Summary Organic Chemistry A Level - Amino acids and Proteins

Amino Acids
Structure
• An amino acid has an amine (-NH2) group, a carboxyl (-COOH)
group and an organic side chain (-R).

• In glycine, the R-group is hydrogen.
• Cysteine has a thiol (-SH) group in its side chain.

• Almost all natural amino acids are α-amino acids (or 2-amino
acids), where the –NH2 is attached to the α-carbon.

• There are 20 naturally occurring amino acids that differ only by their side chains.
• Some amino acids have extra amine and carboxylic acid groups in their side chain.

Zwitterions
• A zwitterion has both a positive charge (-NH3+) and negative charge (-COO-).

• Each amino acid has a unique isoelectric point – the pH at which the zwitterion exists.
• It varies between amino acids due to the effect of different R-groups.




• If an extra carboxylic acid or amine group is present in the side chain, then this group will also react
and change form in alkaline or acidic conditions.


Ionic Properties
• Amino acids form crystalline solids with high melting points that dissolve readily in water – they are
significantly polar in nature.

• In both solid and solution state, amino acids exist as the zwitterion form, where the H atom of the
COOH group protonates the basic amine group.

• There are strong ionic interactions between zwitterions (as opposed to the weaker hydrogen bonding
that would occur in the no charge form) which cause the high melting points.

, Amino Acids
Acid & Base Properties
• The amine group is basic and the carboxylic acid group is acidic – amino acids are amphoteric.
• Amphoteric: Able to act as both a base and an acid.
• However, amino acids are both weak acids and bases.

• Amino acids act as weak buffers
• They will only gradually change pH if small amounts of acid and alkali are added.
o E.g. +NH3CH2COO- + HCl ! Cl- +NH3CH2COOH
o E.g. +NH3CH2COO- + NaOH ! NH2CH2COO- Na+ + H2O

Optical Activity
• All amino acids, except glycine, have 4 different groups around
the α-carbon.
• This makes all other amino acids chiral, so they exhibit optical
activity and will rotate plane-polarised light.
• Only the L-isomers are found naturally.

Solubility
• Amino acids are generally soluble in water, as the charged regions of the zwitterion are attracted to
the δ+ hydrogen atoms or δ- oxygen atom of water.

Nomenclature
• The carbon atom in the carboxylic acid group will always be carbon-1.
• Carboxylic acids take priority when naming, so the suffix should be –
oic acid.
• The amine group will typically be on carbon-2; use the prefix amino-.

• For molecules with 2 carboxylic acid groups, the carbon chain in between the 2 carboxyl groups is
used as the longest chain and the suffix is –dioic acid.
• (Note: there should be an ‘e’ before the dioic acid, e.g. butanedioic acid and not butandioic acid)

Reactions
• The carboxylic acid and amine groups in amino acids can undergo the usual reactions of these
functional groups:
o Condensation reactions with other amino acids to form dipeptides and
polypeptides/polyamides (COOH and NH2 groups).
o Esterification reactions with alcohols in the presence of a strong acid catalyst (COOH
group).
o Nucleophilic addition-elimination reactions with acyl chlorides (NH2 group).
o Nucleophilic substitution reactions with haloalkanes (NH2 group).
o Acid / Base reactions (COOH or NH2 group).