740 7. MERCURIALS 



and, in the calculations of the constants, only that fraction of the amino 

 acid at the pH used was considered. 



The fact that simple amines complex with Hg++ to approximately the 

 same degree as the amino acids indicates that the amino group is the im- 

 portant ligand, the carboxylate group perhaps contributing slightly to the 

 stability. Ring nitrogen atoms are probably not as effective as amino groups. 

 Hg++ reacts with both the amino and imidazole groups of histidine, but 

 more tightly with the former, the pj^'s being 10.6 and 7.5, respectively 

 (Simpson, 1961). The effect of the pH on the stability of these complexes 

 is well illustrated by the constants for the following equilibria with histidine 

 (Brooks and Davidson, 1960): 



Hg++ + 2 hist- ±5 Hg (hist)2 p/ff^ = 21.2 



Hg++ + hist- + H-hist i? Hg (hist) (H-hist)+ Pi92 = 18.4 



Hg++ + 2 H-hist 15 Hg (H-hist)2++ pp^ = 15.0 



where hist designates the ~00C — R — NHg form and H-hist the OCC — 

 R — -NH3+ form. These complexes with histidine were assumed to be linear 

 and it was claimed that chelation must play only a small role in Hg++ 

 complexes due to the tendency of Hg++ to form linear complexes. This 

 brings up an interesting point of importance in understanding the reactions 

 of HgClg with proteins and enzymes. Certainly some of the most stable 

 complexes of Hg++ — as with 1,2,3-triaminopropane, triethylenetetramine, 

 and EDTA — must be chelates and nonlinear, and it is also well known 

 that Hg++ reacts with dimercaprol (BAL) to form a ring with the two SH 

 groups. Whether chelation is or is not important in any case probably de- 

 pends on several factors, such as the spatial arrangement of the ligand 

 groups, the intrinsic affinity of the Hg++ for these groups, and the entropy 

 changes accompanying the formation of the complex. Certainly the third 

 and fourth ligands generally add to the HgLg complex much less readily 

 than the first two, as we have seen for CI". This is also true for ammonia, 

 the successive constants being given by pifj = 8.8, piiTg = 8.7, pK-^ = 1.0, 

 and TpK^ = 0.78. Thus the formation of HgLg and UgL^ type of complexes 

 must involve some additional factors increasing the stability. 



In any event it is clear that the complexes of Hg++ with amino acids 

 and many other compounds are stable enough so that, when these sub- 

 stances are present in the media used for the study of inhibition, a signifi- 

 cant fraction of the Hg may be in the form of such complexes. For example, 

 it may be calculated for a solution containing 1 mM glycine and a total 

 concentration of Hg of 0.1 mM that (Hg++) = 6.3 x lO-i^ M. If the Cl" 

 concentration is appreciable, this will reduce the binding to these other 

 ligands. One may visualize the situation somewhat as follows. In most 

 media there will be several substances — Cl~, OH", buffers, amino acids, 

 substrates, etc. — capable of complexing with Hg++. The end result will 



