Chemistry Labs

Biochemistry and biomedical chemistry

Structure–activity relationships (SAR)

A structure–activity relationship (SAR) compares related molecules to learn how chemical changes alter a measured biological response.

1 Lower secondary2 Upper secondary3 Undergraduate4 Advanced5 Research

IntuitionIntuition: the central idea

A structure–activity relationship (SAR) compares related molecules to learn how chemical changes alter a measured biological response.

Explore the schematic model; it illustrates a concept rather than replacing experimental evidence.

SchoolSchool level: key concepts and a first application

Definition: Core concept

Matched molecular pairs, substituent constants and quantitative models can reveal trends, but assay protocol, solubility and exposure must be controlled.

SAR emerges when related compounds differ at defined positions. Hydrophobic, steric and electronic descriptors capture properties relevant to binding, permeability and metabolism.

Key terms and meaning
TermInterpretation
pIC₅₀Negative logarithm of half-maximal inhibitory concentration.
logPOctanol–water partition coefficient on a logarithmic scale.
Substituent constantA descriptor such as σ or π used to compare substituent effects.

Example: Apply the idea

A compound has IC50=1IC_{50}=1 μM. What is its pIC₅₀?

Solution

Convert 11 μM to 10−610^{-6} M; −log⁡10(10−6)=6-\log_{10}(10^{-6})=6. This is an assay-dependent potency measure, not automatically a binding affinity.

UndergraduateUniversity level: quantitative description

pIC50=−log⁡10(IC50[M])\mathrm{pIC}_{50}=-\log_{10}(IC_{50}[\mathrm{M}])

Hansch analysis separates electronic, hydrophobic and steric contributions. Interpret coefficients cautiously: they describe the analogue set and assay that produced them.

References