458 /. van Overbeek 



film is not entirely predictable from their two dimensional structural 

 formulae. After we have studied penetration into membranes, per- 

 haps then we could answer that cjuestion. I am not proposing here a 

 ready-made theory, but I am simply asking the question, "In which 

 direction are we looking?" Are we going to continue to look for an 

 answer to the primary mode of auxin action in the direction of co- 

 valent bonding, activation of a single enzyme system, water soluble 

 systems, etc., or are we now going to try to look at it from the point 

 of view of surface chemistry, membranes, etc., and in the direction of 

 things such as Dr. Freed and I discussed? 



Dr. Wain: Well, Dr. van Overbeek, I accept that. May I go on 

 to my second point? This is in relation to the phenylacetic acids. Now, 

 according to your concepts, the phenylacetic acids are active when 

 you have 2,6 positions blocked because the 2,6-dichloro atoms sit on 

 top of the cytoskeleton and leave the carboxyl group in precisely the 

 right position. Reference to our results, however, shows that quite 

 good activity is found in 2,3-dichlorophenylacetic acid and there, ac- 

 cording to your idea, the compound should be able to penetrate well 

 down into the cytoskeleton. Exactly the same principles apply to the 

 benzoic acids, where 2,3-dichlorobenzoic acid is active. This com- 

 pound, according to your concept, would penetrate deeply into the 

 membrane, leaving the carboxyl group far too low to join up Avith 

 the oscillating hydrogen bonding network. 



Dr. van Overbeek: When one studies these problems Avith molecule 

 models and especially when one tries to fit these into 3-dimensionaI 

 holes, one finds that the 2,3-dichlorobenzoic and phenylacetic acids do, 

 indeed, fit the theory very nicely, contrary to what one might expect 

 from 2-dimensional reasoning with structural formulae. This type of 

 3-dimensional hole is shown in figure 3 and Avas suggested several years 

 ago by Dr. Mullins (cf. van Overbeek, Bot. Rev. 25:300. 1959). 



The phenylacetic acids have the added feature that the side chain 

 sticks out of the plane of the ring in a very prominent fashion. This 

 gives this type of molecule a tendency to hang itself up, thus favoring 

 activity. Incidentally, the prominent perpendicular side chain may 

 also be a reason why indole-3-propionic acid is highly active, as you 

 have heard dining the discussions on the first day. It, too, has a 

 natural tendency, on account of the angular side chain, to hang itself 

 up and, therefore, stabilize itself in the cytoskeleton system. 



Not only does my view of looking at the primary mode of action 

 of auxin as a solubilization phenomenon explain puzzles such as why 

 the 2,3-dichlorobenzoic and phenylacetic acids possess activity, but 

 it also gives a plausible explanation for the activity of the structurally 

 totally unrelated 2-heptadecanol, which Dr. Crosby told us the first 



