Chemical structure and biological activity 



results are presented in Table 5. We hope that an exact comparison of the 

 affinity values with the measurements of the molecule in respect to electrical 

 and chemical (electronic) configuration, may add to our knowledge of the 

 relations between chemical constitution and activity of growth substances and 

 inhibitors. Without referring to our more recent results which are too 

 extensive to discuss here, we can establish already the following facts: 

 A growth-promoting molecule may not have a ring length (/?,) of more 

 than 9-0 to 9-6 A, a ring breadth (/?^) of more than 7-1 and 7-6 A and a 

 side-chain distance {S j) of more than 7-0 A {Table 6). The general state- 

 ment that the molecular length may not exceed 9-1 A could also be made, 



Table 6 



The differences in dimensions of the most important growth promoters and growth inhibitors 



t Dimensions of molecule in side by side positions of the rings; when taken as U'-twin the dimensions are as 

 follows: Mj = 14-4; /?j = 8-7 ; R^, = 14-4; S^ = 70. 



with the single exception of 2-oxybenzthiazoleacetic acid. This substance, 

 with a molecular length of about 10 A, would be expected to be a growth 

 inhibitor like the /i-naphthoxyacetic acid, which shows nearly the same 

 measurements, but we found it to be active as a promoter. It is possible 

 that the substance we tested was not really pure and it will be necessary 

 to repeat the tests with extremely pure samples of 2-oxybenzthiazoleacetic 

 acid. 



W^e are not yet able to assume that the growth-promoting molecule must 

 be able to fall into a certain position, called a standard position, because 

 2:3:6-trichlorobenzoic acid is not able to do so, in spite of its activity as a 

 growth promoter. For this reason, we are forced to suppose that there are 

 two different standard forms of growth-promoting molecules. One group is 

 characterized by the form of indole-3-acetonitrile, and includes indole- 

 3-acetic acid, a-naphthaleneacetic acid, 2:4-D, 2:4:5-trichlorophenoxyacetic 

 acid, 2-oxybenzthiazoleacetic acid, and dimethylthiuramacetic acid. 



156 



