Chemical structure and biological activity 



substance occurs by filling in a gap in the structure of the appropriate part of 

 the growing system, not by means of chemical reactions and the formation of 

 new chemical bonds, but only by the action of intermolecular forces. The 

 intensity of these forces, which depends on the gap as well as on the molecule 

 fitting in, is called 'affinity'. This affinity will be high, if the form of the gap 

 agrees with the form of the molecule, or, to be more precise, if the form of the 

 gap agrees with the form of the active group of the molecule with which it is 

 to be filled. The molecule is able to enter into the gap only when its form 

 corresponds to that of the gap. It follows that it must be of some importance 

 to measure the affinity (which we accept as hypothesis for our heuristic 

 purpose) of different growth substances and inhibitors, as well as of different 

 chemically related inactive substances. The mathematical formulation of 

 the concentration-action curves evaluated by Kaindl (1954) enables us to 

 obtain values from the experimental data of these curves, which are called X 

 and are proportional to the size of the affinity between gap and molecule. 

 These values for A are designated A„- for a growth-promoting part of the 

 molecule and as Ijj for the growth-inhibiting part of the same molecule. 

 We do not stipulate that these two parts must be separated. They may be 

 partly or entirely the same. These values "k^r and Ij^ have been calculated 

 for the main types of growth substances and growth inhibitors and are shown 

 in Table 1. 



The most active growth substances show the highest affinities in the order 

 shown in Table 2. Apart from the differences in the side chains with the 

 different salts and esters of acid products, which may be altered by the plant 

 metabolism and which lead to differences in ease of penetration into the cells, 

 the most active promoters show the following order: 



2:4:5-trichlorophenoxyacetic acid, 



indole-3-acetic acid, 



2 : 5-dichlorophenoxyacetic acid, 



2:3:6-trichlorobenzoic acid, 



2:4-dichlorophenoxyacetic acid, 



a-naphthaleneacetic acid, 



tetrachlorophenoxyacetic acid. 

 It is surprising that the 2: 5-dichlorophenoxyacetic acid, which has no strong 

 weed-controlling properties, shows a higher affinity Ajj- than the very active 

 weed-killer 2:4-D. The same is the case with the inactive weed killer 

 indole-3-acetic acid, which shows a higher Ay,- value than 2:4-D. 



The most active growth inhibitors can be ranged as shown in Table 3. 

 We see that the highest affinity of a growth inhibitor is only about half the 

 highest affinity of a growth promoter. The affinities of the inhibiting group 

 of growth substances (A^^) range between the affinities of the different growth 

 substances as shown in Table 4. This table shows that the most active growth 

 promoters possess A^ values which are very small, compared with those of most 

 of the inhibitors, but high enough to show effects as the less active inhibitors do. 

 It is not easy to correlate these values for the affinities and the steric forms of 

 different active molecules, because of the difficulties in comparing different 

 molecular shapes. The main difficulty is that the organic molecules with a 

 molecular weight of 100-500 have no fixed form. Free rotation is possible 

 about certain bonds and their actual forms depend on the relative positions 



148 



