title: Reactive sulfur species: kinetics and mechanism of the equilibrium between cysteine sulfenyl thiocyanate and cysteine thiosulfinate ester in acidic aqueous solution.
abstract: 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.
authors: Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
authors: Kelemu
authors: K
authors: Lemma
authors: Michael T
authors: MT
authors: Ashby
mesh_terms: D000143:Acids / Q000737:chemistry*; D003553:Cystine / Q000737:chemistry*; D004952:Esters / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008623:Mercaptoethanol / Q000737:chemistry; D012996:Solutions; D013455:Sulfur / Q000737:chemistry*; D013861:Thiocyanates / Q000737:chemistry*; D013886:Thiosulfonic Acids / Q000737:chemistry*
chemical_list: D000143:Acids; D004952:Esters; D012996:Solutions; D013861:Thiocyanates; D013886:Thiosulfonic Acids; D003553:Cystine; D008623:Mercaptoethanol; D013455:Sulfur; C031760:thiocyanate