INTERACTION OF ORGANIC MOLECULES WITH PROTEINS 



97 



— .7 



a. 

 E 



00 

 fO 



m 



z 



r .2 



0. 



o 



EFFECT OF pH ON MEDIATION 

 OF DYE BINDING BY Zn** 



10 



pH 



Fig. 5. Photometric titration of protein-zinc-dye complex 



I would like to consider each of those regions in more detail and see if we can 

 understand them. The increase at the lower pH end is fully understandable, 

 since we already have data which show that the amount of zinc bound by the 

 protein increases in this region, because the protein itself acquires a more nega- 

 tive charge and so binds these metallic cations more strongly. 



The next illustration (Fig. 6) demonstrates that if you do measure the extent 

 of the binding of zinc as a function of pH, where you have a fixed total amount 

 of zinc in the solution, the number of zinc ions bound is found to increase from 

 5 ions per mole of protein at pH 6 to about 12 or 13 at a pH a little over 7, 

 and then to about 28 at approximately 8. So there is a substantial increase 

 in the zinc binding with increasing pH. There are more sites then at which the 

 dye can be chelated to the protein and that, essentially, is why the binding will 

 go up at first. 



Fig. 6. Effect of pH on binding of zinc 



