Editing Disulfide (section)

===Properties===
Disulfide bonds are strong, with a typical [[bond dissociation energy]] of 60&nbsp;kcal/mol (251&nbsp;kJ&nbsp;mol<sup>−1</sup>). However, being about 40% weaker than {{chem2|[[C–C bond|C\sC]]}} and {{chem2|[[Carbon–hydrogen bond|C\sH]]}} bonds, the disulfide bond is often the "weak link" in many molecules. Furthermore, reflecting the [[polarizability]] of divalent sulfur, the {{chem2|S\sS}} bond is susceptible to scission by polar reagents, both [[electrophile]]s and especially [[nucleophile]]s (Nu):<ref>{{cite book|first=R. J.|last=Cremlyn|title=An Introduction to Organosulfur Chemistry|publisher=John Wiley and Sons|location=Chichester|date=1996|isbn=0-471-95512-4}}</ref> <chem display=block>RS-SR + Nu- -> RS-Nu + RS-</chem> 

The disulfide bond is about 2.05&nbsp;[[ångström|Å]] in length, about 0.5&nbsp;Å longer than a {{chem2|C\sC}} bond. Rotation about the {{chem2|S\sS}} axis is subject to a low barrier. Disulfides show a distinct preference for [[dihedral angle]]s approaching 90°. When the angle approaches 0° or 180°, then the disulfide is a significantly better oxidant.

Disulfides where the two R groups are the same are called symmetric, examples being [[diphenyl disulfide]] and [[dimethyl disulfide]]. When the two R groups are not identical, the compound is said to be an asymmetric or mixed disulfide.<ref name=Sevier>{{cite journal | doi = 10.1038/nrm954 |last1=Sevier |first1=C. S. |last2=Kaiser |first2=C. A. | title = Formation and transfer of disulphide bonds in living cells | journal = [[Nature Reviews Molecular Cell Biology]]  | year = 2002 | volume = 3 | issue = 11 | pages = 836–847 | pmid = 12415301|s2cid=2885059 | doi-access = free }}</ref>

Although the [[hydrogenation]] of disulfides is usually not practical, the equilibrium constant for the reaction provides a measure of the standard redox potential for disulfides:
:<chem>RSSR + H2 -> 2 RSH</chem>
This value is about −250&nbsp;mV versus the [[standard hydrogen electrode]] (pH&nbsp;=&nbsp;7). By comparison, the standard reduction potential for [[ferrodoxin]]s is about −430&nbsp;mV.