Amino acids are the monomers used to make proteins. There are many different types of proteins found inside living organisms from the collagens found in tendons and arteries to the keratine in hair, nails and animal hoof. Enzymes, the biological catalysts used in living organisms to control many essential bodily functions also consist of proteins.

Amino acids are the small monomer units that are used to build the larger protein polymer molecules. As the name implies amino acids contain two functional groups; the basic amino group (-NH₂) and the acidic carboxyl group (-COOH).

Around 20 amino acids are found inside the human body, they are all α-amino acids (that is they have the basic amino and the acidic carboxyl groups attached to the same carbon atom). With the exception of glycine all the amino acids are optically active molecules with two different enantiomeric form for each amino acid, however in nature only the S-enantiomer for each amino acid is used in building up proteins and polypeptide molecules. It is quite remarkable to think that from these 20 amino acids that the huge range and types of proteins found in living organism can be  synthesised, but think how many words can be made from the 26 letters in the alphabet! The structure of four common α-amino acids found in the human body are shown in the diagram below:

Zwitterion ions

I am sure you may have noticed a problem with the structures of the amino acids drawn above. How can we have a molecule with an acidic group on one end and a basic group on the other end? Surely they will just neutralise each other in an acid-base reaction? Before we answer that question here a few facts that might help with an answer!

Amino acid do not behave as you might expect a typical organic compound to, for example:

• Amino acids are crystalline solids which have high melting and boiling points.
• Amino acids are water soluble and insoluble in most non-polar organic solvents.
• Amino acids are less acidic than most carboxylic acids and less basic than most amines.
• Amino acids have large dipole moments.

What could explain this unusual behaviour? The reason was hinted at above, amino acids undergo an internal acid-base reaction to form a dipolar ion or zwitterion. This zwitterion has ionic charges present and so has many of the properties you might expect from an ionic salt.

The diagram below shows how the amino acid alanine undergoes an internal neutralisation reaction to form the zwitterion.

The isoelectric point

The zwitterion ion of an amino acid contains acidic and basic groups. Though perhaps not the groups you were expecting! The basic group is the carboxylate anion (-COO^-) which is the conjugate base of the carboxylic acid (-COOH) and the acidic group is the ammonium ion (-NH₃⁺), which is the conjugate acid of the amine group (-NH₂). Since it contains both acidic and basic groups the amino acid zwitterion ion is amphoteric, that is it can react with both acids and bases.

To summarise: If the zwitterion is:

• placed in an acidic solution, that is a low pH the basic carboxylate ion on the zwitterion accepts a proton (H⁺) and forms a positively charged cation
and
• if sodium hydroxide or another strong alkali is added to the zwitterion, that is at high pH the acidic ammonium ion (-NH₃⁺) will donate a proton (H⁺) to the hydroxide ion (OH^-) and form the basic amine group (NH₂), this leaves the zwitterion as an anion.

This means then at some intermediate pH value between these acidic and basic conditions the zwitterion must exist as a neutral ion with both positive and negative ends, that is a dipolar ion. The pH at which this occurs is called the isoelectric point. This is shown in the image below:

A common misconception among students is that the isoelectric point will be at pH 7, but this is not the case. The acidic ammonium group (-NH₃⁺) is a stronger acid than the basic carboxylate group. You must also consider the nature of the side chain R on the amino acid. In some amino acids this side chain may contain acidic and basic groups which will affect the pH at which the neutral zwitterion exists as a neutral ion. However for most amino acids the isoelectric point is around pH 5-6.5.

Key Points

• Amino acids are the monomers or building blocks for proteins. Amino acids are the monomers and proteins are condensation polymers formed the polymerisation of amino acid monomers.
• Amino acids are crystalline salts which are :
  • Water soluble
  • Have high melting points
  • Are soluble in water and polar organic solvents but insoluble in non-polar organic solvents
• A zwitterion is a neutral salt formed by the internal acid-base neutralisation of an amino acid.

Practice questions

Check your understanding - Questions on amino acids.

Next