Chemistry:Thiyl radical

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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 regularly invoked as intermediates in some biochemical reactions.

Formation

Thiyl radicals are generated by hydrogen-atom abstraction from thiols. The radical initiator AIBN is proposed to generate thiyl radicals from thiols:[2]

RN=NR → 2 R• + N2
R• + R′SH → R′S• + RH

Thiyl radicals rapidly formation by the action of OH· radical (k = 6.8 x 109 M−1s−1) on thiols.[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).

Structure

Thiyl radicals have rarely been isolated and purified. Claims of their existence have been often disputed, reflecting their high reactivity.[4]

Reactions

The main reaction of thiyl radicals their reversion to disulfides:[4]

2 RS · → RS–SR

Thiyl radicals are intermediates in the thiol-ene reaction, which is the basis of some polymeric coatings and adhesives. Thiyl radicals catalyze diverse reactions involving unsaturated substrates.[5]

Thiyl radical in biology

Thiyl radicals in vivo primarily are derived from the amino acid residue cysteine.

Thiyl radicals are involved in the mechanism of action of ribonucleotide reductase, an enzyme that catalyzes the formation of deoxyribonucleotides from ribonucleotides.[6] It catalyzes this formation by removing the 2'-hydroxyl group of the ribose ring of nucleoside diphosphates (or triphosphates depending on the class of RNR).

Other important substrates of thiyl radicals in biological systems are lipids, where thiyl radicals promote peroxidation.[7] In this process, thiyl radicals act as chain transfer catalysts by transferring the unpaired electron to a new lipid, thereby.[7] Other substrates of thiyl radicals include other proteins (k = 1.4 x 105 M−1s−1),[8] monounsaturated fatty acids (MUFAs) (k = 1.6 x 105 M−1s−1),[9] and ubiquinone (k = 2.5 x 103 M−1s−1). The addition of lipophilic thiols in cell culture or administration to C. elegans accelerated lipid peroxidation, caused damage to membrane proteins and was associated with a decline in polyunsaturated fatty acids (PUFAs) and a shortened lifespan.[10][11]

Elimination of thiyl radicals

Phenolic antioxidants, such as ubiquinone or α-tocopherol, are inefficient scavengers of thiyl radicals.[7][12][13][3] 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.[14][15][16] Isoprenoid polyenes, such as carotenoids like lycopene, react at very high rates with thiyl radicals (up to 109 M−1s−1).[17] In aqueous media ascorbic acid and glutathione react rapidly with thiyl radicals (>108 M−1s−1) and are present in high concentrations. Thus, in aqueous environments, thiyl radicals can be effectively neutralized by these antioxidants.

