Chemical structure and biological activity 



chemical structure of the substances we found to be active as growth pro- 

 moters and inhibitors. Although we know that the form of a molecule is only 

 one of the different properties which must be taken into consideration if we 

 want to get a complete knowledge of the relation between activity and 

 chemical structure of different substances, nevertheless, we think that this 

 point of view has so far been too much neglected. It will be seen that there are 

 some surprising similarities in the form of substances of similar physiological 

 action which are very different chemically. We cannot believe that the 

 factor 'form' is of no importance when a molecule enters a living system; 

 indeed, we have good reason to assume that the analogy of key and lock is 



Figure I. The concentration-action curves of 

 indole-3-acetic acid (lAA) and indole-i- 

 acetonitrile {IAN) in the paste test. 



log concn. 



very relevant. We consider that each molecule must be well matched with 

 its corresponding gap by its form, and by its electrical and chemical 

 configuration. 



In this communication, I will first present some recent results we obtained 

 by investigating the activity of different synthetic substances using our paste 

 method. Secondly, I will give a comparison of some values for the affinities 

 and other significant constants which we evaluated using Kaindl's formula 

 for the concentration-action curves of growth substances and growth 

 inhibitors (Kaindl, 1954). In addition, photographs are included showing 

 similarities of the molecular form and size of different growth substances and 

 growth inhibitors. 



The first important growth substance examined was indole-3-acetonitrile, 

 a compound first isolated from natural sources by Jones and his co-workers 

 (1952). It was tested in comparison with indole-3-acetic acid and showed a 

 similar concentration-action curve, though it was found to be about ten 

 times more active than indole-3-acetic acid {Figure 1). I have to thank 

 Prof. Jones of Manchester and Dr. Thoma of Linz for the two samples of 



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