Biological Activity of Compounds in Homologous Series

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 homologues 1 . 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 organism 2 . 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.