Explain Free Wilson method
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Free and Wilson reasoned that the biological activity for a set of analogues could be described by the contributions that substituents or structural elements make to the activity of a parent structure.
Free-Wilson equations do not require the use of substituent constants such as , m, p, F, R, ES, and MR, and are represented by the general equation:
Free Wilson equation
where aj is the group contribution of the structural feature X in position j in molecule i and mu.gif is the theoretical biological activity value of a reference or parent compound. The descriptor Xij has a value of 1 if the feature is present in position j in molecule i and 0 in the absence of that feature. There are advantages and disadvantages of Free-Wilson type analyses that are discussed in Exploring QSAR.
The presence and absence of structural elements is indicated by the values 1 and 0, respectively. It is possible to add indicator variables to the Hansch equation as a combination of Free-Wilson analysis and Hansch analysis. The indicator variables can be used to describe a variety of structural features, such as substructures, chiral centers and special substituents.