Kinetics and mechanism for oxidation of L-proline by bis(hydrogen periodato)argentate(III) complex anion

Oxidation of L-proline by bis(hydrogen periodato)argentate(III) complex anion, [Ag(HIO6)(2)](5-), has been studied in aqueous alkaline medium in the temperature range of 25-40 V by use of conventional spectrophotometry. Under the conditions for kinetic measurements, the oxidation results in decarboxylation of proline, giving rise to gamma-aminobutyrate as identified by mass spectrometry. The oxidation kinetics is first order with respect to the silver(III) and proline concentrations; the second-order rate constants, decreasing with increasing [periodate], are essentially independent of [OH-] The kinetic results were interpreted in terms of a reaction mechanism which involves a pre-equilibrium between [Ag(HIO6)(2)](5-) and [Ag(HIO6)(H2O)(OH)](2-), a mono-periodate coordinated silver(III) complex. Both Ag(IIII) complexes are reduced parallelly by the fully deprotonated form of proline in rate-determining steps (described by k(1) for the former Ag(III) and k(2) for the latter one). The determined rate constants and their associated activation parameters are k(1)(25 degrees C)=(1.87 +/- 0.04) L.mol(-1).s(-1), Delta H-1(#)=(45 +/- 4) kJ.mol(-1), Delta S-1(#)=(-90 +/- 13) J.K-1.mol(-1) and k(2)(25 degrees C)= (3.2 +/- 0.5) L.mol(-1).s(-1), Delta H-2(#) =(34 +/- 2) kJ.mol(-1), Delta S-2(#) = (-122 +/- 10) J.K-1.mol(-1). This is also the firsttime to find that [Ag(HIO6)(2)](5-) is also reactive toward a particular reductant and shows a lower reactivity than [Ag(HIO6)(OH)(H2O)](2-).