Chemical structure and biological activity 



These results, then, are fully consistent with the view that the side-chain of 

 these acids is degraded within the flax plant by ^-oxidation. The 9-phenoxy- 

 nonane-1-carboxylic acid {n = 9) showed exceptional behaviour in yielding 

 phenol, a result which can be explained by side-chain degradation involving 

 oj-oxidation as follows: 



C6H50CH2(CH2)8COOH^[C6H50CO(CH2)8COOH]^^-^C6H50H 



fi — 9 phenol 



This findinor for the nonane acid correlates with the results of animal meta- 

 holism studies (Verkade and Lee, 1934; Breusch, 1948). 



Further chemical evidence that /i-oxidation can occur in plants has been 

 provided by recent work on the metabolism of aryloxy-acids in pea-stem tissue. 

 We have found that phenoxyheptanoic acid {n = 6) is degraded more readily 

 than phenoxyvaleric acid {n = 4) to phenoxypropionic acid (n = 2), a result 

 which correlates with the growth-regulating activity shown by these com- 

 pounds in the pea test (Fawcett et. al., 1954). Furthermore, in these experi- 

 ments the heptanoic and valeric derivatives were converted to phenol more 

 readily than the propionic analogue. 



In another aspect of this work, three homologous series of chloro-substituted 

 phenoxy acids with chlorine substituents in the 4-, 2:4-, and 2:4:5- positions 

 respectively, and each consisting of the first six members, were examined in 

 the wheat cylinder and pea curvature tests (Wain and Wightman, 1954). 

 In the former test, all series showed a typical alternation in activity which was 

 consistent with the /i-oxidation hypothesis. In the pea test, however, 

 although this alternation was shown in two series, in the 2:4:5-trichloro- 

 phenoxy series only the acetic derivative was active. Also in this investiga- 

 tion, by using chromatographic and biological methods, clear evidence was 

 obtained that 2:4:5-trichlorophenoxyacetic acid was produced by treating 

 the corresponding butyric and caproic acids with wheat coleoptile tissue. 

 The most important conclusion from this work (Wain and Wightman, 1954) 

 was that whereas different types of plant tissue may all possess /^-oxidase 

 enzyme systems, whether or not these are able to degrade the side chain of 

 specific fo-phenoxyalkanecarboxylic acids depends on the nature and posi- 

 tion of the nuclear substituents present. Such considerations have led 

 logically to developments in the field of selective toxicity (Wain, 1955). 



Evidence that /5-oxidation of the side chain of co-(l-naphthyl)alkanecarb- 

 oxylic acids can occur in plant tissues has been obtained at Wye. The 

 effect on activity of substituting alkyl groups into the a- or /3- positions of the 

 side chain of certain of these compounds was also investigated and found 

 to be consistent with the concept of /i-oxidation (Wain and Wightman, 1954). 



More recently, a detailed study has been undertaken of the influence of 

 ring substitution in to-phenoxyalkanecarboxyhc acids on the /5-oxidation of 

 these compounds in different plant tissues. This work, to which 

 Miss M. B. Pybus and Miss R. M. Pascal have also contributed, has involved 

 the synthesis of the first six members of each of fifteen homologous series of 

 phenoxy acids. The results, which will be published in full elsewhere, 

 provide further evidence that /i-oxidase enzyme systems are present in 

 wheat coleoptile and pea stem tissues and that these enzymes can show 

 considerable substrate specificity. In this connection, they confirm and 



188 



