How do amino acids behave as both acids and bases, and how do they join to form proteins?
The structure of amino acids, their amphoteric behaviour and zwitterions, the effect of pH and the isoelectric point, peptide bond formation and hydrolysis, and protein structure.
An Eduqas A-Level Chemistry OA3.2 answer on amino acid structure, amphoteric behaviour and zwitterions, the isoelectric point, peptide bond formation and hydrolysis, and protein structure.
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What this topic is asking
Eduqas topic OA3.2 covers amino acids and the proteins built from them: the dual acid-base structure of amino acids, their amphoteric behaviour and zwitterions, the effect of pH and the isoelectric point, the formation and hydrolysis of the peptide bond, and the levels of protein structure. It links the amine and carboxylic-acid chemistry of earlier topics into a biologically important whole.
The structure of amino acids
Zwitterions and the isoelectric point
The effect of pH
The peptide bond
Two amino acids join when the of one condenses with the of another, forming a peptide (amide) bond () and releasing water. Repeating this builds a polypeptide. The reverse, hydrolysis, is achieved by refluxing with hot concentrated acid or alkali, breaking the chain back into its amino acids (used to analyse a protein's composition).
Protein structure
The primary structure is the sequence of amino acids; the secondary structure (such as the alpha helix and beta pleated sheet) arises from hydrogen bonding along the backbone; and the tertiary structure is the overall 3D fold, held by hydrogen bonds, ionic interactions and other side-chain interactions. The shape determines the protein's function.
Examples in context
Example 1. Electrophoresis. Because an amino acid's charge depends on pH, a mixture can be separated by electrophoresis: at a chosen pH each amino acid carries a different net charge and migrates toward a different electrode, a direct use of the isoelectric point.
Example 2. Digestion of proteins. Enzymes in the stomach and gut hydrolyse the peptide bonds of dietary protein into individual amino acids, the biological equivalent of the acid or alkaline hydrolysis carried out in the laboratory.
Try this
Q1. Draw the zwitterion of the amino acid glycine, . [1 mark]
- Cue. (the amine is protonated and the carboxylic acid deprotonated).
Q2. Name the bond formed when two amino acids react, and the type of reaction. [2 marks]
- Cue. A peptide (amide) bond, formed by a condensation reaction (with loss of water).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksThe amino acid alanine, , is amphoteric. (a) Explain what is meant by amphoteric and draw the zwitterion of alanine. (b) Draw the species present in strongly acidic and strongly alkaline solution.Show worked answer →
(a) Amphoteric means it can react as both an acid and a base; alanine has a basic group and an acidic group (1). The zwitterion is (1).
(b) In strongly acidic solution the is protonated, giving (a positive ion) (1). In strongly alkaline solution the loses its proton, giving (a negative ion) (1).
Eduqas 20214 marksTwo amino acids can join to form a dipeptide. (a) Name the bond formed and the type of reaction. (b) Describe how a dipeptide can be hydrolysed back to the amino acids, stating the conditions.Show worked answer →
(a) A peptide (amide) bond is formed by a condensation reaction, with the loss of a water molecule (2).
(b) The dipeptide is hydrolysed by refluxing with a hot acid (such as hydrochloric acid) or hot alkali (2); this breaks the peptide bond, regenerating the constituent amino acids.
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)