Free Wilson-type indicator variable

For DHFR inhibition by a large group of 2,4-diaminopyrimidines, Hansch derived Eq. (67), which includes six Free Wilson-type indicator variables and one interaction term [106] ... 

In the topological pharmacophore methods [527 — 531], e.g. LOGON [527, 528, 530], LOGANA [528 — 530], and EVAL [531], Free Wilson-type indicator variables... 

The use of Free Wilson-type indicator variables in Hansch analysis has been discussed in chapters 3.8, 4.3, and 7 [21, 390, 391, 393]. Nonadditivities in Free Wilson analyses due to nonlinear lipophilicity-activity relationships have been discussed in chapter 4.3 [22, 390—392, 394]. 

Indicator (Free Wilson-type, dummy) variables. 

Like the continuous physico-chemical descriptor Z variables, indicators of the presence or absence of certain substructures have also been treated by multiple regression analysis. As modified by Fujita and Ban (Seydel and Schaper, 1979), this group contribution method can be a useful alternative to the LFER approach, if only limited knowledge is available about the relevant molecular properties or no uniform physico-chemical descriptors for the various compounds in the data set are accessible. For activities and properties of compounds that may be attributed to the occurrence of certain substructures in the molecules (e.g. biodegradation section 4.8), Free-Wilson-type substructure models have their major application in environmental sciences. 

Indicator variables which encode certain structural features that cannot otherwise be described are very important parameters. From a theoretical point of view, they are Free-Wilson-type parameters. Their additional consideration in Hansch equations broadens the scope and applicability of this approach (cf. equation 5, Section 2.2). " ... 

The epoch of QSAR (Quantitative Structure-Activity Relationships) studies began in 1963-1964 with two seminal approaches the a-p-7i analysis of Hansch and Fujita " and the Free-Wilson method. The former approach involves three types of descriptors related to electronic, steric and hydrophobic characteristics of substituents, whereas the latter considers the substituents themselves as descriptors. Both approaches are confined to strictly congeneric series of compounds. The Free Wilson method additionally requires all types of substituents to be suflficiently present in the training set. A combination of these two approaches has led to QSAR models involving indicator variables, which indicate the presence of some structural fragments in molecules. 

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