Amino Acids (A2 Only)
Chemistry: AQA A Level, topic 6.7.1
Amino Acids
There are about 20 naturally-occurring amino acids in animal cells.
Amino acids
- Amino acids have the general formula RCH(NH2)COOH.
- Amino acids contain an acidic group (carboxylic acid) and a basic group (amine) in the same molecule.
- The -NH2 group is on the carbon next to the -COOH group and so they are known as α-amino acids.
Acid and base behaviour
- The -COOH group is acidic and can react with bases by donating a proton (H+).
- The -NH2 group is basic and can react with acids by accepting a proton (H+).
- Amino acids are amphoteric because they contain both acidic and basic groups in the same molecule.
Side chain
- An amino acid contains an -R group which is a side chain.
- This is different for different amino acids.
- E.g. Alanine has an -R group of -CH3.
The Zwitterion Form
An amino acid can exist in the zwitterion form.
Zwitterion form
- Molecules or ions having separate positively and negatively charged groups are known as zwitterions.
- An internal acid-base reaction in an amino acid causes the nitrogen of the amine group to accept a proton from the carboxylic acid group.
Zwitterion form
- The zwitterion makes the amino acid have ionic charges.
- A positive charge is on the protonated nitrogen.
- A negative charge is on the deprotonated oxygen.
- The ionic nature of amino acids gives them higher melting points than expected.
pH on the zwitterion
- At low pH, the carboxylic acid will not be deprotonated (no charge) and the nitrogen will have accepted a proton (positive charge).
- At high pH, the amine group will not be protonated (no charge) and the carboxylic acid group will have been deprotonated (negative charge).
- At an intermediate pH, which depends on the amino acid, the main species present is the zwitterion.
- This intermediate pH is known as the isoelectric point.
Test yourself on Amino Acids (A2 Only)
All amino acids have the same two functional groups. So what is different about the different types of α-amino acids?
Zwitterion form is changed by which of the following?