Amino Acids 🌙

Define zwitterion

1. A zwitterion is a dipolar compound that contains both cationic and anionic groups and is electrically neutral.

Describe and explain how zwitterions are formed

1. Amino acids form zwitterions as a result of an internal acid-base reaction where the acidic COOH group donates a proton to the basic NH2 group.

Describe and explain the physical properties of amino acids

1. High melting/boiling point.

• In the solid state, amino acids exists as zwitterions. Zwitterions have a giant ionic lattice structure. A large amount of energy is required to overcome the strong ionic bonds between zwitterions.

2. Soluble in water.

• Formation of ion-dipole interactions between zwitterions and water molecules releases sufficient energy to overcome ionic bonds between zwitterions and hydrogen bonds between water molecules.

Define isoelectric point

1. Isoelectric point is the pH at which a particular amino acid exists as a zwitterion. It is unique for each amino acid (e.g. glycine: 5.97, the only alpha amino acid that does not contain a chiral carbon and is optically inactive).

2. Isoelectric point = Isoelectric pH = 1/2(pKa1 + pKa2).

3. The amino acid has the least buffering effect at the isoelectric point.

Describe and explain the acid base properties of amino acids

1. Amino acids are amphoteric hence they can function as both an acid and a base.

2. When placed in a solution of pH lower than its isoelectric point (acidic), the amino acid acts as a base and the basic COO- groups accepts a proton hence the amino acid exists as a cation.

3. When placed in a solution of pH higher than its isoelectric point (alkaline), the amino acid acts as an acid and the acidic NH3+ group donates a proton hence the amino acid exists as an anion.

Describe and explain how amino acids act as buffers (+1 cation and zwitterion in equal concentrations)

1. Amino acids can act as buffer solutions within certain pH ranges to resist changes in pH upon addition of small amounts of acid or alkali.

2. The buffer contains equal concentrations of the +1 cation and zwitterion.

3. When a small amount of acid is added, the basic COO- group in the zwitterion neutralises the acid added hence there is negligible change in pH.

4. When a small amount of base is added, the alpha COOH group in the zwitterion neutralises the base added hence there is negligible change in pH.

Describe and explain how amino acids act as buffers (-1 anion and zwitterion in equal concentrations)

1. Amino acids can act as buffer solutions within certain pH ranges to resist changes in pH upon addition of small amounts of acid or alkali.

2. The buffer contains equal concentrations of the -1 anion and zwitterion.

3. When a small amount of acid is added, the alpha NH2 group neutralises the acid added hence there is negligible change in pH.

4. When a small amount of alkali is added, the acidic NH3+ group neutralises the base added hence there is negligible change in pH.

Describe the method to determining species at various pH

1. Assign the pKa values to each group.

2. Protonated all ionisable groups in the amino acid.

3. If there are more than one acidic or basic groups, infer which has a lower pKa value (stronger acid).

4. In all amino acids, the alpha COOH group is always the most acidic group hence has the lowest pKa. This is because it is closest to the electron withdrawing NH2 group, dispersing the negative charge on the alpha carboxylation anion hence stabilising it.

5. If pH() < pKa (acidic solution), the group will remain protonated.

6. If pH () > pKa (basic solution), the group will remain deprotonated.

Case: glutamic acid

pH 7 is higher than pKa values of 2.1 and 4.1 but lower than pKa value of 9.5 hence the two COOH groups should be deprotonated to basic COO- groups while the NH3+ should remain protonated.