References

  1. ↑ Dénès, F.; Pichowicz, M.; Povie, G.; Renaud, P. (2014). "Thiyl Radicals in Organic Synthesis". Chemical Reviews 114 (5): 2587–2693. doi:10.1021/cr400441m. PMID 24383397. 
  2. ↑ Hoyle, C. E.; Lee, T. Y.; Roper, T. (2004). "Thiol–enes: Chemistry of the Past with Promise for the Future". Journal of Polymer Science Part A: Polymer Chemistry 42 (21): 5301–5338. doi:10.1002/pola.20366. Bibcode: 2004JPoSA..42.5301H. 
  3. ↑ 3.0 3.1 3.2 Tartaro Bujak, Ivana; Mihaljević, Branka; Ferreri, Carla; Chatgilialoglu, Chryssostomos (2016-11-01). "The influence of antioxidants in the thiyl radical induced lipid peroxidation and geometrical isomerization in micelles of linoleic acid" (in en). Free Radical Research 50 (sup1): S18–S23. doi:10.1080/10715762.2016.1231401. ISSN 1071-5762. PMID 27776460. https://www.tandfonline.com/doi/full/10.1080/10715762.2016.1231401. 
  4. ↑ 4.0 4.1 Sneeden, Eileen Y.; Hackett, Mark J.; Cotelesage, Julien J. H.; Prince, Roger C.; Barney, Monica; Goto, Kei; Block, Eric; Pickering, Ingrid J. et al. (2017). "Photochemically Generated Thiyl Free Radicals Observed by X-ray Absorption Spectroscopy". Journal of the American Chemical Society 139 (33): 11519–11526. doi:10.1021/jacs.7b05116. PMID 28750509. Bibcode: 2017JAChS.13911519S. 
  5. ↑ Kosaka, S.; Kurebayashi,K.; Yamato, N.; Tanaka, H.; Haruta, N.; Yamamoto, M. (2025). "Thiyl Chemistry: Cysteine-Catalyzed Maleate Isomerization via Aqueous Thiyl Radical Processes". Green Chemistry 27 (10): 2743–2750. doi:10.1039/d4gc06310d. 
  6. ↑ Stubbe, Joanne; Nocera, Daniel G.; Yee, Cyril S.; Chang, Michelle C. Y. (2003). "Radical Initiation in the Class I Ribonucleotide Reductase: Long-Range Proton-Coupled Electron Transfer?". Chemical Reviews 103 (6): 2167–2202. doi:10.1021/cr020421u. PMID 12797828. 
  7. ↑ 7.0 7.1 7.2 Moosmann, Bernd; Hajieva, Parvana (2022-04-29). "Probing the Role of Cysteine Thiyl Radicals in Biology: Eminently Dangerous, Difficult to Scavenge" (in en). Antioxidants 11 (5): 885. doi:10.3390/antiox11050885. ISSN 2076-3921. PMID 35624747. 
  8. ↑ Nauser, Thomas; Pelling, Jill; Schöneich, Christian (2004-10-01). "Thiyl Radical Reaction with Amino Acid Side Chains: Rate Constants for Hydrogen Transfer and Relevance for Posttranslational Protein Modification" (in en). Chemical Research in Toxicology 17 (10): 1323–1328. doi:10.1021/tx049856y. ISSN 0893-228X. https://pubs.acs.org/doi/10.1021/tx049856y. 
  9. ↑ Chatgilialoglu, Chryssostomos; Ferreri, Carla (2005-06-01). "Trans Lipids: The Free Radical Path" (in en). Accounts of Chemical Research 38 (6): 441–448. doi:10.1021/ar0400847. ISSN 0001-4842. PMID 15966710. https://pubs.acs.org/doi/10.1021/ar0400847. 
  10. ↑ Heymans, Victoria; Kunath, Sascha; Hajieva, Parvana; Moosmann, Bernd (2021-11-08). "Cell Culture Characterization of Prooxidative Chain-Transfer Agents as Novel Cytostatic Drugs" (in en). Molecules 26 (21): 6743. doi:10.3390/molecules26216743. ISSN 1420-3049. PMID 34771157. 
  11. ↑ Kunath, Sascha; Schindeldecker, Mario; De Giacomo, Antonio; Meyer, Theresa; Sohre, Selina; Hajieva, Parvana; von Schacky, Clemens; Urban, Joachim et al. (September 2020). "Prooxidative chain transfer activity by thiol groups in biological systems" (in en). Redox Biology 36. doi:10.1016/j.redox.2020.101628. PMID 32863215. 
  12. ↑ Denisova, T. G.; Denisov, E. T. (May 2009). "Reactivity of natural phenols in radical reactions" (in en). Kinetics and Catalysis 50 (3): 335–343. doi:10.1134/S002315840903001X. ISSN 0023-1584. http://link.springer.com/10.1134/S002315840903001X. 
  13. ↑ Chatgilialoglu, Chryssostomos; Zambonin, Laura; Altieri, Alessio; Ferreri, Carla; Mulazzani, Quinto G; Landi, Laura (December 2002). "Geometrical isomerism of monounsaturated fatty acids: thiyl radical catalysis and influence of antioxidant vitamins" (in en). Free Radical Biology and Medicine 33 (12): 1681–1692. doi:10.1016/S0891-5849(02)01143-7. PMID 12488136. https://linkinghub.elsevier.com/retrieve/pii/S0891584902011437. 
  14. ↑ Granold, Matthias; Hajieva, Parvana; Toşa, Monica Ioana; Irimie, Florin-Dan; Moosmann, Bernd (2018-01-02). "Modern diversification of the amino acid repertoire driven by oxygen" (in en). Proceedings of the National Academy of Sciences 115 (1): 41–46. doi:10.1073/pnas.1717100115. ISSN 0027-8424. PMID 29259120. Bibcode: 2018PNAS..115...41G. 
  15. ↑ Traber, Maret G.; Atkinson, Jeffrey (July 2007). "Vitamin E, antioxidant and nothing more" (in en). Free Radical Biology and Medicine 43 (1): 4–15. doi:10.1016/j.freeradbiomed.2007.03.024. PMID 17561088. 
  16. ↑ Ohlow, Maike J.; Granold, Matthias; Schreckenberger, Mathias; Moosmann, Bernd (2012-03-23). "Is the chromanol head group of vitamin E nature's final truth on chain-breaking antioxidants?" (in en). FEBS Letters 586 (6): 711–716. doi:10.1016/j.febslet.2012.01.022. ISSN 0014-5793. PMID 22281199. Bibcode: 2012FEBSL.586..711O. https://febs.onlinelibrary.wiley.com/doi/10.1016/j.febslet.2012.01.022. 
  17. ↑ Mortensen, Alan; Skibsted, Leif H.; Sampson, Julia; Rice-Evans, Catherine; Everett, Steven A. (1997-11-24). "Comparative mechanisms and rates of free radical scavenging by carotenoid antioxidants" (in en). FEBS Letters 418 (1–2): 91–97. doi:10.1016/S0014-5793(97)01355-0. ISSN 0014-5793. PMID 9414102. Bibcode: 1997FEBSL.418...91M. https://febs.onlinelibrary.wiley.com/doi/10.1016/S0014-5793%2897%2901355-0. 



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