3.13.2Organic Chemistry
Proteins — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 47 of 55
Part 1 of 5Peptide Bonds
1
The –COOH of one amino acid reacts with the –NH₂ of another in a condensation reaction, losing a molecule of water.
2
The new link –CONH– is called a peptide bond, and it is an amide bond.
3
Two amino acids give a dipeptide, three give a tripeptide, and a long chain is a polypeptide.
4
A chain of n amino acids has n − 1 peptide bonds, and the chain still has a free –NH₂ at one end and –COOH at the other.
Part 2 of 5Protein Structure
Primary structure
- The sequence of amino acids in the chain, held together by covalent peptide bonds.
Secondary structure
- Hydrogen bonds between the C=O and N–H groups of the backbone fold the chain into an α-helix or a β-pleated sheet.
Tertiary structure
- The chain folds again into a 3D shape through interactions between R groups, as in globular proteins and enzymes.
Disulfide bridges
- Covalent S–S bonds form between the side chains of two cysteine residues.
Ionic interactions
- Attractions form between oppositely charged R groups, such as –NH₃⁺ and –COO⁻.
Other interactions
- Hydrogen bonds between R groups and weak hydrophobic interactions between non-polar side chains help to hold the shape.
| Bond type | Primary | Secondary | Tertiary |
|---|---|---|---|
| Peptide | yes | yes | yes |
| Hydrogen bonds | no | between backbone groups | between R groups too |
| Disulfide | no | no | yes |
| Ionic | no | no | yes |
Part 3 of 5Hydrolysis of Proteins
Hydrolysis of Proteins
Reverse of condensation
- Adding water across each peptide bond splits a protein back into its amino acids.
Chemical conditions
- Heat with 6 mol dm⁻³ hydrochloric acid under reflux for many hours;
- the amino acids are then present as their salts.
Enzyme conditions
- Protease enzymes cut peptide bonds at room temperature and body temperature, which is how digestion works.
Analysis
- The mixture of amino acids is separated by TLC and identified using ninhydrin and Rf values.
Part 4 of 5Worked Example
Question Calculate the relative molecular mass of the dipeptide formed from glycine (Mr = 75) and alanine (Mr = 89).
- The dipeptide is formed by a condensation reaction that removes one molecule of water.
- Mr of the dipeptide = 75 + 89 − 18 = 146.
- Checkthe formula C₅H₁₀N₂O₃ gives 5 × 12 + 10 + 2 × 14 + 3 × 16 = 146.
Answer Mr = 146.
Part 5 of 5Exam Checklist & Tips
3.13.2 – Key Points for the Exam
- Peptide bond The amide link –CONH– between two amino acids.
- Polypeptide A long chain of amino acids joined by peptide bonds.
- Primary structure The sequence of amino acids in a protein.
- α-helix A coiled secondary structure held by hydrogen bonds in the backbone.
- Disulfide bridge A covalent S–S link between two cysteine side chains.
Common Mistakes
- Saying hydrogen bonds hold the primary structure; covalent peptide bonds do.
- Forgetting to subtract water when working out the Mr of a peptide.
- Confusing hydrolysis with condensation; hydrolysis uses water to break the bond.
- Stating that secondary structure involves the R groups, when it involves the backbone.
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