Document Detail

Reactive sulfur species: kinetics and mechanism of the equilibrium between cysteine sulfenyl thiocyanate and cysteine thiosulfinate ester in acidic aqueous solution.
MedLine Citation:
PMID:  18351774     Owner:  NLM     Status:  MEDLINE    
The kinetics and mechanism of the hydrolysis of cysteine sulfenyl thiocyanate (CySSCN) to give cysteine thiosulfinate ester (CyS(=O)SCy) have been investigated between pH 0 and 4. The reaction is reversible. The hydrolysis of CySSCN is second-order in [CySSCN] and inverse first-order in [H+] and [SCN-]. The following mechanism is proposed for the hydrolysis of CySSCN (where the charge depends upon the pH): CySSCN0/+ + H2O <==>CySOH0/+ + SCN- + H+, CySOH0/+ + CySSCN0/+ --> CyS(=O)SCy0/+/2+ + SCN- + H+; k1 = 3.36 +/- 0.01 x 10-3 s-1, K1k2 = 0.13 +/- 0.05 Ms-1 (which yields k2/k-1 = 39 M). The observed rate law rules out alternative mechanisms for 1 <or= pH <or= 4, including the condensation of two molecules of cysteine sulfenic acid (CySOH) to give CyS(=O)SCy. The equilibrium can be approached from the opposite direction. The reaction of CyS(=O)SCy with SCN- to give CySSCN is first-order in [CyS(=O)SCy], [SCN-], and [H+] (for [H+] > 0.4 M). The following mechanism is proposed: CyS(=O)SCy2+ + H+ <==> CyS(OH)=SCy3+, Ka; CyS(OH)SCy3+ + SCN- --> CySOH+ + CySSCN+, k-2 = 0.239 +/- 0.007 M-2s-1/Ka M-1. Since cysteine sulfenic acids are known to play an important function in many enzymes, and SCN- exists in abundance in physiologic fluids, we discuss the possible role of sulfenyl thiocyanates in vivo.
Kelemu Lemma; Michael T Ashby
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Publication Detail:
Type:  Journal Article; Research Support, N.I.H., Extramural; Research Support, Non-U.S. Gov't; Research Support, U.S. Gov't, Non-P.H.S.     Date:  2008-03-20
Journal Detail:
Title:  The Journal of organic chemistry     Volume:  73     ISSN:  0022-3263     ISO Abbreviation:  J. Org. Chem.     Publication Date:  2008 Apr 
Date Detail:
Created Date:  2008-04-11     Completed Date:  2008-06-09     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  2985193R     Medline TA:  J Org Chem     Country:  United States    
Other Details:
Languages:  eng     Pagination:  3017-23     Citation Subset:  IM    
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
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MeSH Terms
Acids / chemistry*
Cystine / chemistry*
Esters / chemistry*
Hydrogen-Ion Concentration
Mercaptoethanol / chemistry
Sulfur / chemistry*
Thiocyanates / chemistry*
Thiosulfonic Acids / chemistry*
Grant Support
1 R21 DE016889-01A2/DE/NIDCR NIH HHS
Reg. No./Substance:
0/Acids; 0/Esters; 0/Solutions; 0/Thiocyanates; 0/Thiosulfonic Acids; 302-04-5/thiocyanate; 56-89-3/Cystine; 60-24-2/Mercaptoethanol; 7704-34-9/Sulfur

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