HYDROGEN-ION CONCENTRATION 455 



The negative logarithm of the ionization constant of Equation 2, signi- 

 fied by pK a , is numerically equal to the pH at which the acid is 50 

 per cent dissociated. The pK a is characteristic of the molecule, and 

 one of the commonly overlooked effects of addition of substituents to 

 an organic molecule is that the substituent may change pK a . Exactly 

 the same considerations, of course, apply to weak bases. 



Acidic toxicants almost invariably are more effective at low pH than 

 at high; as discussed later, the effect of pH is most noticeable near 

 pK a , so that data on very strong and very weak acids will not normally 

 show an effect of hydrogen-ion concentration under the usual experi- 

 mental conditions. Data on inhibition of fungi which fit the rule of 

 greater activity at low pH are extremely numerous, although they do 

 not include spore germination results. As examples, we may cite the 

 action of fatty acids (161, 350, 354, 456), aromatic acids (75, 76, 78, 

 162, 339, 390), acid dyes (205), substituted phenols (41, 187, 383), and 

 nitrous acid (Chapter 8). 



The usual explanation for the effect of dissociation on toxicity is 

 that only the undissociated molecule enters the cell. This reflects the 

 general experience that ions penetrate the cell at least much more 

 slowly than do neutral molecules (84). Several experiments with 

 fungi have shown that the ester of a toxic acid is more effective than 

 the free acid (24, 62, 140, 178, 301). The ester almost certainly pene- 

 trates the cell more rapidly than the partially ionized acid and is then 

 split by esterases. More direct evidence would, however, be desirable, 

 since esterification of an acid may have other effects, for example on 

 reactivity of groups near to the carbonyl carbon. 



It is often stated that one can conclude from these pH-activity studies 

 that it is only the undissociated molecule which is toxic, i.e., action 

 within the cell is visualized as caused solely by some combination of 

 the neutral molecule of the acid or base with a sensitive element. For 

 this conclusion to hold, the biologically effective dose in terms of 

 neutral molecules must be invariant with pH; Simon and Blackman 

 (383) review several studies on echinoderm eggs in which this relation 

 prevails for both weak acids and weak bases. 



However, studies on a wide variety of compounds — phenols, cresols, 

 fluoride, azide, cyanide, etc. — show quite conclusively that the ion 

 must also play a role in toxicity within the sensitive volume of the 

 cell (382, 383). Figure 6 exemplifies the data: it is seen that as the 

 ionization increases with increasing pH the equi-effective concentration 

 of undissociated molecules actually declines. The total drug — ions 

 plus neutral molecules — rises of course, but it is the fall in undissoci- 

 ated toxicant which is decisive. 



