ANTIGENS, HAPTENS, ANTIBODIES, ETC. 



407 



position. The electric fields clue to the polarity of the 

 groupings (see p. 33) are sufficiently alike in the case of 

 o-amino -benzoic acicl and o-amino -benzene -sulphonic acid 

 to allow of either of them reacting with the antiserum 

 prepared against the other ; but altering the position of 

 the substituent to the 7/i-position is sufficient to change 

 the electric field so much that cross -reactions can no 

 longer occur. The difference is, of course, even more 

 enhanced when the p- and o-derivatives are compared. 

 The addition of a second substituent is also sufficient 

 to alter the charge distribution to a great enough extent 

 to abolish the reactions ; thus antisera to o-amino - 

 benzene -sulphonic acid-azo -proteins will give no reaction 

 with chloro -amino -benzene -sulphonic acid-azo -protein (1), 



>N=N— Protein 



SO,H 



, nor with the corresponding methyl 



derivative (2), 



CH, 



)X=X — Protein 



SO,H 



, and vice versa. 



The fields of (1) and (2) are nearly enough alike, however, 

 for antisera to (1) to react with (2), and vice versa. 



Landsteiner also made some interesting experiments 

 by coupling peptides to proteins. He diazotised 2^-amino- 

 benzoyl-glycyl -leucine, 



CO. 



NHa 



giycyi [G] 



.XH.CH.COOH 



I 

 CH, 



I " 

 CH 



Clfg CM3 



leucine [L] 



coupled it to proteins and made antisera in the usual 

 way. He also prepared similar compounds with different 

 arrangements of the amino -acids in the peptide, namely, 

 LG, LL and GG, where L and G represent leucine and 

 glycine respectively. The various combinations of 



27 



