In chemistry, a thiyl radical has the formula RS, sometimes written RS^• to emphasize that they are free radicals. R is typically an alkyl or aryl substituent. Because S–H bonds are about 20% weaker than C–H bonds, thiyl radicals are relatively easily generated from thiols RSH. ^[1] Thiyl radicals are intermediates in the thiol-ene reaction, which is the basis of some polymeric coatings and adhesives. They are generated by hydrogen-atom abstraction from thiols using initiators such as AIBN: ^[2]

RN=NR → 2 R^• + N₂

R^• + R′SH → R′S^• + RH

Thiyl radicals are also invoked as intermediates in some biochemical reactions.

The formation of thiyl radicals in vivo primarily occurs through the action of various radicals on the amino acid cysteine incorporated into proteins. The rate of radical formation is highest with the OH^· radical (k = 6.8 x 10⁹ M^-1s^-1) ^[3] and decreases through the H^· radical (k = 6.8 x 10⁹ M^-1s^-1) ^[3] down to peroxyl radicals R-CHOO^· (k = 4.2 x 10³ M^-1s^-1). One of the most important substrates of thiyl radicals in biological systems is lipids, where thiyl radicals play a crucial role in lipid peroxidation ^[4]. In this process, thiyl radicals act as chain transfer catalysts by transferring the unpaired electron to a new lipid, thereby accelerating lipid peroxidation ^[4]. Other substrates of thiyl radicals include other proteins (k = 1.4 x 10⁵ M^-1s^-1) ^[5], monounsaturated fatty acids ( MUFAs) (k = 1.6 x 10⁵ M^-1s^-1) ^[6], and ubiquinone (k = 2.5 x 10³ M^-1s^-1). Interestingly, the addition of lipophilic thiols in cell culture or administration to C. elegans accelerated lipid peroxidation at the same initiation rate, caused damage to membrane proteins, and was associated with a decline in polyunsaturated fatty acids ( PUFAs) and a shortened lifespan ^{[7] [8]}.

The most important phenolic antioxidants, such as ubiquinone or α-tocopherol, are not suitable scavengers of thiyl radicals ^[4]. Both substances are not sufficiently reactive ^{[9] [10] [11]}, and α-tocopherol is also not present in sufficient quantities to scavenge thiyl radicals. Nonetheless, both compounds have high rate constants for their reaction with peroxyl radicals, highlighting their evolutionary importance as scavengers ^{[12] [13] [14]}. Isoprenoid polyenes, such as carotenoids or lycopene, exhibit very high rate constants regarding thiyl radicals (up to 10⁹ M^-1s^-1) ^[15]. However, even with excessive supplementation, the effect of lycopene is not sufficient to adequately counteract lipid peroxidation ^[4]. The situation is significantly more promising in aqueous media: ascorbic acid and glutathione also have high rate constants (>10⁸ M^-1s^-1) and are present in sufficiently high concentrations, so in aqueous environments, thiyl radicals can be effectively neutralized by the aforementioned antioxidants.

This organic chemistry article is a stub. You can help Wikipedia by expanding it.

• v
• t
• e