p-AMINOBENZOIC ACID 



505 



k is a proportionality constant derived empirically from the experimental 

 maximum activity of sulfonamides at pH 7, as indicated in Figure 7. 



The curve for this expression when plotted as indicated in Figure 7 

 agrees closely with the experimental curve for Escherichia coli obtained 

 from the data given in Table 8. Similar experimental data are obtained 

 with other organisms, as indicated in Table 8. This theory, of course, 



p Ka 



Figure 7. Experimental and theoretical relationships of activity of sulfonamides to 

 acid dissociation constants. 



Experimental curve. 204 



_._._._. Theoretical (Negativity of S0 2 group). 204 

 Theoretical (Ionization). 99 



does not account for the high activity of certain sulfonamides which do 

 not ionize. 



Correlation with Negativity of the S0 2 Group. Bell and Roblin 204 

 have presented a theory correlating the negativity of the S0 2 group of 

 ISP-substituted sulfonamides with their ability to prevent the growth of 

 Escherichia coli. According to the theory, the more negative the S0 2 group 

 of an N^substituted sulfanilamide derivative, the greater its bacterio- 

 static power. Since p-aminobenzoic acid is more than 99 per cent ionized 

 in a medium buffered at pH 7, the first formula of Figure 8 indicates the 

 main form in which the vitamin exists in the medium and also within 



