Chemistry 4.7 - Amino acids, peptides and proteins

NOTE: apart from aminoethanoic acid, all a-amino acids show CHIRALITY. Its 'R' group is another hydrogen atom.

There is an internal transfer of a hydrogen ion between the -COOH group and the -NH2 group by the use of its lone pair, creating a zwitterion.

Zwitterion = a molecule with an atom/group bearing a positive charge and an atom/group bearing a negative charge.

Melting point

a-Amino acids are solid at room temperature. Therefore, the strong ionic attractions between the positively and negatively charged ions requires a large energy input to overcome the forces: high melting temperature. 

Solubility

a-Amino acids are soluble in water due to the presence of zwitterions. The ionic attractions that usually occur between the ions of the solid a-amino acid are replaced by the strong attractions between polar water molecules and zwitterions. 

The larger the molecule, the less soluble it is.

a-Amino acids are amphoteric: they can act as both an acid and as a base. 

Aqueous solutions of the a-amino acids contain zwitterions BUT for the compound itself will only exist as a zwitterion at a certain pH. This is the ISOELECTRIC POINT. The isoletric point varies for each a-amino acid, e.g. aminoethanoic acid = pH 6.0.

If the pH is less than the isoelectric point, then the amino acid will act as a BASE: it accepts a proton. The zwitterion is now a positive ion (cation).

If the pH is greater than the isoelectric point, then the amino acid will act as an ACID: it donates a proton. The zwitterion is now a negative ion (anion).

Dipeptides are formed via a condensation reaction, where two amino acids join, forming a peptide bond and releasing water. 

NOTE: if two different amino acids are used, there are two different products to the reaction.

Polypeptide = long chain of condensed amino acids joined by peptide bonds. 

A polypeptide is formed via many condensation reactions.

PRIMARY STRUCTURE: the sequence and order of amino acids in a polypeptide chain. 

SECONDARY STRUCTURE: the shape formed by the polypeptide chain due to hydrogen bonds. This can either be an a-helix, which occurs when the polypeptide chain is coiled into a spiral, or a B-pleated sheet. 

TERTIARY STRUCTURE: when an a-helix is foled and twisted to form a more compact, 3D shape. This shape is maintained by ionic, disulpher and hydrogen bonds, alongside hydrophobic and hydrophilic interactions. 

Globular proteins, such as enzymes, have a tertiary structure. They have a polar molecules on their surface, increasing their solubility. 

• Proteins are enzymes which are biological catalysts. Their role in the body is essential. For example, amylase (found in human saliva) breaks down starch into maltose. 

Enzymes have commercial uses. For example:

• Rennin - used in the dairy industry to hydrolyse protein in cheese making. 
• Amylases, proteases and lipases are found in laundry detergent to remove stains from proteins, fats and starch on clothes. 
• Some proteins act as hormones, e.g. insulin.