Describe the primary structure of a protein.

The primary structure of a protein refers to the sequence of amino acids along a protein chain. A protein's primary structure is held together by peptide links (amide bonds) between amino acids.


A sketch of a protein's primary structure, containing 5 amino acids, is shown below.

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Describe the secondary structure of a protein.

The secondary structure of a protein refers to the formation of a helix (the α-helix) or a folded sheet (called a β-pleated sheet) in a protein chain.


Sketches of an α-helix (left) and a  β-pleated sheet (right) are shown below.

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Describe the tertiary structure of a protein.

The tertiary structure of a protein refers to the three-dimensional shape formed by the α-helix or β-pleated sheet.


A sketch of a protein's tertiary structure is shown below.

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What is a disulfide bond?

A disulfide bond is a type of bond that forms between the -CH2SH side chains of two cysteine amino acids under suitable oxidising conditions.

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How are the secondary and tertiary structures of a protein maintained?

The secondary and tertiary strutures of a protein are maintained by:

  1. Hydrogen bonds e.g., between the -OH groups of two amino acids.
  2. Disulfide bonds between the -SH groups of two cysteine residues.


Hydrogen bonds and disulfide bonds are represented in the sketch below.


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How can temperature and pH affect protein structure?

Temperature and pH can affect hydrogen bonding and the formation of disulfide bonds, which can lead to protein denaturation i.e. a change in the protein's 3D shape.

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What is thin layer chromatography?

Thin layer chromatography (TLC) is a technique used to separate the components in a mixture. The components travel at different rates through the stationary phase, allowing them to be separated.

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How does thin-layer chromatography separate amino acids?

Different amino acids have different solubilities in the same solvent, thus they move at different rates on the chromatography plate. This allows them to be separated.


More soluble amino acids travel faster along the chomatography plate. Less soluble amino acids travel slower along the chromatography plate.

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How are the spots of amino acids made visible on the chromatography plate?

By spraying the plate with a developing agent such as ninhydrin (which turns the spots purple), or by spraying UV light on the plate. 

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What is the formula to calculate the Rf value of an amino acid spot in thin-layer chromatography?

Rf =distance moved by solventdistance moved by amino acid spot

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How can you use experimental Rf values to identify the amino acids in a mixture?

By comparing experimental Rf values to a table of known amino acid Rf values run in the same solvent mixture.

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What type of reaction forms a peptide bond?

A condensation reaction.


This reaction results in the elimination of a small molecule, in this case, water.

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What are polypeptides?

Polypeptides are long chains of amino acids.


When polypeptides contain more than 50 amino acids they are known as proteins.

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Describe how proteins can be broken down.

Proteins can be broken down by acid hydrolysis. When a protein is heated with an acid such as hydrochloric acid, all the peptide linkages are hydrolysed and the protein breaks down into its constituent amino acids.


The acid hydrolysis of a dipeptide is shown below.

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What is a dipeptide?

A dipeptide is a molecule that results from the reaction of two amino acids.


The acidic -COOH group in one amino acid react with the basic -NH2 group in another, as shown below. Three amino acids comine to form a tripeptide.

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What are the stationary and mobile phases in thin-layer chromatography?

The stationary phase is a thin layer of silica gel or alumina on a plate.

The mobile phase is a solvent or solvent mixture.

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