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CHEMISTRY The thiol group is the sulfur analog of the s, Thioacetal s and Thioester s, which are analogous to Ethers , Acetal s, and Esters . Furthermore, a thiol group can react with an Alkene to create a thioether. (In fact, biochemically, thiol groups may react with Vinyl groups to form a Thioether Linkage . The sulfur atom of a thiol is quite Nucleophilic , rather more so than the oxygen atom of an alcohol. The thiol group is fairly acidic with a usual PKa around 10 to 11. In the presence of a Base , a thiolate Anion is formed which is a very powerful nucleophile. The group and its corresponding anion are readily Oxidized by reagents such as Bromine to give an organic Disulfide (R-S-S-R), or by more powerful reagents such as Sodium Hypochlorite to yield Sulfonic Acid s (RSO3H). Because of the small Electronegativity difference between sulfur and hydrogen, an S-H bond is practically Nonpolar Covalent . Thiols show little association by Hydrogen Bonding . They have lower Boiling Point s and are less Soluble in water and other Polar Solvents than alcohols of similar molecular weight. Many thiols are colorless Liquid s having an Odor resembling that of Garlic . The odor of thiols is often strong and repulsive, particularly for those of low molecular weight. Thiols bind strongly to Skin Protein s, and are responsible for the intolerable, persistent odor produced by Feces , rotting flesh and the spraying of Skunk s. Natural Gas distributors began adding various forms of pungent thiols, usually Ethanethiol , to natural gas, which is naturally odorless, after the deadly 1937 New London School Explosion in Texas , United States . Thiols are also responsible for a class of Wine Fault s caused by an unintended reaction between sulfur and Yeast . However, not all thiols have unpleasant odors. For example, Grapefruit Mercaptan , a Monoterpenoid thiol, is responsible for the characteristic scent of Grapefruit . BIOLOGICAL IMPORTANCE As the functional group of the Amino Acid Cysteine , the thiol group plays an important role in biological systems. When the thiol groups of two cysteine residues (as in monomers or constituent units) are brought near each other in the course of Protein folding, an Oxidation Reaction can create a Cystine unit with a Disulfide Bond (-S-S-). Disulfide bonds can contribute to a protein's Tertiary Structure if the cysteines are part of the same Peptide chain, or contribute to the Quaternary Structure of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. The Heavy and Light Chain s of Antibodies are held togehter by disulfide bridges. Also, the kinks in curly Hair are a product of cystine formation. Permanent s take advantage of the oxidizability of cysteine residues. The chemicals used in hair straightening are Reductant s that reduce cystine disulfide bridges to free cysteine sulfhydryl groups, while chemicals used in hair curling are Oxidant s that oxidize cysteine sulfhydryl groups to form cystine disulfide bridges. Sulfhydryl groups in the Active Site of an Enzyme can form Noncovalent bonds with the enzyme's Substrate as well, contributing to Catalytic Activity . Active site cysteine residues are the functional unit in Cysteine Proteases . NOMENCLATURE When a thiol group is a Substituent on an Alkane , there are several ways of naming the resulting thiol:
ETYMOLOGY The term ''mercaptan'' comes from the Latin ''mercurius captans'', meaning 'laying hold of mercury,' because the –SH group binds tightly to the element Mercury . EXAMPLES OF THIOLS
SEE ALSO EXTERNAL LINKS
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