478 



CHEMICAL AGENTS 



other fungi have a direct bearing on the toxicity of quinones and other 

 o,/?-unsaturated ketones. It is proposed that any substituent which 

 lends to withdraw electrons from a double bond increases toxicity (262, 

 301). Thus, derivatives of w -nitrostyrene, C C H 5 — CH = CH— N0 2 , are 

 much more active than styrene, C 6 H 5 — CH = CH 2 . The similarity to 

 the a,/3-unsaturated ketone structure, 



O 



\ I H 



c=c— c— 



is obvious. The same relation holds among the lachrymators: keto, 

 aldehyde, ester (but not carboxyl), and nitro groups all confer ability 

 on the double bond to attack sulfhydryl compounds; i.e., all increase 

 activity toward nucleophilic reagents (94, 95). 



10. CHELATION AND FUNGISTATIC ACTIVITY 



A number of chemicals which are known to be strong chelating 

 agents for metals are also found to be toxic to cells of fungi and other 

 organisms. Since the most effort has gone into the problem of the 

 mode of action of the dithiocarbamates and of 8-hydroxyquinoline 

 (oxine), we shall limit this section largely to these compounds, after 

 preliminary consideration of general topics. 



Chelate compounds are compounds in which a metal ion combines 

 with two or more electron-donating groups of a ligand in such a way 

 that one or more rings are formed (276). As an example, the stepwise 

 reaction of oxine with copper may be written: 



Cu 



+ + 



Oxine 



1 : 1 complex 



#\ 



\_/" \ " <5> 



1:2 complex 



In this example, the final product, the 1:2 chelate, has no exposed polar 



