758 



7. MERCURIALS 



form crystals with Hgl^g, the interactions being purely electrostatic and 

 not involving SH groups (Lewin, 1951). 



Reaction 1 is quite rapid, but reaction 2 is slow because it involves two 

 large molecules of similar charge. The dimerization requires about 25 min 

 for half-reaction and 2 hr for equilibrium when Hg++ is mixed with mer- 



~D 



Fig. 7-5. Possible forms of the Hg-mercaptalbumin dimer. 

 The small solid circle represents Hg++. (From Low, 1952.) 



captalbumin in a 0.5 : 1 ratio (Edelhoch et al., 1953). Dimerization is an 

 endothermic reaction, rise in the temperature favoring formation of the 

 dimer, AH^ being about 7 kcal/mole. The temperature effect indicates 

 an activation energy for dimerization of 17-21 kcal/mole. The constant 

 for the equilibrium (alb— S— HgCl) (alb— SH)/(alb— S— Hg— S— alb) was 

 found to be 3.2 X 10^^^ at pH 4.5 and 25°. Reaction 1 is reversible by sub- 

 stances forming stable compounds with Hg++, and reaction 2 is reversible 

 by ligands forming HgX„ complexes. Dimerization is also reversed by reac- 



AMOUNT 

 OF DIMER 

 (TURBIDITY) 



0.5 



MOLE Hg/MOLE PROTEIN 



Fig. 7-6. The formation and dissociation 

 of the Hg-mercaptalbumin dimer as the 

 molar ratio of Hg++ to protein is in- 

 creased. (From Edelhoch et al., 1953.) 



tion 3 (Fig. 7-6), Hg++ competing with mercaptalbumin for the alb — S — Hg 

 monomer. The dissociation of the dimer by cysteine is very rapid (appar- 

 ently within a few seconds) and yet the dimer is not split nor the Hg++ 

 dissociated readily by dialysis, so that this would seem to be one of those 

 interesting situations in which a complexer appears to take the metal from 

 the protein rather than merely combining with free metal ions (Straessle, 



