REDOX ACTIVITY OF CAFFEIC ACID TOWARDS IRON(III) COMPLEXED IN A POLYGALACTURONATE NETWORK

The transfer of several metal ions from the soil to the plant absorbin g cells is mediated principally by organic molecules of low molecular weight with complexing and reducing activity, among which caffeic acid (CAF) is particularly important. Here we report the results of a surv ey which deals with the oxidation of CAF by the Fe(III) ions bound to a polygalacturonate network (Fe(III)-PGA network). The interaction bet ween Fe(III) and CAF was studied by using Fe(III)-PGA networks equilib rated in the 2.4-7.0 pH range by means of kinetic and spectroscopic me thods. The reducing power was found to depend on the nature of the Fe( III)-PGA network complexes: when the ferric ion was complexed only by the PGA carboxylic groups, a high redox activity was observed, whereas the Fe(III) reduction was found to be lower when a hydroxylic group w as inserted in the Fe(III) coordination sphere. The iron complexed in the network was protected from hydrolysis reactions, as shown by the h igh pH values at which its reduction occurred. Two different fractions of Fe(II) produced were identified, one diffusible and another exchan geable with CaCl2 6.0 mM. The existence of the exchangeable form was a ttributed to the electrostatic interaction of the Fe(II) ions with the carboxylate groups of the fibrils and with the degradation products o f CAE The arrangement of the fibrils was altered following the substit ution of Ca(II) by Fe(III) ions and was restored following the seducti on of Fe(III) by CAF.