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.
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.
