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Links to the current specifications
AQA
3.4.8 Amino acids: acid and base properties; proteins.
Edexcel
Topic 5.4 – Organic chemistry
5.4.2a Give examples of:
i. molecules that contain amine and amide functional groups.
ii. amino acids.
5.4.2h Comment on the physical properties of polyamides in terms
of hydrogen bonding.
5.4.2i Describe and carry out, where appropriate, experiments to
investigate the characteristic behaviour of amino acids. This is
limited to:
i. acidity and basicity and the formation of zwitterions.
iv. formation of peptide groups in proteins by condensation
polymerization.
OCR
4.2.1 Amino acids and chirality: amino acids; peptide formation
and hydrolysis of proteins.
4.2.2 Polyesters and polyamides: condensation polymers;
hydrolysis and degradable polymers.
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Proteins and amino acids
Proteins are a diverse group of large and complex polymer
molecules, made up of long chains of amino acids.
Amino acids contain both amine
(NH2) and carboxyl (COOH)
functional groups.
In alpha amino acids, these groups
are attached to the same carbon atom.
glycine (gly)
The R group, also attached to the same carbon atom, can vary.
There are 22 amino acids that are used to make proteins
(proteinogenic). The simplest is glycine, where R = H.
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Zwitterions
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Acid–base properties of amino acids
The presence of a carboxyl group and an amine group mean
that amino acids have both acidic and basic properties.
NH2 group
acts as a base
COOH acid group
acts as an acid
When acting as an acid, the COOH group loses a H+ ion:
H2NCHRCOOH + OH-  H2NCHRCOO- + H2O
When acting as a base, the NH2 group gains a H+ ion:
H2NCHRCOOH + H+  H3N+CHRCOOH
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Effect of pH on amino acids
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Peptide formation
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Hydrolysis of peptide bonds
A peptide bond can be split by refluxing with hydrochloric acid.
During hydrolysis, the water molecule adds across the peptide
bond, forming a mixture of the two amino acids.
Peptide links can also be broken using a solution of alkali,
such as aqueous sodium hydroxide at above 100°C.
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