KENNETH V. THIMANN 29 



acid was found to be 30 times as active as the (— ). On straight growth 

 of immersed sections, however, the two isomers have the same activity. 





CH 



H H 



vviy 



0^ /^"s ^^ ^0. /COOH 



^CH C ^^1 CH 



CllL ^Cl iOOH CllL ^Cl CHj 



The difference is not one of primary activity therefore, a fact which was 

 confirmed by direct demonstration of a difference in transport rate and 

 in rate of inactivation by tissue brei. The isomers of 2,4-dichlorophenoxy- 

 a-propionic acid (XXX) present a different picture. In this case the 

 d- and racemic acids* show differences in the pea test, the d-iorm being 

 almost exactly twice as active as the racemic, which means that the 

 activity of the /-form is very low (11). This, then, is a difference in 

 primary activity. The third case is that of the 1,2,3,4-tetrahydro- 

 naphthoic acids (XXII) of which the (-) form has nearly the activity 

 of indoleacetic acid, while the activity of the (+) form is very low; this 

 again refers to activity in the pea test (14). Optical isomerism, therefore, 

 does influence primary activity. 



The geometrical isomerism is perhaps more interesting because it has 

 given rise to a consistent theory. Veldstra (13) has pointed out that in 

 /ra;2^-cinnamic acid the dipole of the carboxyl is held in the plane of the 

 ring, while in the aV-isomer it is held at an angle to it. He has thus 

 generalized the five requirements of Koepfli, Thimann, and Went to 

 two, namely, "a non-polar part (ring-system) active interfacially, carry- 

 ing an acidic polar group (preferably a carboxyl group) in such a spatial 

 position with respect to the nucleus that this group is situated out of the 

 ring system as far as (or as frequently as) possible" (14). To the cmnamic 

 acids, tetrahydronaphthyhdeneacetic (XXXI) and a-naphthaleneacrylic 

 (XXXII) have been added. In each case one form is active and the other 



*Kindly supplied to me by Dr. Franklin Jones. 



