The highest level of confidence can be placed in calculated log P values when (1) the log P of a parent solute is known, (2) pi constants for the required substituent(s) are available, and (3) the substituents either do not have an effect on groups already present in the parent or else this effect has been previously determined. In some instances there are no values available for any related structures which could serve as a parent; then, rather than substitute groups for hydrogen, it is easier to begin "from scratch", as suggested by Nys and Rekker, and assemble the structure from fragments, each of which has been assigned a hydrophobic value. In the present paper some new log P values for the lower alkanes and the inert gases are analyzed with the view of separating hydrophobic effects according to volume (including branching and flexibility) and polarity. Modified fragment values appear to enable reliable calculations to be made for a wider range of structures than was possible with the originally proposed constants.

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Correlation equations between logP (P = octanol water partition coefficient) and the biological activity of alcohols has been derived for 101 examples on all sorts of systems, from simple proteins to whole animals. This provides an overview of the toxic nature of hydrophobic compounds which can be used as a basis for comparison of more complex chemicals. About 100 examples of the hydrophobic effects of chemicals, other than alcohols, to various living systems or their parts are presented for comparison. It is clear that hydrophobic xenobiotics are toxic to almost every form of life, including humans (or parts there of).

A quantitative structure-activity relationship study on a set of anxiolytic benzodiazepines was carried out using the Hansch approach (QSAR) and the Comparative Molecular Field Analysis (CoMFA).The aim of the study was to investigate the nature of some of the main factors influencing the binding of the benzodiazepines to the CNS receptors. Since our preliminary QSAR results provided evidence of an involvement of a specific ligand-receptor hydrophobic interaction, the methodological problem of how to properly include the scalar descriptor x in the development of the CoMFA model was extensively examined. The following explanatory variables were considered in the regression analyses: the HOMO and LUMO energies, the total dipole moment, the substituent constants x, . ' 7 , BI , B5, L, MR, the steric and the electrostatic CoMFA fields. The QSAR and the CoMFA were performed with Multilinear Regression Analysis and Partial Least Squares (PLS) Analysis, respectively. In each of the two methods the cross-validation procedure was systematically applied to evaluate the significance of the correlation and to establish the most appropriate parametric dimensionality of the calibration models. A deeper insight into the meaning of the CoMFA electrostatic field as explanatory variable of the binding affinity was obtained by correlating the HOMO and LUMO energies with the electrostatic field itself. The results of this study suggest the existence of a specific interaction between the substituents which are in the "C7-position", formally corresponding to the 7-chlorine atom in the structure of diazepam, and a receptor hydrophobic pocket. These substituents can additionally promote, through an electron-withdrawing effect, a charge-transfer interaction between the ligand and an electronegative receptor site.

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