656 5. OXIDANTS 



actual precipitation, upon oxidation has occasionally been taken as evidence 

 for intermolecular disulfide bonding, but this is perhaps not always valid, 

 since the oxidation may bring about a dissolution of the protein structure 

 leading to such intermolecular reactions as occur during any type of de- 

 naturation. If reversal of aggregation can be induced by reducing agents, it 

 is more likely that disulfide bonds are responsible. One factor of primary 

 importance in determining whether a disulfide bond can be formed is the 

 steric relationship between the interacting groups. The C — S — S — C group- 

 ing is not linear, or even planar; the S — S — C bond angle is around 107° and 

 the dihedral angle between the two C — S bonds is close to 90° (due to the 

 electrostatic repulsion between unbonded electron pairs). Thus such bonds 

 will be formed readily only when the residues to which the sulfur atoms are 

 attached can assume the proper orientations. 

 The thiol-disulfide equilibrium: 



R— SH ^ 1/2 (R— S— S— R) + H+ -f e- 



has not been easy to determine, due to abnormal electrode reactions and 

 the usual sluggishness of such systems, and hence values for the oxidation- 

 reduction potential vary with the method used. It appears that Eq at pH 7 

 for various low molecular weight thiols generally lies between —0.35 and 

 0.0 (Calvin, 1954; Clark, 1960, p. 486). The values of E^' for protein SH 

 groups are not known, but it is likely that they would lie mainly in this 

 range also. It is possible, however, that some SH groups, due to their par- 

 ticular molecular environment, may have positive potentials, i.e., would be 

 less easily oxidized than most SH groups of the smaller compounds. It is 

 certainly true that certain SH groups on proteins, although readily acces- 

 sible to alkylation or mercaptide formation, are not oxidized readily, but 

 whether this is due to an especially high oxidation-reduction potential or 

 steric factors, as discussed above, is not known. The values of Eq depend 

 strongly on the pH, which must be taken into account when experiments 

 are run at pH's varying from neutrality. In any event, the oxidant should 

 have a rather high potential (probably 0.2 or higher) in order to oxidize 

 the susceptible SH groups to virtual completeness. On the other hand, it is 

 usually desirable to oxidize the SH groups only to the disulfide stage. Strong 

 oxidants can occasionally not only oxidize SH groups to sulfonate but at- 

 tack enzyme groups other than SH so that specificity is lost. Thus iodate 

 oxidizes gluten and thiolated gelatin mainly to the disulfide stage: 



6 R— SH + lOg^ -► 3 R— S— S— R + 1+3 H^O 



but further oxidation also occurs simultaneously: 



R— SH + IO3- -> R— SO3- + H+ + I- 



