Environmental control of reactions. Influence of poly(L-glutamate) on the kinetics of decomposition of hydrogen peroxide catalysed by quaterpyridineiron (III) complex ions

Abstract

The decomposition of hydrogen peroxide catalysed by quaterpyridineiron(III) complex ions has been studied within the pH range 6.5–8. The reaction obeys total third-order kinetics with a partial order of two with respect to the substrate. At pH ≈ 7.6, where the reaction velocity exhibits a maximum asymptotic value, the apparent activation energy is 29.3±2.9 kJ mol^–1. Addition of poly (L-glutamate)(PLG) in solution determines a change in the overall kinetics. In this case, the process follows a total second-order equation and has an apparent activation energy of 53.1±6.3 kJ mol^–1. At a fixed concentration of complex, saturation phenomena are observed in all cases on increasing the concentration of substrate. As expected, the Lineweaver–Burk plot of the polymer-free complex-catalysed reaction is nonlinear, whereas it is linear when the complex + PLG system is used. At 25°C, the rate constant for the irreversible decomposition of the Michealis “complex”, which represents the rate-determining step, is k₃= 4.9±0.2 and 2.7±0.2 s^–1 for [Fe(tetpy) X₂]ⁿ⁺ and [Fe(tetpy) X₂]ⁿ⁺+ PLG catalytic systems, respectively. Implications of the role played by the poly-peptide matrix as environmental controller of the catalysis under study are briefly discussed.