Reactive sulfur species: kinetics and mechanism of the equilibrium between cysteine sulfenyl thiocyanate and cysteine thiosulfinate ester in acidic aqueous solution.
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.
Lemma
Kelemu
K
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
Ashby
Michael T
MT
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*
D000143:Acids; D004952:Esters; D012996:Solutions; D013861:Thiocyanates; D013886:Thiosulfonic Acids; D003553:Cystine; D008623:Mercaptoethanol; D013455:Sulfur; C031760:thiocyanate