INTERACTION OF ORGANIC MOLECULES WITH PROTEINS 



99 



P-Me-Dye 4- OH" = P-Me-OH + Dye (I) 

 P-Me-Dye + 0H~ == P + HO-Me-Dye (2a) 

 HO-Me-Dye + 0H~ =. Me(0H)2 + Dye (2b) 



^P-Me 



'^Me- 



is known, then we should at least get a parallelism between that affinity and 

 the onset of the pH effect. 



For example, for copper [which also can act as a bridge here (Fig. 4), as we 

 know from independent experiments], which has a much greater affinity for 

 OH ions, you could expect this competition of OH and dye to occur at an earlier 

 pH; and indeed it does, as the next illustration (Fig. 9) shows. 



Here (Fig. 9) we have the same type of photometric titration with copper as 



0.6 



0.5 





0.3 



Q- 0.2 

 o 



0.1 



J_ 



I 



6 



10 II 



7 8 9 



pH 



Fig. 9. Photometric titration of protein-copper-dye 



