3.3.13 Amino acids, proteins and DNA (A-level only)
Amino acids, proteins and DNA are the molecules of life. In this section, the structure and bonding in
these molecules and the way they interact is studied. Drug action is also considered.
3.3.13.1 Amino acids
Amino acids have both acidic and basic properties,
including the formation of zwitterions.
Students should be able to draw the structures of amino
acids as zwitterions and the ions formed from amino acids:
•• in acid solution
•• in alkaline solution.
3.3.13.2 Proteins
Proteins are sequences of amino acids joined by peptide
links.
The importance of hydrogen bonding and sulfur-sulfur
bonds in proteins.
The primary, secondary (α-helix and β-pleated sheets) and
tertiary structure of proteins.
Hydrolysis of the peptide link produces the constituent
amino acids.
Amino acids can be separated and identified by thin-layer
chromatography.
Amino acids can be located on a chromatogram using
developing agents such as ninhydrin or ultraviolet light and
identified by their Rf values.
Students should be able to:
•• draw the structure of a peptide formed from up to three
amino acids
•• draw the structure of the amino acids formed by
hydrolysis of a peptide
•• identify primary, secondary and tertiary structures in
diagrams
•• explain how these structures are maintained by hydrogen
bonding and S-S bonds
•• calculate Rf values from a chromatogram.
3.3.13.3 Enzymes
Enzymes are proteins.
The action of enzymes as catalysts, including the concept
of a stereospecific active site that binds to a substrate
molecule.
The principle of a drug acting as an enzyme inhibitor by
blocking the active site.
Computers can be used to help design such drugs.
Students should be able to explain why a stereospecific
active site can only bond to one enantiomeric form of a
substrate or drug.
3.3.13.4 DNA
The structures of the phosphate ion, 2-deoxyribose (a
pentose sugar) and the four bases adenine, cytosine,
guanine and thymine are given in the Chemistry Data
Booklet.
A nucleotide is made up from a phosphate ion bonded to
2-deoxyribose which is in turn bonded to one of the four
bases adenine, cytosine, guanine and thymine.
A single strand of DNA (deoxyribonucleic acid) is a polymer
of nucleotides linked by covalent bonds between the
phosphate group of one nucleotide and the 2-deoxyribose
of another nucleotide. This results in a sugar-phosphatesugar-
phosphate polymer chain with bases attached to the
sugars in the chain.
DNA exists as two complementary strands arranged in the
form of a double helix.
Students should be able to explain how hydrogen bonding
between base pairs leads to the two complementary
strands of DNA.
3.3.13.5 Action of anticancer drugs
The Pt(II) complex cisplatin is used as an anticancer drug.
Cisplatin prevents DNA replication in cancer cells by a
ligand replacement reaction with DNA in which a bond is
formed between platinum and a nitrogen atom on guanine.
Appreciate that society needs to assess the balance
between the benefits and the adverse effects of drugs, such
as the anticancer drug cisplatin.
Students should be able to:
•• explain why cisplatin prevents DNA replication
•• explain why such drugs can have adverse effects.