Descriptors to Characterize Properties of Substituents and Spacers

In order to calculate the similarity between fragments (substituents, spacers, or rings) that one wants to replace in the process of bioisosteric design, it is necessary to quantify somehow their properties and express them as a set of numerical values -descriptors. In the classical years of quantitative structure-activity relationship (QSAR), the properties of substituents were mostly characterized by experimentally derived parameters. Hammett sigma constants a (and several variations of this parameter) played a prominent role in characterizing the electron-donating or electron-accepting power of substituents 7], and the Hansch n parameter, defined 

1) PubChem, http //pubcbeni.ncbi.iilm.iiih.gov/. 

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