Inchemistry,persulfide refers to thefunctional group R-S-S-H.[1] Persulfides are intermediates in the biosynthesis ofiron-sulfur proteins[2] and are invoked as precursors tohydrogen sulfide, a signaling molecule.
The nomenclature used for organosulfur compounds is often non-systematic. Sometimes persulfides are called hydrodisulfides to further avoid confusion withdisulfides with the grouping R-S-S-R, by emphasizing the presence of an H at one end of adisulfide bond.
Compared tothiols (R-S-H), persulfides are uncommon. They are thermodynamically unstable with respect to loss of elemental sulfur:
Nonetheless, persulfides are often kinetically stable.
The S-H bond is both more acidic and more fragile than in thiols. This can be seen in thebond dissociation energy of a typical persulfide, which is 22 kcal/mol weaker than a typical thiol, and the lower pKa of about 6.2 for persulfides compared to 7.5 for thiols. Thus, persulfides exist predominantly in the ionized form at neutral pH. This effect is attributed to the stability of the RSS· radical.[1]
The structure of trityl persulfide has been determined byX-ray crystallography. The S-S bond length is 204 picometers and the C-S-S-H dihedral angle is 82°. These parameters are unexceptional.[3] (C6H5)3CSSH behaves as a source of sulfur, illustrated by its reaction withtriphenylphosphine to givetriphenylphosphine sulfide andtriphenylmethanethiol:
The cofactors4-thiouridine andthiamine are produced by the action of persulfides.Cystathionase generates the persulfide of cysteine (sometimes called thiocysteine) fromcystine.
Persulfides have been invoked as intermediates in the biodegradation ofcarbon disulfide[4] andmercaptopyruvate.
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