Amino Acids

Amino Acids and Proteins

Amino Acids and Proteins

Structure of Amino Acids

Amino acids consist of an amine group and a carboxylic acid group. An amino acid that occurs naturally has the following general formula:

The central carbon atom is attached to:

  • a hydrogen
  • a carboxylic acid group
  • an amine group
  • an alkyl group

Therefore, apart from glycine where the R is another H atom, amino acids are all chiral.

Amino acids occur extensively in the human body. Examples include glycine aminoethanoic acid and alanine 2-aminopropanoic acid.

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Acid-Base Properties of Amino Acids

Amino acids have both acidic and basic properties:

  • carboxylic acids possess acidic properties
  • amines possess basic properties

Reaction with bases

Amino acids are able to react with strong bases like sodium hydroxide. Therefore, in a high pH amino acids are anionic.

Reaction with acids

Amino acids are able to react with strong acids like hydrochloric acid. Therefore, in a low pH amino acids are cationic.

Reaction with itself

On the same molecule amino acids possess both a proton accepting and a proton donating group. They can, therefore, undergo acid-base reactions with themselves. The resulting product is a double ion called a Zwitterion.

This reaction takes place in a solid state which means that amino acids must be ionic. This is the reason why they have a high melting point.

Summary

Therefore, it is possible for amino acids to exist in four forms:

  • molecular
  • cationic
  • anionic
  • Zwitterion

As they can react with both acids and alkalis they also make good buffer solutions.

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Condensation Reactions of Amino Acids

Amines can undergo condensation with acid anhydrides and acyl chlorides.

Although less reactive, carboxylic acids can undergo a similar reaction with amines in conditions present within living organisms.

Due to the fact amino acids possess both an acid and an amine group they can react with one another. The end result is a molecule consisting of an amide or peptide link. The reason why this is called a condensation reaction is between two amino acid molecules bond and one molecule of water is lost.

The resulting molecule is called a dipeptide. Dipeptides can also be formed from two different amino acids in which case two different molecules are produced.

As dipeptides also possess amine and carboxylic acid groups they can take part in further condensation reactions and, eventually, form polymers.

In the body, DNA lines up amino acids in a specific order so that condensation can take place. The resulting polymer is called a protein.

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Proteins

Proteins are naturally occurring polyamides. Under the careful supervision of DNA they are formed by the condensation of numerous amino acids.

Structure

The primary structure of a protein is the sequence the amino acids are in. This is different from protein to protein and depends on the function of the protein.

The code of an amino acid is made up of three letters. The sequence of these letters represents the order they are in to make up a particular protein. For example:

gly – ala – leu – iso – gln

It is possible for a protein to be composed of thousands of amino acids.

Due to the hydrogen bonding that makes up the structure, protein molecules are not straight. Instead, they have a coiled structure. This is because the hydrogen atom on one peptide link forms a hydrogen bond with the oxygen or nitrogen atoms on the other peptide link.

This coiling structure is known as the secondary structure of a protein.

Hydrolysis

The peptide link found in proteins is the same as that found in N-substituted amides. Therefore, it is possible to break it with heat in the presence of a strong acid or a strong alkaline. In general, 6 moldm-3 HCl is used.

This in an example of a hydrolysis reaction and in acidic conditions amino acids in cationic form are produced.

Using this reaction it is possible to deduce which amino acids are present in a protein sample. The amino acids can then be identified through chromatography. Chromatography involved placing the amino acid sample onto chromatography paper. Here the amino acids separate. The Rf values (the distance that each amino acid moves up the paper compared to the solvent) of the amino acids are then compared to those listed in a database of known amino acids.