V. Solinas

Reduction of the Fe(III)-desferrioxamine-B complexes by caffeic acid: A reduction mechanism of biochemical significance

Abstract The reduction of Fe(III) by caffeic acid (CAF) in the presence of desferrioxamine-B (DFOB) was studied as a function of pH and time. The HPLC analyses, UV/vis spectra and kinetic data provide evidence that the reduction of Fe(III) by caffeic acid is strongly inhibited by DFOB and that it can be induced by the siderophore at low pH values. The stoichiometry and the possible mechanism of the reduction process are reported.

Complexing and redox properties of the system D-galacturonic acid-iron(III)

Abstract The stoichiometry of the reduction reaction and D -galacturonic acid with iron(III) has been determined, leading to a 4:1 ratio between the quantities of reduced iron and oxidized D -galacturonic acid, respectively. This last molecule is oxidized to formic acid, and the residue is thought to present a further terminal carboxylic group. The reducing predominant species appears to be the FeGal 3 complex.

Analytical study of the interactions of D-galacturonic acid with iron(III) and iron(II) in solution and with iron(III)-bentonite

Abstract The interaction of d -galacturonic acid (HG) with iron(III) and iron(II) ions was studied in aqueous 1 M sodium perchlorate solutions. At pH 3 where the ligand molecules coordinate essentially by the carboxylic group. At higher pH values, complexes with further deprotonated ligand molecules are the main species in the system. There is some evidence that the chelates formed involve the open form of the sugar molecule, the iron(III) ions binding the oxygen corresponding to the ring oxygen of the closed form. In the heterogeneous d -galacturonic acid/Fe(III)-clay system, the sugar molecule appears to interact strongly with the bentonite, suggesting the formation of complex species in which all three compounds of the ternary system are involved.

Interaction of Oxidation Products from Caffeic Acid with Fe(III) and Fe(II)

Abstract Phenolic substances in the soil–plant system can be oxidized by metal ions, inorganic components, molecular oxygen as well as by phenoloxidases, 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(III)‐, 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 ≥ 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.

Role of the caffeic acid oxidation products on the iron mobilization at the soil-root interface

Previous results show that the reduction of Fe(III) by caffeiic acid (CAF) is strongly influenced by the pH of the reaction medium (DEIANA et al., 1995). In particular, it has been found that al pH > 3.8 the reducing activity of CAF towards the Fe(III) ions in solution is low, but it increases when Fe(III) is complexed as Fe(III)-polygalacturonate. The mobilization of the Fe(II) ions, which form upon the Fe(III) reduction, has been shown to depend mainly on the nature of the Fe(III)-polygalacturonate complexes (DEIANA et al., 1994a). In particular, it bas been observed a high redox activity of CAF towards Fe(III) when the metal ion is coordinated by the carboxilic groups of the macromolecule. The Fe(III) reduction has been found to decrease when an oxydrilic group was inserted in the Fe(III) coordination sphere. The Fe(II) produced partly diffuses into the external solution and partly is still strongly held by the polysaccharidic matrix. The oxidation of CAF by Fe(III) gives rise to the formation of products (OP) with different polymerization degree, some of which are similar to those found in natural systems (DEIANA et al., 1994b). In order to determine the role of these products in the Fe(III) reduction as well as in the mobilization of the Fe(II) produced it was set up an electrochemical method to synthetize these products (DEIANA et al., 1994b). Here are reported some results about the interaction which establish between the CAF oxidation products and iron in both oxidized and reduced form.

Influence of pH and of several organic acids on the interaction between esculetine and iron(III).

The availability of iron to plants is influenced by organic molecules of low molecular weight with complexing or reducing capacity such as organic acids and phenolic acids and their derivates. To provide information about the interactions between Fe(III) and esculetine (ESC) and the influence of organic acids on these interactions, the redox activity of ESC at different pH values in aqueous solution was investigated in the presence and in the absence of citric, malic, oxalic and pyruvic acid on systems with different Fe(III)/ESC molar ratios. At pH oxalic >malic >>pyruvic acid.

A molecular modeling study of the urease active site

Abstract Urease catalyzes the decomposition of urea to ammonium and carbamate ions by its active site which contains two nickel(II) atoms. Here we report the results of a molecular dynamics investigation performed on a system constituted of the cavity of the enzyme and one hydroxamic acid molecule which acts as an inhibitor of the protein. The results agree with experimental data and represent a valid starting point for the design of more efficient urease inhibitors.

Determination of vanadate(V) by conductometric anion chromatography

Analyse quantitative du vanadate(V) par chromatographie ionique avec detection conductimetrique