Molecular structure and penetration of growth regulators 



When the amino phosphine oxides of this series are put in aqueous solutions 

 in which Elodea leaves are present, only the higher homologues show evidence 

 of physiological activity. This is manifested under lOOOx magnification by 

 blue green droplets in the chloroplasts. These droplets are quite character- 

 istic for the presence of amino phosphine oxides in the chloroplasts, and thus 

 serve as an index for penetration. The lower homologues of the series, by 

 contrast, do not penetrate into the cell and the chloroplasts, or only do so 

 when supplied at a much higher concentration. 



H3C 



HaC- 



\H 



N- 



H3C- 



/ 



/H 



H3C 











Figure 5. Structures, partition coefficient, and physiological activity of dii^opropylamino phosphine 

 oxides. 



When partition coefficients (water/olive oil) were made for this series of 

 amino phosphine oxides, it was found that the more the compound partitions 

 into the oil phase the more it penetrated into the chloroplasts. This is not 

 surprising because chloroplasts are highly fatty bodies (Chibnall, 1939). 



CONCLUSION 



It has thus been demonstrated on these two examples, maleimides and amino 

 phosphine oxides, that physiological activity of the molecule (as contrasted with 

 biochemical activity inside the cell) depends on the presence of a lipoid radical 

 whose function it is to carry the molecule into the lipoid phases of the cell. 

 It has further been shown that simple aliphatic chains will often perform this 

 function. By analogy it is thus concluded that the increased auxin activity of 

 the alpha substituted growth regulators oi" Figure 1 is also due to increased oil 

 solubility. 



It has further been shown that chlorination of the benzene ring caused 

 increased lipophilic properties in the maleimide molecule. By analogy it is 

 concluded that at least part of the increased auxin activity caused by a 

 similar chlorination in phenyl- and phenoxy acetic acids is simply due to 

 increased lipoid solubility. 



We compared the water/oil partition coefficients of phenoxy- and phenyl- 

 acetic acids and found that the phenyl partitioned far more into the oil 

 {Figure 1). It would seem, therefore, that the long-known difference in auxin 

 activity between these two acids is also a logical result of their difference in 

 capacity to penetrate into the lipoid phases of the plant. 



R. C. Brian substantiated this conclusion (during the discussion of another 



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