Kinetics and mechanisms: Reaction of iodine and triiodide with thiosulfate; decomposition of copper(III)glycylglycyl-L -histidine and copper(III)glycylglycyl-alpha -hydroxyhistamine

William Michael Scheper, Purdue University

Abstract

Aqueous iodine and triiodide ion react rapidly with thiosulfate ion in a multistep mechanism: (UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\eqalign{&\rm I\sb2 + I\sp- \ {{k\sb1\atop\longrightarrow}\atop{\longleftarrow\atop K\sb{-1}}}\ I\sbsp{3}{-}\cr&\rm I\sb2 + S\sb2O\sbsp{3}{2-}\ {{k\sb2\atop\longrightarrow}\atop{\longleftarrow\atop K\sb{-2}}}\ I\sb2S\sb2O\sbsp{3}{2-}\cr&\rm I\sbsp{3}{-} + S\sb2O\sbsp{3}{2-}\ {{k\sb3\atop\longrightarrow}\atop{\longleftarrow\atop K\sb{-3}}}\ I\sb2S\sb2O\sbsp{3}{2-} + I\sp-\cr&\rm I\sb2S\sb2O\sbsp{3}{2-}\ {{k\sb4\atop\longrightarrow}\atop{\longleftarrow\atop K\sb{-4}}}\ IS\sb2O\sbsp{3}{-} + I\sp-\cr&\rm IS\sb2O\sbsp{3}{-} + S\sb2O\sbsp{3}{2-}\ {k\sb5\atop\longrightarrow}\ I\sp- + S\sb4O\sbsp{6}{2-}\cr}$$(TABLE/EQUATION ENDS) Rate constants (25.0 $\pm$ 0.2$\sp\circ$C, $\mu$ = 0.10) measured by pulsed-accelerated-flow and stopped-flow methods are: k$\sb2$ = 7.8 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$, k$\sb{-2}$ = 2.5 $\times$ 10$\sp2$ s$\sp{-1}$, k$\sb3$ = 4.2 $\times$ 10$\sp8$ M$\sp{-1}$ s$\sp{-1}$, k$\sb{-3}$ = 9.5 $\times$ 10$\sp3$ M$\sp{-1}$ s$\sp{-1}$, k$\sb4$/k$\sb{-4}$ = 0.26 M, k$\sb5$ = 1.28 $\times$ 10$\sp6$ M$\sp{-1}$ s$\sp{-1}$. The I$\sb2$S$\sb2$O$\sb3\sp{2-}$ adduct is thermodynamically stable (k$\sb2$/k$\sb{-2}$ = K$\sb2$ = 3.2 $\times$ 10$\sp7$ M$\sp{-1}$), but is kinetically reactive. It dissociates rapidly to give IS$\sb2$O$\sb3\sp-$ which reacts with S$\sb2$O$\sb3\sp{2-}$ to eliminate I$\sp-$ and form S$\sb4$O$\sb6\sp{2-}$. Cu(III) (H$\sb{-2}$GlycylGlycyl-L-Histidine) has a reduction potential of 0.98 V (vs NHE) and decomposes rapidly, 0.7 s$\sp{-1}$ (pH = 6.3, $\mu$ = 1.0 M, 5$\sp\circ$C). The oxidation products are CO$\sb2$ and Cu(II) (GlycylGlycyl-NHCH(OH)CH$\sb2$IM) or Cu(II) (GGP). This Cu(II)product has a reduction potential of 1.02 V. The Cu(III) (H$\sb{-2}$GGP) decomposes more slowly than Cu(III) (H$\sb{-2}$GGH). From kinetic data, the pK$\sb{\rm a}$ of terminal amine deprotonation for Cu(III) (H$\sb{-2}$GGP) is 9.5 (5$\sp\circ$C, $\mu$ = 1.0). This is higher than the value of 8.7 measured for Cu(III) (H$\sb{-2}$GGH) by electrochemical methods. The oxidation product of the Cu(III) (H$\sb{-2}$GGP) decomposition is Cu(II) (GlycylGlycyl-N(OH)CH(OH)CH$\sb2$IM). The corresponding Cu(III) complex has a reduction potential of 1.2 V, and is slower to decompose than Cu(III) (H$\sb{-2}$GGH). It is proposed that the Cu(III) (H$\sb{-2}$GG-NHCH(OH)CH$\sb2$IM)$\sp+$ and the Cu(III) (GG-N(OH)CH(OH)CH$\sb2$IM)$\sp{2+}$ complexes decay by the loss of an proton from the $\alpha$-carbon of the histamine residue, while the Cu(III) (H$\sb{-2}$GGH) decays by the loss of carbon dioxide from the $\alpha$-carbon of histidine.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry|Chemistry

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