What is primary structure?

The sequence of amino acids in a polypeptide chain.

Name two secondary structures.

The α-helix and the β-pleated sheet.

What holds secondary structure in place?

Hydrogen bonds between the N–H and C=O groups of peptide links.

What is tertiary structure?

The further folding of the whole polypeptide into a specific 3D shape.

What maintains tertiary structure?

Hydrogen bonds between side chains and S–S bonds between cysteine residues.

What is an S–S bond?

A covalent bond between sulfur atoms of two cysteine side chains.

How are peptides hydrolysed?

Heated with aqueous acid (such as 6 mol dm⁻³ HCl) or alkali; every peptide link breaks.

What is the formula for Rf?

Distance moved by the spot ÷ distance moved by the solvent front.

Why is ninhydrin used in TLC of amino acids?

Amino acids are colourless; ninhydrin gives coloured spots. UV light with a fluorescent plate also works.

How is an amino acid identified from TLC?

Compare its Rf with known values measured under the same conditions.

ProteinsAQA A-Level Chemistry: Flashcards

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What is a peptide link?

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What is a peptide link?: The –CONH– link formed when the –COOH of one amino acid reacts with the –NH₂ of another, losing water.
What is primary structure?: The sequence of amino acids in a polypeptide chain.
Name two secondary structures.: The α-helix and the β-pleated sheet.
What holds secondary structure in place?: Hydrogen bonds between the N–H and C=O groups of peptide links.
What is tertiary structure?: The further folding of the whole polypeptide into a specific 3D shape.
What maintains tertiary structure?: Hydrogen bonds between side chains and S–S bonds between cysteine residues.
What is an S–S bond?: A covalent bond between sulfur atoms of two cysteine side chains.
How are peptides hydrolysed?: Heated with aqueous acid (such as 6 mol dm⁻³ HCl) or alkali; every peptide link breaks.
How many H₂O are lost in forming a tripeptide?: Two.
What is the formula for Rf?: Distance moved by the spot ÷ distance moved by the solvent front.
Why is ninhydrin used in TLC of amino acids?: Amino acids are colourless; ninhydrin gives coloured spots. UV light with a fluorescent plate also works.
How is an amino acid identified from TLC?: Compare its Rf with known values measured under the same conditions.
Draw Gly–Ala.: H₂NCH₂CONHCH(CH₃)COOH

Exam questions on Proteins

Glycine, H₂NCH₂COOH, and alanine, CH₃CH(NH₂)COOH, can join together to form dipeptides.
Give the structural formula of the dipeptide in which the –COOH group of glycine has reacted with the –NH₂ group of alanine, and identify the peptide link.2 marks
A mixture of amino acids from the hydrolysis of a protein is separated by thin-layer chromatography (TLC). The plate is then sprayed with ninhydrin. The solvent front moved 8.0 cm from the baseline. The centre of spot 1 is 3.6 cm above the baseline and the centre of spot 2 is 5.2 cm above the baseline.
Calculate the Rf value of spot 2 and explain how it can be used to identify the amino acid.2 marks
Keratin, the protein in hair, contains regions that are coiled into an α-helix. Its shape is also held by sulfur–sulfur bonds between cysteine residues.
Describe what is meant by the primary structure and the secondary structure of a protein such as keratin.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).