Chemical structure and biological activity 



substituted fluoresceins, we see that their action as growth inhibitors seems 

 to depend on a specific M'-form of the molecule [Figure 11). This form is due 

 to the substituent halogens and it is immaterial which of the different 

 halogens is substituted. It is not true, however, that all strong inhibitors are 



^b 



Rk 



^lAi 



Figure 13. The dimensions of the molecule 

 in projection on the ring-plane in the standard 

 position of the side-chain. Mi = length of the 

 molecule; M,, = breadth of the molecule; 

 Rj = length of the ring; R^ = breadth of 

 the ring; S^ = distance of the side-chain from 

 the opposite side of the ring. 



Figure 12. Molecular models of 2:3: 5-tri- 

 iodobenzoic acid (top). 4:6-dinitroorthocresol 

 [middle) and dinitro-sec.butjilphenol {bottom) . 



similar in their form to that of cosine ; we know that compounds with one 

 ring-system, like 2:3:5-triiodobenzoic acid, are also strong inhibitors; 

 it is therefore of interest to see that the inhibiting substance 4:6-dinitrocresol 

 is very similar in its form to 2:3:5-triiodobenzoic acid {Figure 12).^ 



Our work on the comparison of structural forms and active positions of the 

 molecules of growth substances and growth inhibitors is, as yet, in an early 

 stage and I regret that I cannot give you definitive and conclusive results but 

 only point out some possibilities for proceeding further in the field of our 

 research. In the standard position every molecule shows different measure- 

 ments (see Figure 13). We have carried out measurements in a long series of 

 different molecules, promoting, inhibiting, and inactive ones, and our 



154 



