Chemical structure and biological activity 



Prof, den Hertog kindly afforded us an opportunity to test a number of his 

 compounds and with his consent a preliminary account of the results is 

 given (see Figure 9 and Table 1). 



Cl- 



-Cl 



0=xN/ ^j^/-OCH.3COOH ^j^/-OGH,COOH 



I 

 CH2COOH 



XXVIII XXIX XXX 



Whereas iV-(2-pyridonyl) acetic acid (XXVIII) is inactive, 2-pyridoxy- 

 acetic acid (XXIX) and its 3 : 5-dichloro-derivative (XXX) are active both 

 in the pea test and in the straight growth test, the activity of the latter 

 approaching the level of lAA. It is noteworthy that, as compared with 

 2:4-D, the pyridine analogue shows a far lower phyto-toxicity, at least 

 towards these tissues. 



After the other isomeric pyridoxyacetic acids and their derivatives have 

 been investigated, this group of compounds, possibly in connection with the 

 dithiocarbamates, might well enable us to analyse further the details of 

 structure-activity relationships. 



In these compounds we are dealing with more precise variations in the 

 ring-system and its influencing by the location of the side chain than have 

 so far been studied with naphthalene and indole ring-systems. Their effect 

 on activity may provide a possibility to 'reconnoitre' some aspects of the 

 receptor surface. 



ON THE FUNCTION OF GROW^TH SUBSTANCES 



As stated above, for fundamental reasons we cannot expect to solve the 

 problem of auxin function by exclusively studying their form (structure) and 

 its influence on activity. Such work may certainly provide information, 

 however, as to dimensions and nature of the receptor(s) as well as to the type 

 of auxin action. 



The various interpretations of results obtained in the structure-activity 

 domain have led to different attempts to solve the primary question. 

 We have to admit, however, that despite all these efforts, our joint explora- 

 tions have so far not yielded real basic information as to the mode of action 

 (cf. the reviews by Bonner and Bandurski, 1952, and by Gordon, 1953) ; for, 

 as Gordon has expressed it, 'the biochemical mechanism of auxin action still 

 remains as one of the pressing and challenging problems in plant physiology'. 



So apparently more data are needed and a constant re-evaluation of 

 those already obtained (even if affecting beloved theories) will be compulsory 

 too. 



When deciding to shift the accent from an analysis of structure-activity 

 to a biochemical one (in the cell), our way of analysing will obviously be 

 determined to a large extent by the conclusions about the type of action 

 referred to above. 



Thus, when trying to 'visualize' auxin function as a physico-chemical 

 action of an amphipatic organic anion with a particular spatial pattern, an 



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