CHEMICAL PROPERTIES 737 



Complexes of Hg++ with Various Ligands 



Most metal ions, including Hg++, form strongly ionic covalent bonds with 

 ligand atoms capable of donating electron pairs, both Cl~ and 0H~ being 

 simple examples of this. We would expect that Hg++ might complex readily 

 with a variety of substances, many of which occasionally occur in media 

 used for inhibition studies. It is usually stated that Hg++ reacts with SH 

 groups selectively and that other groups on proteins seldom contribute to 

 the binding; it is necessary to look into this matter quantitatively, and ob- 

 tain some idea of the relative affinities of the various groups for Hg++. 

 Complexes of Hg++ with ammonia are well known so that binding to amino 

 groups might be predicted and, since some interaction with carboxylate 

 groups is likely, it may be anticipated that amino acids would provide ef- 

 fective ligands. Indirect evidence for such complexes was obtained by Salle 

 and Ginoza (1943) by showing that several amino acids reduce the bacte- 

 ricidal activity of HgClg. The minimal lethal concentration of HgClg is in- 

 creased 6 times by glycine, aspartate, glutamate, arginine, and lysine at 

 67 inM, and 120 times by cysteine. This indicates appreciable complexing 

 with amino acids under physiological conditions, although the reaction with 

 the SH group of cysteine is evidently stronger than with other groups. 

 Haarmann (1943 a,b) claimed that whereas 1 equivalent of Hg is bound to 

 certain amino acids at pH 7, as much as 4 to 8 equivalents may be bound 

 at pH 11, some loosely and some tightly. A definitive investigation was 

 made by Perkins (1952, 1953) and a number of stability constants were 

 determined. Two major complexes were assumed, probably with the follow- 

 ing structures: 



OC— O OC— O N 



H, 



Complex I Complex II 



The composite constant, ^2 = K^K^, where K^ and K^^ are defined by the 

 following equilibria: 



Hg++ + AA- ±s HgAA+ K, 



HgAA+ + AA- ±^ Hg(AA)2 K, 



(Hg++) (AA-) 

 (HgAA+) 



(HgAA+) (AA-) 

 Hg(AA), 



was determined in each case, and these values are given in Table 7-4 along 

 with the dissociation constants for a number of ligand complexes. The form 

 of the amino acid necessary for chelation with Hg++ is "OOC — E — NHg 



