Quantitative structure-activity relationship Taft steric parameter

The fundamental assumption of SAR and QSAR (Structure-Activity Relationships and Quantitative Structure-Activity Relationships) is that the activity of a compound is related to its structural and/or physicochemical properties. In a classic article Corwin Hansch formulated Eq. (15) as a linear frcc-cncrgy related model for the biological activity (e.g.. toxicity) of a group of congeneric chemicals [37, in which the inverse of C, the concentration effect of the toxicant, is related to a hy-drophobidty term, FI, an electronic term, a (the Hammett substituent constant). Stcric terms can be added to this equation (typically Taft s steric parameter, E,). 

BR is the relative biologic activity under investigation, for example the molar EDr>o or the percent response at a given dose. Log P is the logarithm of the octanol-water partition coefficient, a is the approprite Hammett signa constant (electronic in nature), and Es is the Taft steric parameter. With the important demonstration by Hansch that the partition coefficient is often an additive constitutive property, all of the physical parameers may be obtained from the literature. A second important contribution by Hansch is the recognition that the use of statistical techniques is essential to the analysis of quantitative structure-activity relationships. To develop an equation such as that above, one feeds the... 

Integrating various factors, namely Taft s steric factor, resonance, inductive, Verloop steric parameters with the partition behaviour of drug molecules Hansch and Fuj ita exploited these principles in determining the establishing quantitative structure-activity relationship (QSAR) of drugs, which has rmdergone a sea change both in expansion and improvement with the help of computer researched softwares. 

The alternatives to mathematical descriptors derived from molecular graphs or molecular geometry are the traditional QSAR (quantitative structure-activity relationship) descriptors and quantum chemically computed parameters. The former include the partition coefficient for oil/water (often octanol/water) (log P), the Hammet sigma value (electronic parameter that measures the electron withdrawal from and the electron release to the aromatic ring by a substituent, the Taft s parameters for the electronic effects of substituents in aliphatic compounds (a ), and a steric parameter for the proximity of substituents on reaction sites (Es)- Also selected molecular properties, such as molar refractivity (MR), polarizability (a), molecular weight (MW), and density (d), have been used. 

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