Investigation of the mechanism of oxidation of l-cystine by diperiodatoargentate(III) in aqueous alkaline medium by stopped flow technique

Abstract

The kinetics of oxidation of l-cystine by diperiodatoargentate(III) (DPA) in alkaline medium at a constant ionic strength of 0.10 mol dm⁻³ was studied spectrophotometrically. The reaction exhibits a 1:2 stoichiometry (l-cys:DPA) and is first order in [DPA]. The order in both [l-cystine] and [alkali] changes from first to zero order as their concentrations increase. Added periodate retards the rate of reaction. The effects of added products have been investigated. The active species of silver(III) is identified as monoperiodatoargentate(III) (MPA). The oxidation is thought to proceed via an MPA–l-cystine complex, which decomposes in a rate-determining step to give a free radical followed by a fast step to give the products. The products were identified by spot test, IR and GC–MS. The reaction constants involved in different steps of the mechanism were evaluated. The activation parameters with respect to the slow step of the mechanism were computed and discussed.

Appendix

According to Scheme 1.

$$ \begin{gathered} {\frac{{ - {\text{d[DPA]}}}}{\text{dt}}} = {\text{k[C]}} = {\frac{{{\text{kK}}_{ 1} {\text{K}}_{ 2} {\text{K}}_{ 3} [ {\text{DPA][OH}}^{ - } ] [ {\text{{\sc l}\hbox{-}cys]}}}}{{[{\text{H}}_{3} {{\text{IO}}_{6}^{}}^{2 - } ]}}} \hfill \\ [ {\text{DPA]}}_{\text{T}} = [ {\text{DPA]}}_{\text{f}} + [ {\text{Ag(H}}_{ 3} {\text{IO}}_{ 6} ) ( {\text{H}}_{ 2} {\text{IO}}_{ 6} ) ]^{ 2- } + [ {\text{Ag(H}}_{ 2} {\text{IO}}_{ 6} ) ( {\text{H}}_{ 2} {\text{O)}}_{ 2} ]+ [ {\text{C]}} \hfill \\ \end{gathered} $$

(1)

where T and f refer to total and free concentrations.

$$ \begin{gathered} = {\text{[DPA]}}_{\text{f}} {\text{\{1}} + {\text{K}}_{ 1} [ {\text{OH}}^{ - } ]+ {\frac{{{\text{K}}_{ 1} {\text{K}}_{ 2} [ {\text{OH}}^{ - } ]}}{{ [ {\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - } ]}}} + {\frac{{{\text{K}}_{ 1} {\text{K}}_{ 2} {\text{K}}_{ 3} [ {\text{OH}}^{ - } ] [ {\text{{\sc l}\hbox{-}cys]}}}}{{ [ {\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - } ]}}} \hfill \\ [ {\text{DPA]}}_{\text{f}} = {\frac{{ [ {\text{DPA]}}_{\text{T}} [ {\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - } ]}}{[{{\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - }] + {\text{K}}_{ 1} [ {\text{OH}}^{ - } ] [ {\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - } ]+ {\text{K}}_{ 1} {\text{K}}_{ 2} [ {\text{OH}}^{ - } ]+ {\text{K}}_{ 1} {\text{K}}_{ 2} {\text{K}}_{ 3} [ {\text{OH}}^{ - } ] [ {\text{{\sc l}\hbox{-}cys]}}}}}. \hfill \\ \end{gathered} $$

(2)

$$ {\text{Similarly}},\; [ {\text{OH]}}_{\text{T}} = {\text{[OH]}}_{\text{f}} + [ {\text{Ag(H}}_{ 3} {\text{IO}}_{ 6} ) ( {\text{H}}_{ 2} {\text{IO}}_{ 6} ) ]^{ 2- } + [ {\text{Ag(H}}_{ 2} {\text{IO}}_{ 6} ) ( {\text{H}}_{ 2} {\text{O)}}_{ 2} ] $$

$$ = {\text{[OH]}}_{\text{f}} + {\text{K}}_{ 1} [ {\text{DPA] [OH}}^{ - } ]+ {\frac{{{\text{K}}_{ 1} {\text{K}}_{ 2} [ {\text{DPA][OH}}^{ - } ]}}{{[{\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - }] }}}. $$

In view of the low concentration of DPA and H₃IO₆ ²⁻ used,

$$ \begin{aligned} [{\text{OH}}]_{\text{T}} = & [{\text{OH}}]_{\text{f}} \\ [{\text{{\sc l}\hbox{-}cys}}]_{\text{T}} = & [{\text{{\sc l}\hbox{-}cys}}]_{\text{f}} + [{\text{C}}] \\ = & [{\text{{\sc l}\hbox{-}cys}}]_{\text{f}} + {\frac{{{\text{K}}_{1} {\text{K}}_{2} {\text{K}}_{3} [{\text{DPA}}] \, [{\text{OH}}^{ - } ][{\text{{\sc l}\hbox{-}cys}}]}}{{ [ {\text{H}}_{3} {{\text{IO}}_{6}^{}}^{2 - } ]}}}. \\ \end{aligned} $$

(3)

In view of the low concentration of DPA, OH and H₃IO₆ ²⁻ used,

$$ [ {\text{{\sc l}\hbox{-}cys]}}_{\text{T}} {\text{ = [{\text{{\sc l}\hbox{-}cys}}]}}_{\text{f}} . $$

(4)

Substituting (2), (3) and (4) in (1) and omitting the subscripts T and f, we get

$$ {\text{Rate}}={\frac{{{\text{k K}}_{ 1} {\text{K}}_{ 2} {\text{K}}_{ 3} {\text{ [DPA][{\text{{\sc l}\hbox{-}cys}}] [OH}}^{ - } ]}}{[{{\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - }] + {\text{K}}_{ 1} [ {\text{OH}}^{ - } ] [ {\text{H}}_{ 3} {{\text{IO}}_{6}^{}}^{2 - } ] + {\text{K}}_{ 1} {\text{K}}_{ 2} [ {\text{OH}}^{ - } ]+ {\text{K}}_{ 1} {\text{K}}_{ 2} {\text{K}}_{ 3} [ {\text{OH}}^{ - } ][{\text{{\sc l}\hbox{-}cys}}]}}}. $$