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Amino Acids

Lajoy Tucker

Teacher

Lajoy Tucker

Contents

Introduction to Amino Acids

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Amino Acids Explainer Video

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Zwitterions

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Behaviour in Acidic and Basic Conditions

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Optical Isomerism in Amino Acids

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Peptide Formation

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Practice Questions

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Introduction to Amino Acids

Amino acids are organic compounds containing:

  • An amine group  

  • A carboxylic acid group  

  • A variable R group (side chain)

General structure:

A general structure of an amino acid is shown, highlighting the amino group (–NH₂), carboxyl group (–COOH), hydrogen, and variable R side chain attached to the central carbon.

Amino Acids Explainer Video

Zwitterions

Due to the presence of both acidic and basic groups, amino acids exist as zwitterions – molecules with both a positive and negative charge

These charges cancel out to give an overall neutral molecule.

Zwitterion structure:

Zwitterion structure

  •  accepts a proton →

  •  donates a proton → 

Note – zwitterions must have no overall charge.

A diagram shows a negatively charged amino acid (aspartate) zwitterion with two carboxylate groups, illustrating overall charge and ionic form.

Behaviour in Acidic and Basic Conditions

The pH will determine the structure of an amino acid.

A diagram shows how an amino acid changes with pH, existing as a negatively charged ion in alkaline conditions, a zwitterion at neutral pH, and a positively charged ion in acidic conditions.

Condition

Behaviour

Ion Form

Acidic (pH  )

Gains 

Basic (pH  )

Loses  

Optical Isomerism in Amino Acids

Many amino acids are chiral, meaning the carbon bonded to ,  ,  , and   has four different groups.

  • Results in non-superimposable mirror images (enantiomers)

A diagram shows a tetrahedral carbon centre with four different groups forming non-superimposable mirror images, illustrating optical isomerism (enantiomers).

Most amino acids exist naturally as the L (left-handed) enantiomer.

Peptide Formation

A peptide bond forms via a condensation reaction between:

  • The   group of one amino acid

  • The   group of another

Forms a dipeptide + water:

A diagram shows two amino acids joining via a condensation reaction, forming a peptide bond and releasing a molecule of water.

Peptides are covered in the Proteins and Enzymes note

Practice Questions

Question 1

Give the structural formula of the zwitterion formed by alanine.

Answer

Question 2

Explain why glycine does not show optical isomerism, but most other amino acids do.

Answer

  • Glycine has two hydrogen atoms attached to the central carbon (1 mark)

  • So it is not chiral (1 mark)

Question 3

Give the structure of the main species present in an aqueous solution of lysine at pH 13?

Answer

Alkaline solution therefore -COOH group deprotonated

A structural formula of an amino acid (lysine) shows the α-amino and carboxylate groups with a long side chain ending in an additional –NH₂ group.

Question 4

Draw the two possible dipeptides formed from serine and cysteine

Answer

A diagram compares two dipeptides (Ser–Cys and Cys–Ser), showing how changing the order of amino acids results in different structures due to different side chains.

Practice Question Video

Proteins and Enzymes