chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, persulfide refers to the

functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...

R-S-S-H. Persulfides are intermediates in the biosynthesis of iron-sulfur proteins and are invoked as precursors to

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

, a signaling molecule.

Nomenclature

The nomenclature used for organosulfur compounds is often non-systematic. Sometimes persulfides are called hydrodisulfides to further avoid confusion with

disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

s with the grouping R-S-S-R, by emphasizing the presence of an H at one end of a

disulfide bond In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inor ...

Properties

Compared to

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

s (R-S-H), persulfides are uncommon. They are thermodynamically unstable with respect to loss of elemental sulfur: :RSSH → RSH + 1/8 S₈ 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 the

bond dissociation energy The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical ...

of a typical persulfide, which is 22 kcal/mol weaker than a typical thiol, and the lower pK_a 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.

Structure and reactions

The structure of trityl persulfide has been determined by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. The S-S bond length is 204 picometers and the C-S-S-H dihedral angle is 82°. These parameters are unexceptional. (C₆H₅)₃CSSH behaves as a source of sulfur, illustrated by its reaction with

triphenylphosphine Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is versatile compound that is widely used as a reagent in organic synthesis and as a l ...

to give

triphenylphosphine sulfide Triphenylphosphine sulfide (IUPAC name: triphenyl-''λ''5-phosphanethione) is the organophosphorus compound with the formula , usually written (where Ph = phenyl). It is a colourless solid, which is soluble in a variety of organic solvents. Struc ...

and

triphenylmethanethiol Triphenylmethanethiol is an organosulfur compound with the formula (C6H5)3CSH. It is the thiol derivative of the bulky substituent triphenylmethyl (called trityl). The compound forms a number of unusual derivatives that are more stable than les ...

: :(C₆H₅)₃CSSH + P(C₆H₅)₃ → (C₆H₅)₃CSH + SP(C₆H₅)₃

Biosynthetic and catabolic roles

The cofactors

4-thiouridine 4-Thiouridine is an atypical nucleoside formed with the 4-thiouracil base found in transfer RNA Transfer ribonucleic acid (tRNA), formerly referred to as soluble ribonucleic acid (sRNA), is an adaptor molecule composed of RNA, typically 76 to 9 ...

and

thiamine Thiamine, also known as thiamin and vitamin B1, is a vitamin – an Nutrient#Micronutrients, essential micronutrient for humans and animals. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosp ...

are produced by the action of persulfides.

Cystathionase The enzyme cystathionine γ-lyase (EC 4.4.1.1, CTH or CSE; also cystathionase; systematic name L-cystathionine cysteine-lyase (deaminating; 2-oxobutanoate-forming)) breaks down cystathionine into cysteine, 2-oxobutanoate ( α-ketobutyrate), an ...

generates the persulfide of cysteine (sometimes called thiocysteine) from

cystine Cystine is the oxidized derivative of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of redox reactions and a mec ...

. Persulfides have been invoked as intermediates in the biodegradation of

carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is an inorganic compound with the chemical formula and structure . It is also considered as the anhydride of thiocarbonic acid. It is a colorless, flammable, neurotoxic liquid that is used as ...

and

mercaptopyruvate 3-Mercaptopyruvic acid is an intermediate in cysteine metabolism. It has been studied as a potential treatment for cyanide poisoning, but its half-life is too short for it to be clinically effective. Instead, prodrug A prodrug is a pharmacolo ...

References

{{Functional groups Thiols Functional groups