Classification of Bioisosterism

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Chapter: Medicinal Chemistry : Bioisosterism

Bioisosteres are classified into the following two types: 1. Classical bioisosteres 2. Nonclassical bioisosteres


CLASSIFICATION

Bioisosteres are classified into the following two types:

  1. Classical bioisosteres

  2. Nonclassical bioisosteres


1. Classical bioisosteres: 

Classical bioisosteres have similarities in shape and electronic configuration of atoms, groups, and molecules, which they replace. Actual applications of bioisosteres in the successful design of a specific given molecule interacting with particular receptor is one glaring example, and very often either fails or negates the biological characteristics in another environment. Therefore, it is pertinent to state at this juncture that the logical use of biological replacement (classical or nonclassical) in the design of a new target drug molecules is solely and significantly depend on the specific biological system under critical investigation. Hence, there are no predetermined, well-established, predictable hard and fast guidelines, or laid generalized rules that may be useful to a medicinal chemist to affect biosteric replacement gainfully towards improved biological activity. Various classical bioisosteres with their appropriate examples are listed as follows:

i. Monovalent atoms and groups

F, H 

OH, NH

F, OH, NH, or CH3 for H 

SH, OH

Cl, Br, and CF3


ii. Divalent atoms and groups

–C=S,–C=O, –C=NH, –C=C–


iii. Trivalent atoms and groups

–CH =, –N =

–P =, –As =

iv. Tetravalent atoms and groups

=N+=, =C=, =P+=, =As+=

v. Ring equivalents



2. Nonclassical bioisosteres: 

They do not obey the steric and electronic definition of classical isosteres. Also, they do not have the same number of atoms as replacement. Although, many of these functional moieties practically just behave as one, they have one of the following characteristic features, such as

  • electronic properties

  • physicochemical properties

  • spatial arrangements

  • functional moiety critical for biological activity

1. Exchangeable groups


2. Cyclic versus noncyclic structure



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