CHEMICAL PROPERTIES 749 



nation of an amino group on cysteine or glutathione reduces somewhat the 

 affinity of the thiol for Hg++, as might be anticipated, while ionization of 

 the thioglycolate carboxyl groups has little effect. With the constants in 

 Table 7-7 it is possible to predict the relative concentrations of the various 

 species present; when complexing ligands are in significant concentration, 

 appropriate corrections must be made (page 737). The variations with tem- 

 perature allow the calculation of certain basic thermodynamic parameters. 

 For the formation of the mercaptides, /iF is — 55 to — 59 kcal/mole and 

 the entropy changes are positive and usually rather large; for the equilibria 

 expressed by y>Ki (see legend in Table 7-7), AS is +27 cal/deg for cysteine, 

 -|- 54 cal/deg for glutathione, and + 68 cal/deg for thioglycolate. 



We must now inquire into the effect of pH on mercaptide formation and 

 particularly consider the reactions with the SH group and the ionized S^ 

 group. Taking the two following equilibria: 



R— Hg+ + R'— S- ±^ R— Hg— S— R' K^ 



R— Hg+ + R'— SH ±^ R— Hg— S— R' + H+ K^„ 



(R— Hg^)(R--S^) 

 (R— Hg— S— R') 



(R-Hg+) (R--SH) 

 (R_Hg-S-R')(H+) 



and the ionization of SH — i.e., K,, - (H+) (R'— S-)/(R'— SH) — it is 

 easy to show that: 



pATg = pK^^ + pK, (7-2) 



The p^^ for SH groups varies from 7 to 10, and in Volume I a mean value 

 of 8.7 was assumed for protein SH groups. In any event, pKf- and p^sh 

 will differ quite markedly. This is, of course, essentiallj- a competition be- 

 tween H+ ions and the mercurial for the S~ group. At physiological pH, SH 

 will predominate over S~, and the apparent pK for mercaptide dissociation 

 will be smaller than those given in Tables 7-6 and 7-7. 



Just as H+ competes with the mercurial for the S" groups, so various 

 complexing ligands may compete with the S~ group for the mercurial. 

 Despite the fact that the p/^'s for thiols are much greater than for most 

 other ligands, a very significant effect on the equilibrium may be exerted. 

 Let us write for the usual reaction of mercaptide formation in physiological 

 media: 



R— Hg— X + R'— SH ±i R— Hg— S— R' + X" + H+ 



where X represents some ligand such as CI" or 0H~. The equilibrium is 

 given by: 



(R— Hg— X) (R'— SH) 



K 



(R_Hg-S-R') (X-) (H+) 



