Isosteres

Isosteres are substances with similar molecular and electronic characteristics. They may not be structurally related, but they often have similar physical properties. They are chemical substances, atoms or substituents that possess nearly equal or similar molecular shape and volume, approximately the same distribution of electrons and exhibit similar chemical properties. Benzene and pyridine are classical examples of isosteres.

Can a carcinogenic molecule be made safe by knowing its isostere? Yes. Let’s look at an example.

Are all F isosteric substitutions safe?

Does it depend on the position?

Silicon as an isostere for Carbon

Si is the 2nd most abundant element on the planet. Naturally occurring Si-C bonds are unknown. Even if they were, such bonds would have immediately formed some other bond.

Silicon substitution in acetylcholine analogs

Acetylcholine is a neurotransmitter. A muscarinic receptor agonist is an agent that enhances the activity of muscarinic acetylcholine receptor.

Silane analogue of Polyethylene, Air stable

In the presence of water and/or soil, siloxanes hydrolyze to smaller oligomers and monomeric 1,1-dimethylsilanediol.

Mammals also rapidly oxidize silicon-hydrogen bond, whereas the carbons attached to silicon are metabolized much like simple hydrocarbons.

DDT, DDD & Silicon isosteres

DDT  = dichlorodiphenyltrichloroethane

DDD = dichlorodiphenyldichloroethane

Why is DDT good? Millions of lives were saved against mosquitoes with the help of it. Why is DDT bad? It turned out to be an endocrine disruptor, genetoxic, persistent and exteremely hydrophobic chemical.

Insecticide MTI-800

If you replace C with Si, no mortality of fish is observed at 50 mg/l. While with C in it, Fish LD50 is 3 mg/l.

Further reading

• Application of Bioisosteres in Drug Design
• Drug Design

This article was edited on December 19th, 2017 for accuracy.