556 5. QUINONES 



(F) The introduction of anionic groups, such as sufonate, i^hosphate, 

 or carboxylate, lowers the activity, possibly through impeding penetration 

 into the bacteria, although these groups could also, if they remain on the 

 rings, electrostatically interfere with SH reactions or inductively modify 

 the quinone reactivity. 



Mechanisms of Antibacterial Action 



The bacteriostatic action of the quinones was believed by Cooper (1913) 

 to be associated with the reaction of the quinones with amino or imino 

 groups on the bacterial proteins, since she had demonstrated that the 

 active form is the quinone and that the action is unlike that of the phenols, 

 which are much less potent. Later, Cooper and Haines (1928) altered the 

 original concept because they found p-benzoquinone to be more inhibitory 

 than toluquinone (see accompanying tabulation), and yet the two act 



Organism 



Potency ratio 



(P-Q/TQ) 



similarly on proteins (p-Q/TQ potency ratio is 1.5). On the other hand, the 

 potency or reactivity ratios for the amino acids often fall in the range 

 3-6 (see page 441). It was thus felt that reaction of the quinones with in- 

 tracellular amino acids might be responsible for the growth inhibition. More 

 recent results comparing p-benzoquinone and toluquinone have not provided 

 so marked a difference in potency, and in general the latter seems to be the 

 more potent (even in E. coli, ??-Q/TQ ratios of 0.42 and 1.74 have been ob- 

 tained) (Table 5-9). Although intracellular amino acids are undoubtedly 

 important in bacterial growth, it is doubtful that most of the quinones in 

 bacteriostatic concentrations react appreciably with them. Hilpert (1925) 

 also proposed a mechanism involving the oxidation of protein groups, 

 and Marini-Bettolo and Del Pianto (1947) claimed a parallel between the 

 ability of various quinones to dehydrogenate amino acids and the bacterio- 

 static activity. 



Several workers have suggested that the inhibition of bacterial growth 

 by many substances is related to the redox potential. It is usually assumed 

 that a cell can operate effectively only within a certain redox range and 

 that certain introduced substances can shift the cell out of this range. 



