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Summary Unit 3.3.13 - Amino acids, proteins and DNA (A-level only) NOTES
Notes for AQA A-level Chemistry new spec
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Amino acids, Proteins and DNAStructure of an amino acid:
Amino acids are amphoteric (they have both acidic and basic properties).
They can act as acids because the carboxyl group is acidic (proton donor).
They can act as bases because the amino group is basic (proton acceptor).
Amino acids are chiral molecules (a carbon with 4 different groups attached to it/
asymmetrical). So a solution of a single amino acid ENANTIOMER will rotate polarised light
How to name an amino acid:
1. Find the longest carbon chain that includes the carboxyl group eg.butanoic acid.
2. Number the carbons in the longest chain. The carboxyl carbon is #1
3. The NH2 groups are called amino eg.2-amino.
4. Write down any other functional groups too eg.3-methyl.
, Zwitterions
Amino acids can exist as zwitterions (a dipolar ion). It has both a positive and negative
charge in different parts of the molecule.
Zwitterions only exist in an amino acid’s isoelectric point.
The isoelectric point is the pH where the overall charge on the amino acid is zero (because
the positive and negative charge cancel out). This depends on the R-group.
An amino acid becomes a zwitterion when the amino group is protonated ( ) and its
COOH group is deprotonated ( ).
In conditions more acidic than the isoelectric point, the NH2 group is likely to be protonated
but the COOH group will be unchanged. This means the amino acid will have a positive
charge but no negative charge.
In conditions more basic than the isoelectric point, the COOH group is likely to get
deprotonated but the NH2 group will be unchanged. This means the amino acid will have a
negative charge but not a positive charge.
Only at or near the isoelectric point, the carboxyl and amino group will be ionised (NH2 gets
protonated and COOH gets deprotonated), forming a zwitterion.
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