AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The importance of hydrogen bonding and sulfur–sulfur bonds in proteins.
Full specification wording
Proteins (A-level only): The importance of hydrogen bonding and sulfur–sulfur bonds in proteins.
Two very different attractions hold a protein in shape.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Two very different attractions hold a protein in shape. A hydrogen bond forms between the δ⁺ hydrogen of an N-H and a lone pair on the δ⁻ oxygen of a C=O, or between R groups that carry -OH, -NH₂ or -COOH, as a serine R group does with an aspartic acid R group. Each one is weak, but a protein has thousands and together they hold the helix, the sheet and much of the fold. A disulfide bridge is different in kind: two cysteine R groups, each ending in -CH₂SH, are oxidised so their sulfur atoms bond directly, giving -CH₂-S-S-CH₂-. That is a covalent bond, so it is far stronger than a hydrogen bond and pins together parts of the chain that are far apart in the sequence.
What earns the marks
- Naming cysteine as the amino acid whose R groups form the disulfide bridge.
- Hydrogen bonding between R groups such as those of serine and aspartic acid.
- Stating that the disulfide bridge is the stronger interaction.
- The reason, that a disulfide bridge is a covalent bond whereas a hydrogen bond is not.
Common mistakes and how to put them right
Calling a disulfide bridge a hydrogen bond because it holds two parts of a chain together.
Drawing a hydrogen bond to the hydrogen atom of a C=O rather than to a lone pair on the oxygen.
Describing hydrogen bonds as covalent.
Claiming any pair of R groups can form an S-S bridge, when only cysteine has the -SH group.
Exam tips
- Use the data booklet to read the R groups before you decide what interaction is possible; the R group is what the mark depends on.
- A diagram showing at least -S-S- can earn the disulfide mark on its own.
Try the lesson
The start of the Proteins (A-level only): The importance of hydrogen bonding and sulfur–sulfur bonds in proteins. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of bond holds the α-helix and β-pleated sheet shapes of a protein in place?
Listen
Two very different attractions hold a protein in shape. A hydrogen bond forms between the δ⁺ hydrogen of an N-H and a lone pair on the δ⁻ oxygen of a C=O, or between R groups that carry -OH, -NH₂ or -COOH, as a serine R group does with an aspartic acid R group. Each one is weak, but a protein has thousands and together they hold the helix, the sheet and much of the fold. A disulfide bridge is different in kind: two cysteine R groups, each ending in -CH₂SH, are oxidised so their sulfur atoms bond directly, giving -CH₂-S-S-CH₂-. That is a covalent bond, so it is far stronger than a hydrogen bond and pins together parts of the chain that are far apart in the sequence. A strong answer earns marks for these points. naming cysteine as the amino acid whose R groups form the disulfide bridge. hydrogen bonding between R groups such as those of serine and aspartic acid. stating that the disulfide bridge is the stronger interaction. the reason, that a disulfide bridge is a covalent bond whereas a hydrogen bond is not. Exam tip: Use the data booklet to read the R groups before you decide what interaction is possible; the R group is what the mark depends on.
See it
Key words
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hydrogen bonding
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disulfide bridge
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cysteine R groups
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covalent versus intermolecular
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See every definition
- hydrogen bonding
- A bond forms between the δ⁺ hydrogen of an N-H and a lone pair on the δ⁻ oxygen of a C=O, or between R groups carrying -OH, -NH₂ or -COOH.
- disulfide bridge
- Two cysteine R groups, each ending in -CH₂SH, are oxidised so their sulfur atoms bond directly, giving -CH₂-S-S-CH₂-.
- cysteine R groups
- Each ends in -CH₂SH, and two of them are oxidised so their sulfur atoms bond directly.
- covalent versus intermolecular
- A disulfide bridge is a covalent bond, so it is far stronger than a hydrogen bond, which is not covalent.
Label the diagram
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1Box 1
2Box 2
3Box 3
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