Classical Isosteres are molecules or ions with the same number of atoms and/or the same number of valence electrons. The definition was later revised to include compounds with similarly reactive electron shells. For example, Hydrogen ion and Fluoride are classical isosteres because they both have relatively small nuclei and their outer valence shells are full (or empty in Hydrogen's case) when ionized (0 in Hydrogen, 1s2 + 2p6 = p8 in Fluoride).
Non-Classical Isosteres do not obey the above classifications, but they still produce similar biological effects in vivo. Non-classical isosteres may be made up of similar atoms, but their structures do not follow an easily definable set of rules.
Some examples of isosteres include:
- Sodium and hydrogen cations
- Carbon dioxide (CO2) and nitrous oxide (N2O)
- Silicon and carbon
- methyl, amide, and hydroxyl groups
The isostere concept was formulated by Irving Langmuir in 1919, and later modified by Grimm. Friedrich Erlenmeyer extended the concept to biological systems in 1932. Classical isosteres are defined as being atoms, ions and molecules that had identical outer shells of electrons, This definition has now been broadened to include groups that produce compounds that can sometimes have similar biological activities. Some evidence for the validity of this notion was the observation that some pairs, such as benzene and thiophene, thiophene and furan, and even benzene and pyridine, exhibited similarities in many physical and chemical properties.
A biologically-active compound containing an isostere is called a bioisostere. This is frequently used in drug design: the bioisostere will still be recognized and accepted by the body, but its functions there will be altered as compared to the parent molecule.
- Richard Silverman, The Organic Chemistry of Drug Design and Drug Action, Second Edition, 2004
- Irving Langmuir. Isomorphism, isosterism and covalence. J. Am. Chem. Soc. 1919, 41, 1543-1559. doi:10.1021/ja02231a009
- Mukesh Doble, Anil Kumar Kruthiventi, Vilas Gajanan. Biotransformations and Bioprocesses. CRC Press, 2004, p. 60. ISBN 0-8247-4775-5
- Nathan Brown. Bioisosteres in Medicinal Chemistry. Wiley-VCH, 2012, p. 237. ISBN 978-3-527-33015-7