Kinetics and mechanism of oxidation of apple pectin by CrVI in aqueous acid medium

Selective oxidation of galacturonic acid residues of apple pectin by Cr-VI affords CO2/HCO2H, oxidized pectin, and Cr-III as final redox products. The reaction shows first-order kinetics in [pectin], [Cr-VI], and [H+], at fixed ionic strength and temperature. Kinetic studies show that the redox reaction proceeds through a mechanism combining Cr-VI --> Cr-IV --> Cr-II and Cr-VI --> Cr-IV --> Cr-III pathways. The mechanism is supported by the observation of free radicals, CrO22+ (the formation of which implies involvement of Cr-II and Cr-IV) and Cr-V (formed in monoelectronic redox processes) as reaction intermediates. The reduction of Cr-IV and Cr-V by pectin was independently studied and found to occur more than 10(3) times faster than pectin + Cr-VI reaction, in acid medium. At pH 3-5, apple pectin + Cr-VI redox reaction is slow, oxo-chromate(V)-pectin species stabilize and remain in solution during several hours. The present results show that these abundant and ubiquitous components of the cell walls of all land plants are able to reduce CrIV-VI or stabilize high-valent chromium depending on pH. Copyright (C) 2008 John Wiley & Sons, Ltd.