The structure of hair (4 of 4)
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.
Look at the structure of the amino acid cysteine, Figure 8.
Figure 8: Structure of cysteine
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.
Look at the 20 amino acids in Figure 3 and list those that have.
- acidic side chains and state the type of acid in the side chain in each case.
- basic side chains and state the type of base in the side chain in each case.
- serine, aspartic acid, glutamic acid (tyrosine has a phenolic side chain which would be a weaker acid than those above which have carboxylic acid side chains)
- tryptophan (secondary amine), histidine (secondary and tertiary amine), arginine (primary and secondary amine), lysine (primary amine).