Reactive sulfur species: kinetics and mechanism of the hydrolysis of cysteine thiosulfinate ester.
Reactive sulfur species: kinetics and mechanism of the hydrolysis of cysteine thiosulfinate ester.
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活性硫物质：半胱氨酸硫代亚磺酸酯水解的动力学和机制。
DOI：
10.1021/tx700168z
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发表时间：
2007
影响因子：
4.1
通讯作者：
Ashby,MichaelT
中科院分区：
文献类型：
--
作者：
Nagy,Peter;Ashby,MichaelT
通过停流分光光度法研究了半胱氨酸硫代亚磺酸酯 (CyS(O)SCyx−,x= 0–2) 的水解动力学和机制。pH 6 和 pH 14 之间。在 pH < 13 时观察到氢氧化物的限速反应。在 pH 13 以上观察到更复杂的动力学，其中 CyS(O)SCy2− 的水解相对于后续反应可能更快。在所有反应条件下，水解的最终产物都是摩尔比为 1:1 的胱氨酸 (CySSCy) 和半胱氨酸亚磺酸 (CySO2H)。水解速率取决于 CyS(O)SCyx− 的质子态。此外，半胱氨酸硫代磺酸酯 (CyS(O)2SCy) 在较低 pH 下 CyS(O)SCyx− 水解过程中被观察到作为中间体。 CyS(O)2SCy 最终水解产生化学计量的 CySSCy 和 CySO2H。然而，在某些条件下观察到 CySO2H，其中 CyS(O)2SCy 的水解相对较慢，因此表明 CyS(O)SCyx− 存在多种水解途径。直至限速步骤的机制建议如下： CyS(O)SCy0= H++ CyS(O)SCy−, pKa3= 7.32； CyS(O)SCy−= H++ CyS(O)SCy2−, pKa4= 7.92； CyS(O)SCy0+ OH−→ 产物，P0k0= (5.0 ± 0.01) × 103M−1s−1; CyS(O)SCy−+ OH−→ 产物,P1k1= 60 ± 18 M−1s−1;和 CyS(O)SCy2−+ OH−→ 产物，P2k2= 0.36 ± 0.01 M−1s−1，其中 Px 是一个常数 (1 ≤ Px≤ 3)，它解释了可能的水解途径及其净反应的化学计量之间的分配。
The kinetics and mechanisms of the hydrolysis of cysteine thiosulfinate ester (CyS(O)SCyx−,x= 0–2) have been investigated by stopped-flow spectrophotometry between pH 6 and pH 14. The rate-limiting reaction of hydroxide is observed for pH < 13. More complicated kinetics are observed above pH 13, where the hydrolysis of CyS(O)SCy2−can be fast relative to subsequent reactions. The eventual products of hydrolysis are a 1:1 molar ratio of cystine (CySSCy) and cysteine sufinic acid (CySO2H) under all reaction conditions. The rate of hydrolysis is dependent upon the proton state of CyS(O)SCyx−. Furthermore, cysteine thiosulfonate ester (CyS(O)2SCy) was observed as an intermediate during the hydrolysis of CyS(O)SCyx−at lower pH. CyS(O)2SCy eventually hydrolyzes to give stoichiometric amounts of CySSCy and CySO2H. However, CySO2H is observed under some conditions for which hydrolysis of CyS(O)2SCy is relatively slow, thus suggesting multiple hydrolysis pathways for CyS(O)SCyx−. The mechanism up to the rate-limiting step is proposed to be as follows: CyS(O)SCy0= H++ CyS(O)SCy−, pKa3= 7.32; CyS(O)SCy−= H++ CyS(O)SCy2−, pKa4= 7.92; CyS(O)SCy0+ OH−→ products,P0k0= (5.0 ± 0.01) × 103M−1s−1; CyS(O)SCy−+ OH−→ products,P1k1= 60 ± 18 M−1s−1; and CyS(O)SCy2−+ OH−→ products,P2k2= 0.36 ± 0.01 M−1s−1, wherePxis a constant (1 ≤Px≤ 3) that accounts for the partitioning between the possible hydrolysis pathways and the stoichiometries of their net reactions.