680 5. OXIDANTS 



forms in the oxidation of SH groups is not known. The oxidation-reduction 

 potential for iodine varies with the type of reaction in which it participates 

 and the pH, but is usually sufficiently high to oxidize any accessible SH 

 groups. It is important in certain enzyme studies to realize that iodine may 

 disappear fairly rapidly from solution, independently of reaction with or- 

 ganic materials; such is favored by lack of iodide and high pH. 



Reaction of Iodine with Thiols 



Iodine is able to oxidize SH groups to four different states: the disulfide 

 (S — S), the sulfenate (SO"), the sulfinate (SOg"), and the sulfonate (SO3-). 

 Apparently it is quite easy to oxidize beyond the disulfide state with iodine. 

 The stoichiometry of a particular reaction will depend not only on the state 

 of oxidation of the SH groups, but also on the degree of reduction of the 

 iodine, this varying with the pH. For example, the following reactions can 

 be written for oxidation to the disulfide state: 



I2 + 2 R— SH -> R— S— S— R -I 2 I- + 2 H+ 



2 I2 + 2 R— SH + H2O -> R— S— S— R + HOI + 3 I" + 3 H+ 

 4 I2 + 2 R— SH + 3 H2O -> R— S— S— R + IO3- + 7 I- + 8 H+ 



However, it is likely that near neutrality the first reaction is dominant. In 

 the oxidation of cysteine, 3 equivalents of iodine 'are taken up to form 

 cysteate: 



3 I2 + R— SH + 3 H2O -► R— SO3- + 6 I- + 7 H+ 



It is interesting that, at pH 3.2, iodine oxidizes cysteine well, but does not 

 react with cystine, tyrosine, or histidine. This indicates that the first prod- 

 uct in the oxidation of cysteine is not cystine but free radicals, which can 

 either combine to form disulfides or be further oxidized to sulfonate groups 

 (Anson, 1940). At pH 6.8, cystine is the major product. In most instances, 

 especially with proteins, several reactions will occur and mixed products 

 will be found. In addition to these straightforward oxidations, we shall see 

 that there is now evidence for the formation of sulfenyl iodide groups (SI), 

 so that a certain fraction of the iodine can be incorporated into the thiol 

 molecule. 



Reactions of Iodine with Proteins 



The SH groups of denatured ovalbumin are oxidized by iodine within 5 

 min at pH 3.2 and 37° (Anson, 1940). The rate of the reaction decreases as 

 the pH is raised to 6.8. In acid media iodine does not react with tyrosine or 

 proteins containing tyrosine (e.g. pepsin), whereas at neutrality it readily 

 iodinates tyrosine. By proper choice of pH and iodine concentration it is 

 possible to oxidize the SH groups of denatured ovalbumin without appre- 



