Biological Activity of Compounds in Homologous Series

Abstract

WHEN a biological action may be attributed to a physical mechanism, the equi-effective (equi-toxic, equi-narcotic, etc.) concentrations of compounds in homologous series decrease very rapidly as the number of carbon atoms increases: the molar concentration required to produce a given effect is approximately one third that of the preceding member; that is, the logarithm of the equi-effective concentration is a linear function of the number of carbon atoms. This generalization holds fairly well over a wide range of biological actions and homologous series, and it has been used to predict the activity of higher members of a series from results obtained with the lower homologues1. The decrease in equi-effective concentration does not however, proceed indefinitely. As the homologous series is ascended, a member is reached which has the maximum activity, and the higher members are either entirely inactive or have very greatly reduced activity. The position of this ‘cut-off’ depends on the homologous series, on the nature of the biological action being investigated, and even on the relative resistance of different strains of the same organism2. It is the purpose of this communication to suggest that the position of this ‘cut-off’ can be approximately predicted from the results obtained with lower homologues.