Detailed and comprehensive notes on amino acids and proteins for AQA chemistry A level. Covers amino acid structure/properties, zwitterions, protein structure and enzymes.
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).
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller JunaidAli025. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £2.99. You're not tied to anything after your purchase.
