro or (-), if you deflect it to the left.

The L or D configuration is independent of the optical activity, so an L-amino acid can be left- handed or right-handed, just like another with the D configuration.

Amphoteric behavior

The amino acids dissolved in water have an amphoteric behavior, that is, they can ionize, behaving as an acid or as a base, depending on the pH. This characteristic is due to the existence of the carboxyl group and the amino group:

  • It behaves like acid. The --COOH groups release protons, remaining as -COO-.
  • It behaves as a base. The -NH 2 groups take up protons, remaining as -NH3+.

Due to their amphoteric behavior, amino acids tend to neutralize the pH variations of the medium, since they can behave like an acid or a base, releasing or removing protons from the medium.

  • If the medium is acidic, the amino acid behaves like a base. The -COO- group picks up a proton and loses its negative charge.
  • If the medium is basic, the amino acid behaves like an acid. The group like -NH 3+ releases a proton and loses its positive charge.

Each amino acid has a pH at which it tends to adopt a neutral dipole form (or zwitterion), with both positive and negative charges, which is called the isoelectric point.

Punto isoeléctrico

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Common mistake: amphoteric and amphipathic

A fairly common mistake that appears on tests is to confuse amphoteric and amphipathic.

We have already seen that fatty acids have amphipathic behavior (from the Greek amphi, "both", and pathic, "affection, passion"), that is, they have:

  • The head, a hydrophilic polar zone (the carboxylic end, -COOH) that interacts with water (H2O).
  • The tail, a hydrophobic apolar zone (the hydrocarbon chain) insoluble in water because it does not have polar groups. The longer the chain, the more insoluble it will be in water and the more soluble it will be in polar solvents.

Now, when talking about amino acids, we have seen that they are amphoteric, chemical compounds capable of reacting with both (acids and bases).