Chemistry in your cupboard

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The structure of hair (4 of 4)

Salt formation

Look at the structures of the amino acids. You will see that some of them have side chains (the R groups) that are acidic or basic.

Another type of bonding that helps to hold the secondary structure of proteins is salt formation between amino acids with acidic and basic side chains. A proton (H⁺ ion) is transferred from the acid to the base forming a negative ion and a positive ion - these attract one another, Figure 7.

The protein keratin largely consists of an α-helix. Furthermore two helices wind round each other rather like the strands of a rope. This is called the tertiary structure of the protein and is partly responsible for the fibrous nature of keratin. As well as hydrogen bonds and salt bonds, there is a third form of bonding that helps to hold the tertiary structure of keratin in place – sulfur-sulfur bonding.

Sulfur-sulfur bonding

Look at the structure of the amino acid cysteine, Figure 8.

You will see that its side chain has a S-H group. In certain circumstances, two cysteine molecules can link by the formation of a S-S bond between these two S-H groups. These S-S bonds are often called disulfide bridges.

The resulting ‘double’ amino acid is called cystine. (Take care, the two names are confusingly similar.) Forming the S-S bond involves the removal of two hydrogen atoms and so it is an oxidation process and requires an oxidising agent, represented here by [O], Figure 9.

This reaction can be reversed by using a suitable reducing agent.