Chemical structure and biological activity 



position is the same as that which is necessary for activity and we use it only 

 hypothetically as the standard position for the purpose of comparison. 

 We see {Figures 8, 9) that some other growth-promoting substances like 2:3:6- 

 trichlorobenzoic acid or 3-aminotriazole, are not able to fall into this position, 

 yet a chemical substance without any ring system, such as A'A'-dimethyl- 

 thiuramacetic acid {Figure 9), in spite of the completely different chemical 



Id' id' 



W" 10^ 10^ f^ 10° 10^ % 



Fioiire 10. The molecular model of the 'twin' bis-2 ■A-dichlowphenoxyacetic acid in comparison with 

 the molecular model of tetrachlorofluorescein. Abscissae: log concn. Ordinates : change in length of 

 coleoptiles as per cent oj control. 



Structure, is able to fall into the standard position. A molecule like the 

 Siamese twin (bis-2 : 4-dichlorophenoxyacetic acid) is, in spite of its chemically 

 related structure, not able to fall into the standard position and shows no 

 promoting action. On the other hand, this twin molecule is able to fall into 

 another position which is quite similar to that of the substituted fluorescein 

 molecule {Figure 10) and we know that substituted fluoresceins, such as 

 tetrachloro-, tetrabromo-, and tetraiodofluorescein, all show growth- 

 inhibiting actions {Figure 11) which are stronger than those shown by the 

 twin molecule. Comparing the molecular forms of the unsuJDStituted and 



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