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 ...

, a sulfenic acid is an

organosulfur compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfu ...

and oxoacid 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 about 1000x 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 sulfur hydride hydroxide, 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, a ...

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) 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 thiosulfinates, RS(O)SR, such as allicin from

garlic Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus ''Allium''. Its close relatives include the onion, shallot, leek, chive, Allium fistulosum, Welsh onion and Allium chinense, Chinese onion. It is native to South A ...

. Through the use of X-ray crystallography, 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 (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 hydroperoxides, 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 PRDX ...

are ubiquitous and abundant enzymes 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 and related compounds following tissue damage to

, onions, and other plants of the genus '' Allium''. 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 allicin, 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, esomeprazole, ticlopidine, clopidogrel, and prasugrel is proposed to involve sulfenic acid intermediates. Oxidation of

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

residues in protein to the corresponding protein sulfenic acids is suggested to be important in redox-mediated

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellula ...

. 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, 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 alkenes 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 ligands 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 (binding site) a ...

of the

nitrile hydratase Nitrile hydratases (NHases; ) are mononuclear iron or non-corrinoid cobalt enzymes that catalyse the hydration of diverse nitriles to their corresponding amides R-C≡N + H2O → R-C(O)NH2 Metal cofactor In biochemistry, cobalt is in gener ...

s. 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