Chemical structure and biological activity 



sense the same growth effect can be obtained as with, for example, I AA. both 

 in the pea and in the straight growth tests provided that very high outside 

 concentrations are used (up to 5 X 10^^ mol). Very probably this will have to 

 be explained by a very poor penetration of these completely dissociated 

 strong acids, which in itself would imply that the primary action resides 

 inside the cell. 



In order to analyse further variations of the polar group, in relation to 

 activity, a number of phosphonic and phosphonous acids were synthesized, 

 of which only naphthalene methane phosphonic acid (XIV) had been 

 previously investigated (see Veldstra et al., 1954). 



P(0)(0H)2 PO2H2 CH2P(0)(OH)2 CH0PO2H2 



XII 



XIII 



XIV 



XV 



0C2H, 



-CH2P(0)(OH)2 



y\ 



CH2P(0)(0H) 



H H 



XVI XVII 



Most of them proved to be active again in both tests at high concentrations 

 {see Figure 6 and Table 1). It is worth mentioning that of the P-analogues of 

 the weakly active naphthoic acid only the phosphonous acid (XIII) is 

 active, whereas naphthalenemethanephosphonous acid (XV) at similar 

 concentrations is more active than the phosphonic acid (XIV). Of the indole 

 derivatives the phosphonic acid analogue of lAA (XVI) and its mono ester 

 (XVII) were investigated, both of them being active. 



From these resultsj it is clear that, though COOH is superior as a polar 

 group in that it confers high activity to the compounds at low concentrations, 

 other negative polar groups can perform the same function, and that their 

 intrinsic activity may well come up to the same level, higher concentrations 

 being required only becavise of secondary factors. 



The fact that the structural specificity of the polar group is low, in our 

 opinion is another argument in support of the view that, in functioning, the 

 growth substance is not incorporated by a chemical reaction into an 'active 

 structure'. From all the evidence available from other domains one would 

 expect rather an antagonistic activity of the sulphonic and the phosphonic 

 acids which, chemically, are not equivalent to the carboxylic acids. As in 

 many respects they are equivalent physico-chemically, however, such a type 

 of auxin action would make it comprehensible that all of these chemically 

 different acids are physiologically active. In our opinion this is the most 

 important conclusion to be drawn from this work on the polar group with 

 respect to mode of action. 



I For other phosphoric acid derivatives active as growth substances see Greenham (1953). 



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