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 ...

, a sulfenic acid is an

organosulfur compound Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...

and

oxoacid An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...

with the general formula . It is the first member of the family of organosulfur oxoacids, which also include

sulfinic acid Sulfinic acids are oxoacids of sulfur with the structure RSO(OH). In these organosulfur compounds, sulfur is pyramidal. Structure and properties Sulfinic acids RSO2H are typically more acidic than the corresponding carboxylic acid RCO2H. Su ...

s () and

sulfonic acid In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is kn ...

s (), respectively. The base member of the sulfenic acid series with R = H is

hydrogen thioperoxide Hydrogen thioperoxide, also called oxadisulfane or sulfanol, is the chemical with the structure H–S–O–H. It can be considered as the simple sulfur-substituted analog of the common hydrogen peroxide (H–O–O–H) chemical, and as the simple ...

Properties

In contrast to sulfinic and sulfonic acids, simple sulfenic acids, such as methanesulfenic acid, CH₃SOH, are highly reactive and cannot be isolated in solution. In the gas phase the lifetime of methanesulfenic acid is about one minute. The gas phase structure of methanesulfenic acid was found by microwave spectroscopy (

rotational spectroscopy Rotational spectroscopy is concerned with the measurement of the energies of transitions between quantized rotational states of molecules in the gas phase. The rotational spectrum (power spectral density vs. rotational frequency) of chemical pola ...

) to be CH₃–S–O–H. Sulfenic acids can be stabilized through steric effects, which prevent the sulfenic acid from condensing with itself to form

thiosulfinate In organosulfur chemistry, thiosulfinate is a functional group consisting of the linkage (R refers to organic substituents). Thiolsulfinates are also named as alkanethiosulfinic (or arenethiosulfinic) acid esters. They are the first of the seri ...

s, RS(O)SR, such as

allicin Allicin is an organosulfur compound obtained from garlic and leeks. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic. Allicin is unstable and quickl ...

from

garlic Garlic (''Allium sativum'') is a species of bulbous flowering plants in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chives, Welsh onion, and Chinese onion. Garlic is native to central and south Asia, str ...

. Through the use of

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 structure of such stabilized sulfenic acids were shown to be R–S–O–H. The stable, sterically hindered sulfenic acid 1-triptycenesulfenic acid has been found to have a p''K''_a of 12.5 and an O–H

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 ...

(bde) of 71.9 ± 0.3 kcal/mol, which can be compared to a p''K''_a of ≥14 and O–H BDE of ~88 kcal/mol for the (valence) isoelectronic

hydroperoxide Hydroperoxides or peroxols are Chemical compound, compounds of the form ROOH, where R stands for any group, typically Organic compound, organic, which contain the hydroperoxy functional group (). Hydroperoxide also refers to the hydroperoxide anio ...

s, R O O H.

Formation and occurrence

Peroxiredoxins

Peroxiredoxins Peroxiredoxins (Prxs, ; HGNC root symbol ''PRDX'') are a ubiquitous family of antioxidant enzymes that also control cytokine-induced peroxide levels and thereby mediate signal transduction in mammalian cells. The family members in humans are P ...

are ubiquitous and abundant

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s that detoxify peroxides. They function by the conversion of a cysteine residue to a sulfenic acid. The sulfenic acid then converts to a disulfide by reaction with another residue of cysteine.

Garlic and onions

Sulfenic acids are produced by the enzymatic decomposition of

alliin Alliin is a sulfoxide that is a natural constituent of fresh garlic. It is a derivative of the amino acid cysteine. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma o ...

and related compounds following tissue damage to

onion An onion (''Allium cepa'' , from Latin ), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the onion which was classifie ...

s, and other plants of the genus ''

Allium ''Allium'' is a large genus of monocotyledonous flowering plants with around 1000 accepted species, making ''Allium'' the largest genus in the family Amaryllidaceae and among the largest plant genera in the world. Many of the species are edible, ...

''. 1-Propenesulfenic acid, formed when onions are cut, is rapidly rearranged by a second enzyme, the lachrymatory factor synthase, giving ''syn''-propanethial-''S''-oxide. 2-Propenesulfenic acid, formed from

, is thought to be responsible for garlic’s potent antioxidant activity. Mass spectrometry with a DART ion source were used to identify 2-propenesulfenic formed when garlic is cut or crushed and to demonstrate that this sulfenic acid has a lifetime of less than one second. The pharmacological activity of certain drugs, such as

omeprazole Omeprazole, sold under the brand names Prilosec and Losec, among others, is a medication used in the treatment of gastroesophageal reflux disease (GERD), peptic ulcer disease, and Zollinger–Ellison syndrome. It is also used to prevent up ...

esomeprazole Esomeprazole, sold under the brand name Nexium r Neksiumamong others, is a medication which reduces stomach acid. It is used to treat gastroesophageal reflux disease, peptic ulcer disease, and Zollinger–Ellison syndrome. Its effectiveness ...

ticlopidine Ticlopidine, sold under the brand name Ticlid, is a medication used to reduce the risk of thrombotic strokes. It is an antiplatelet drug in the thienopyridine family which is an adenosine diphosphate (ADP) receptor inhibitor. Research initially ...

clopidogrel Clopidogrel, sold under the brand name Plavix among others, is an antiplatelet drug, antiplatelet medication used to reduce the risk of Cardiovascular disease, heart disease and stroke in those at high risk. It is also used together with aspi ...

, and

prasugrel Prasugrel, sold under the brand names Effient and Efient, is a medication used to prevent formation of blood clots. It is a platelet inhibitor and an irreversible antagonist of P2Y12 ADP receptors and is of the thienopyridine drug class. It ...

is proposed to involve sulfenic acid intermediates. Oxidation of

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

residues in

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

to the corresponding protein sulfenic acids is suggested to be important in

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

-mediated

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...

. Sulfenic acid forms part of the series of chemical reactions that occur when cutting onions. The lachrymal glands are irritated by the end product of the reactions,

syn-Propanethial-S-oxide ''syn''-Propanethial ''S''-oxide (or (''Z'')-propanethial ''S''-oxide), a member of a class of organosulfur compounds known as thiocarbonyl S-oxide, ''S''-oxides (formerly "sulfines"), is a volatile liquid that acts as a lachrymatory agent (trigg ...

, causing tears.

Organic and inorganic chemistry

Sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...

s can undergo thermal elimination via an E_i mechanism to yield vinyl

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

s and sulfenic acids: :R-S(O)CH2CH2-R' -> R-SOH + CH2=CH-R' Compounds which react in this manner are used as polymer stabilizers where they protects against long term heat ageing, structures based on thiodipropionate esters are popular. Sulfenate-based

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s are found at the

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate, the ''binding s ...

of the nitrile hydratases. The group is proposed as the nucleophile that attacks the nitrile.

Other sulfenyl compounds

The prefix sulfenyl in organic nomenclature denotes the RS group (R ≠ H). One example is methane

sulfenyl chloride In organosulfur chemistry, a sulfenyl chloride is a functional group with the connectivity , where R is alkyl or aryl. Sulfenyl chlorides are reactive compounds that behave as sources of . They are used in the formation of and bonds. According ...

, CH₃SCl. Sulfenate esters have the formula RSOR′. They arise by the reaction of sulfenyl chlorides on alcohols. Sulfenate esters are intermediates in the Mislow-Evans rearrangement of allyl sulfoxides.Braverman, S., "Rearrangements involving sulfenic acids and their derivatives," in Sulfenic Acids and Derivatives, 1990, John Wiley & Sons. Sulfenamides have the formula RSNR′₂.

References

{{Functional Groups Organosulfur compounds