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1 of 8 © Boardworks Ltd 2010
2 of 8 © Boardworks Ltd 2010
Links to the current specifications
AQA 3.4.8 Amino acids: acid and base properties; proteins.
Edexcel Topic 5.4 – Organic chemistry5.4.2a 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.
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.
Amino acids contain both amine (NH2) and carboxyl (COOH) functional groups.
In alpha amino acids, these groupsare attached to the same carbon atom.
glycine (gly)
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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:
When acting as a base, the NH2 group gains a H+ ion:
H2NCHRCOOH + OH- H2NCHRCOO- + H2O
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.