Interaction of oxidation products from caffeic acid with Fe(III) and Fe(II)

Phenolic substances in the soil-plant system can be oxidized by metal ions, inorganic components, molecular oxygen as well as by phenol-oxidases, giving rise to the formation of products of low or high molecular weight. Interactions of these products with iron, in both reduced and oxidized form, can affect the iron mobility in soil and rhizosphere, and thus its availability to plants. Here we report the results of a study on the complexing and reducing activity of the oxidation products from caffeic acid (CAF), obtained via electrochemical means, towards Fe(III) and Fe(II) in aqueous solution in the 3.0-6.0 pH range. The HPLC analysis of the filtered solutions after the CAF oxidation showed the formation of two main groups of products: (i) CAF oligomers formed through radicalic reactions which do not involve the double bond of the CAF lateral chain and (ii) products where this bond is involved. These oxidation products (COP) were found to interact with both Fe(III) and Fe(II) with formation of soluble and insoluble Fe(Ill)-, and Fe(II)-COP complexes. The COP were found to be able to reduce Fe(III) to Fe(II) mainly at pH < 4.0. A low redox activity was observed at pH greater than or equal to 4.5 due to Fe(III) hydrolysis reactions as well as to the decrease in the redox potential of the Fe(III)/Fe(II) couple. Formation of hydroxy Fe(III)-COP polymers occurs at pH > 3.5.