chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, persulfide refers to the

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...

R-S-S-H. Persulfides are intermediates in the biosynthesis of

iron-sulfur protein Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur clu ...

s 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 poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...

, 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 biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

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

disulfide bond In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

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 gro ...

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 s ...

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 determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

. 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 widely used in the synthesis of organic and organometallic compounds. PPh3 exists a ...

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 less ...

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

Biosynthetic and catabolic roles

The cofactors

4-thiouridine 4-Thiouridine is an atypical nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) ...

and

thiamine Thiamine, also known as thiamin and vitamin B1, is a vitamin, an essential micronutrient, that cannot be made in the body. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosphorylated forms of thi ...

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), and ...

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 me ...

. Persulfides have been invoked as intermediates in the biodegradation of

carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non ...

and mercaptopyruvate.

References

{{Functional groups Thiols Functional groups