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CSEC Chemistry: Polyamides and Proteins

Polyamides (or polypeptides) are synthetic polymers composed of several amide (also called peptide) groups. The amide group/linkage consists of a carbon to nitrogen bond:

Just like polyesters, polyamides are condensation polymers (prepared by condensation reactions). Polyamides are formed between di-amines and di-acids:

When the diamine and the di-acid react, a water molecule is lost from between, forming an amide linkage:

As you can see, the product contains an amide linkage, but has another amine group and another acid group, that are capable of condensing further with other acids and other amine groups to form a polyamide represented by this repeat unit:

So, when naming condensation polymers, just remember that polyesters contain ester linkages (-COO-) while polyamides (or polypeptides) contain amide/peptide linkages (-CONH-):

Above, we discussed polyamides formed between di-alcohols and di-acids. The amide/peptide exists naturally as well, though, in proteins. Proteins are natural condensation polymers formed by multiple amino acids. Proteins serve several functional and structural purposes in living things, for example, the protein keratin gives structure to hair and nails while the protein insulin is a hormone which helps regulate blood sugar levels.

Proteins are composed of amino acids, compounds containing an amino group and an alkanoic acid group. The amino acid general structure is:

For every different type of amino acid, the group 'R' is different. There are more than 80 amino acids known, however, only 20 of these amino acids form proteins in living systems (known as the proteinogenic amino acids). Here are a few examples from that list of 20:

When we discussed polyamides, the alcohol group from the dialcohol reacted with the amine group from the diamine in a condensation reaction to form the polyamide. What is interesting about amino acids is that each of them has both the amide group and the alkanoic acid group, so they can react with each other to forma polypeptide chain.

Both amino acids have different R groups, so they are different amino acids.

When the dipeptide is formed, there are still functional groups at both ends, meaning that it can still undergo further condensation reactions.

The peptide linkages (highlighted in blue, -CONH-) occur at intervals along the polypeptide chain. When 50 or more amino acids have been condensed together, a protein has been formed.

Protein structures are incredibly complicated. Since 20 amino acids are available, an immense amount of proteins are able to be formed, especially because some proteins comprise 3000 or more amino acid units.

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