These are detailed Revision Notes for Topic 3.13 of AQA A-Level Chemistry (Amino acids, Proteins and DNA). They were written by me using a combination of the textbook and class notes. I will also be uploading the other topics and creating bundles.
Topics Included:
- Introduction to DNA
- Peptid...
Amino Acids, Proteins and DNA
30.1 Introduction to Amino Acids:
- Amino acids have two functional groups – both a carboxylic acid 30.1 Introduction to amino acids
and a primary amine. 30.2 peptides, polypeptides and proteins
- There are 20 naturally occurring amino acids and they are all 30.3 enzymes
alpha-amino acids (the amine group is on the carbon next to the
carboxylic acid group) 30.4 DNA
- This means that the molecule is chiral, almost all natural amino 30.5 The action of anti-cancer drugs
acids are the (-) enantiomer.
- Amino acids contain both an acidic and a basic group.
- There is an internal transfer of a hydrogen ion from the COOH group to the NH2
group to leave an ion with both a negative charge and a positive charge. This is
called a zwitterion.
- This will only occur at a pH which is acidic enough for the carboxylic acid. This is
called the isoelectric point.
- When an amino acid is in an environment where the pH is:
o Greater than its isoelectric point (more basic, low H+ conc). It will behave
as an acid and lose a proton from the carboxylic acid group. .
o Lower than its isoelectric point (more acidic, high H+ conc). It will behave as a
base and accept a proton in the amine group.
30.2 Peptides, Polypeptides and Proteins:
- Amino acids link together to form peptides.
- Proteins or polypeptides are long chains of amino acids. These are formed in condensation reactions
similar to carbohydrates.
- Peptide bonds (amide linkage -CONH-) form between individual amino acid molecules
- Primary structure – the sequence of different amino acids in the protein chain.
- Secondary structure – hydrogen bonds form causing the molecule chain to adopt regular structures
(alpha helix or beta pleated sheet).
- Tertiary structure – hydrophilic/hydrophobic interactions, hydrogen bonds, ionic bonds and disulphide
bonds hold the molecules together in its 3D shape.
- Quaternary structure – same as tertiary but with 2 separate polypeptide chains.
- Hydrolysis
o If a protein or peptide is boiled with 6 mol dm-3 hydrochloric acid for 24 hours, it will break
down to the mixture of amino acids that make it up.
- Amino acids can be separated and identified by thin-layer chromatography (TLC).
- The paper is replaced by a thin plastic sheet called the chromatography plate coated in SiO2
(silica/silicon dioxide).
- The solvent is the mobile phase, the powder is the stationary stage
- Method:
o Spot of amino acid placed on a line 1cm up the plate and placed in a tank with a solvent
0.5cm deep.
o Lid placed on tank to contain the solvent vapour. Solvent rises up the plate carrying amino
acids with it, each lags a different amount depending on its affinity for the solvent compared
to its affinity to the stationary phase.
o The stronger the intermolecular forces between the amino acid and the solvent, the closer the
amino acid is to the solvent.
o When the solvent has nearly reached the top of the plate, the plate is removed from the tank
and the position that the solvent has moved to is marked.
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