Amino acids

Chemistry A2 · Nitrogen compounds. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An amino acid, H₂NCHRCOOH, has a basic –NH₂ group and an acidic –COOH group in the same molecule. The –COOH group gives its H⁺ to the –NH₂ group, forming a zwitterion, ⁺H₃NCHRCOO⁻, which has both charges but no overall charge.

In acid the zwitterion gains H⁺ and becomes a positive ion. In alkali it loses H⁺ and becomes a negative ion. The isoelectric point is the pH at which the amino acid has no overall charge.

Two amino acids join when –COOH reacts with –NH₂, losing water and forming a peptide bond, –CO–NH–. In electrophoresis, ions move in an electric field towards the electrode of opposite charge.

Rules to remember

Zwitterion: ⁺H₃NCHRCOO⁻; ionic attractions between zwitterions give solid amino acids high melting points
pH below the isoelectric point: ⁺H₃NCHRCOOH, positive, moves to the negative electrode
pH above the isoelectric point: H₂NCHRCOO⁻, negative, moves to the positive electrode
pH equal to the isoelectric point: no overall charge, stays near the start
Smaller ions and ions with a larger charge move further in the same time
M_r of a peptide = sum of the M_r of the amino acids − 18 for each peptide bond
Two different amino acids A and B can give four dipeptides: A–A, B–B, A–B and B–A

Common mistake

Students send the ions to the wrong electrode. In a solution more acidic than the isoelectric point the amino acid is protonated, so it is positive and moves to the negative electrode.

Worked example

A mixture of alanine (isoelectric point 6.0) and aspartic acid (isoelectric point 2.8) is placed at the centre of an electrophoresis plate in a buffer of pH 4.5. Which way does each amino acid move?

Alanine: pH 4.5 is below 6.0, so alanine has gained H⁺ and is a positive ion.
Aspartic acid: pH 4.5 is above 2.8, so aspartic acid has lost H⁺ and is a negative ion.
Each ion moves towards the electrode of opposite charge.

Answer: Alanine moves to the negative electrode; aspartic acid moves to the positive electrode

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Watch: 17 | Esters | Amides | Polymers | Peptides

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