702 



coo 



o-Iodobenzoate 



6. 0-IODOSOBENZOATE 



coo' 



I— O" 



o-Iodosobenzoate 



COO 



o-lodoxybenzoate 



It is possible that the aryl iodoso compounds, cp — 1+ — 0~, can add a proton 

 to form 9? — 1+ — OH since the corresponding protonated iodoxybenzene is 

 known, but the ionization constant is unknown. Indeed, the carboxyl p^^ 

 is not accurately known, but is probably around 6.0 to 6.5, in contrast to 

 the piiig for o-iodobenzoic acid (2.86), and o-iodosobenzoic acid may be 

 precipitated from solution by passing COg through solutions of the sodium 

 salt. o-Iodosobenzoic acid may be easily prepared by oxidation of o-iodo- 

 benzoic acid with permanganate, crystallization by cooling, and recrystal- 

 lization with COg, and can be determined iodimetrically (Loevenhart and 

 Grove, 1911; Chinard and HeUerman, 1954). Commercial samples should 

 probably be repurified for accurate work. The m- and p-iodosobenzoates 

 are also strong oxidizing agents and might possibly have certain advantages 

 over o-iodosobenzoate for particular purposes, but they have been almost 

 completely ignored. It is interesting that p-iodosobenzoate reacts like o-iodo- 

 sobenzoate with the SH groups of L-glutamate dehydrogenase, but no de- 

 tailed comparison was made (Hellerman et al., 1958). The o-iodoxybenzoate 

 is also a potentially useful reagent but essentially nothing is known of its 

 actions on proteins or enzymes. 



The oxidation of SH groups by o-iodosobenzoate results in the formation 

 of o-iodobenzoate. Whether the disulfide link is intra- or intermolecular de- 

 pends on the thiol reacted; with cysteine or glutathione it is obviously in- 

 termolecular, but what evidence exists for proteins suggests that intramo- 

 lecular oxidation is dominant. Oxidation apparently does not proceed be- 

 yond the disulfide stage: 



COO 



COO 



I— O 



SH 



SH 



+ R 



/' 



+ H,0 



at pH 7, and under proper conditions accurate titration of SH groups can 

 be achieved. However, at lower pH's or in the presence of excess of o-iodo- 

 sobenzoate, further oxidation to the sulfinate or sulfonate stages may occur, 

 and groups other than SH may be attacked. Whether a free radical mecha- 

 nism is involved in the oxidation of SH groups here is not known, but if so, 

 one might postulate the following types of reaction: 



