Summarised information from topic 3.3.13 for AQA A-Level Chemistry, including key terms and diagrams to help you understand the subject in a concise and accurate way.
3.3.13: Amino Acids, Proteins and DNA
Amino Acids
Amino acids are amphoteric compounds with the general formula RCH(NH2)COOH. They contain both a
basic amine (-NH2) group and an acidic carboxylic acid group (-COOH). The amine group is bonded to the
carbon adjacent to the acid group so these compounds are known as 2-amino acids or alpha-amino acids.
The -R group can be replaced by any organic side-
chain, which is the unique variable that determines the
properties of each amino acid. There are about 20
naturally-occurring amino acids.
The carbon atom joining the four groups is chiral, so
amino acids are optically active and will form
stereoisomers. Often, natural processes will only work
with one of the enantiomers.
The amino acids are crystalline solids with very high melting points because of their ability to form internal
ionic bonds. They are soluble in water because of the ionic attractions between water molecules and the
zwitterions.
These compounds are amphoteric because they have acidic and basic groups in the same molecule. The
acid group COOH can donate a proton, while the basic group NH2 can accept a proton.
Amino acids commonly exist in zwitterion form. These are formed by an internal acid-base reaction in which
the acid group donates a proton to the basic group, giving the molecule ionic charges. Overall, the charge
on each molecule remains neutral.
If you add acid to the solution, pH is lowered. At a low
pH (when H+ is not in excess) the carboxylic acid group
picks up a proton as well as the amine group.
If you add alkali to the solution, pH is increased. At a
high pH (when H+ is not in excess) the carboxylic acid
does lose a proton but a proton is removed from the
amine group.
At an intermediate pH (which is not pH 7), the main species present is the zwitterion. This is known as the
isoelectric point.
Proteins
Amino acids undergo condensation polymerisation reactions to
form proteins. The carboxylic acid and amine groups on separate
amino acids react to form a peptide bond, losing a water molecule
in the process. Two amino acid groups will form a dipeptide, three
form a tripeptide, and so on.
The structure of amino acids is categorised into three groups:
• Primary Structure – how sequence of amino acids that make up the protein polymer. Held together
by strong covalent bonds and peptide links between amino acids. This is represented by a series of
3-letter acronyms describing the order of the amino acids.
• Secondary Structure – how different polypeptide chains are arranged. Causes hydrogen bonding
interactions, leading to a more specific secondary structure.
o α-Helix is a coiled arrangement held together by hydrogen bonds. The chains twist into a
coiled helix shape.
o Β-Pleated Sheets cause amino acid sequences to run in parallel sheets, pulled together into
a pleated configuration.
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