S. Deiana

Urease from the soil bacterium Bacillus pasteurii: immobilization on Ca-polygalacturonate.

Abstract Urease purified from the soil bacterium Bacillus pasteurii was adsorbed and immobilized on a preformed network of Ca-polygalacturonate, a substrate which has a similar composition and morphology to the mucigel present at the root-soil interface. The adsorption proceeded with an essentially quantitative yield, and the immobilized enzyme showed no decrease of specific activity with respect to the free enzyme. The dependence of urease adsorption on NaCl concentration suggested that the enzyme is bound to the carrier gel through electrostatic interactions. The immobilized enzyme showed increased stability, with respect to the free enzyme, with increasing time or temperature, and in the presence of proteolytic enzymes. The pH activity profile revealed that the adsorbed enzyme showed no change in the optimum pH (8.0), but it was more active than the free form in the pH range 5–8. The Michaelis-Menten kinetic pararneters V max and K m were measured for the free ( V max = 1960 ± 250 units ml −1 ; K m = 235 ± 20 mM) and immobilized ( V max = 1740 ± 185 units ml −1 ; K m = 315 ± 25 mM) urease. The substantial similarity of V max in the two cases suggests that there were no conformational changes involving the active site upon enzyme immobilization, while substrate partitioning effects between the bulk solution and the micro-environment surrounding the immobilized enzyme must be operating so as to partly increase its K m . These results suggest that bacterial urease present in plant root mucigel plays a large role in the mobilization of urea N. Its activity is in fact significantly mantained and protected by immobilization on hydrophilic gels such as those produced by root exudates.

Analytical and spectroscopic characterization of humic acids extracted from sewage sludge, manure, and worm compost.

Humic acids extracted from sewage sludges, manure, and worm compost have been characterized by chemical and spec-troscopic methods. Meaningful differences in the composition were revealed by FT IR, 1H, 13C NMR, and visible spectroscopies. These differences allow a differentiation amomg the products depending on the source from which they were obtained. Humic acid extracted from sewage sludges contains the highest percentage of aliphatic carbon, associated with polysaccharides and proteinaceous structures, and has characteristics close to those of aquatic humic acids. On the other hand, humic acids from manure and worm compost are similar to the humic acids originating from soil.

The binding mechanism of urea, hydroxamic acid and N-(N-butyl)-phosphoric triamide to the urease active site. A comparative molecular dynamics study

Abstract Molecular dynamics (MD) calculations have been performed on the active site of urease from Klebsiella aerogenes and its adducts with urea, hydroxamic acid and N -( n -butyl)-phosphoric triamide (NBPT). The catalytic nickel atoms were explicitly included in all the simulations. The nickel atoms, as shown by X-ray analysis, are linked by a carbamate bridge. Average MD structures were calculated starting from the X-ray structure of the urease active site and docking the ligand and the inhibitors from different starting conformations. The urea molecule binds to only one of the Ni atoms, the hydroxamic acid behaves like a monodentate or a bidentate ligand depending on the pH, and NBPT coordinates both the Ni centers and a carbamate residue via the formation of and hydrogen bond thus acting as a tridentate ligand. The proposed binding mechanisms agree with known data about hydroxamic and urea complexes with urease.

Coordination of transition-metal ions by polygalacturonic acid: A spectroscopic study

The interaction between polygalacturonic acid with Cu(II), VO(IV), Mn(II), Ni(II) and Co(II) has been studied with the aid of ESR, electronic and IR spectra. The results show that in fully hydrated gels Cu(II) and VO(IV) ions form inner-sphere carboxylate complexes, whereas [Mn(H2O)6]2+ retains its inner hydration sphere. On dehydration, all the ions are bound to carboxyl groups, whose binding mode is dependent on the nature of the ion. The changes observed in spectral parameters upon dehydration suggest the substitution of water molecules by sugar oxygen atoms in order to preserve the stereogeometry of the ions.

Ca-polygalacturonate as a model for a soil-root interface

The transfer of nutrients from the soil to root cells is mediated by a mucilaginous interface composed predominantly of polysaccharides which are characterized by a fibrillar structure. A network of Ca-polygalacturonate was formed on garlic roots. This artificial network was compared with naturally formed mucilage structures using transmission (TEM) and scanning electron microscopy (SEM). The synthetic Ca-polygalacturonate could serve as a model for the study of the ionic transfer in the soil-root interface.

Copper(II) and lead(II) removal from aqueous solution by water treatment residues

In this study, we investigated the ability of Fe- and Al-based water treatment residues (Fe- and Al-WTR) to accumulate Pb(II) and Cu(II) at pH 4.5. The role of the inorganic and organic fractions of WTRs in metals sorption was also assessed. Sorption isotherms showed a higher sorption of Pb(II) by both WTRs with respect to Cu(II) (e.g. 0.105 and 0.089 mmol g(-1) of Pb(II) and Cu(II) respectively sorbed by Fe-WTR). Fe-WTR revealed a stronger sorbent for both metals than Al-WTR. The amount of Pb(II) and Cu(II) sorbed by Fe-WTR was about the 69% and 63% higher than that sorbed by the Al-WTR. The organic matter of Fe- and Al-WTR contributed to about 26% and 8.5% respectively in the sorption of both metals. The sequential extraction procedure showed that the greatest amount of metals sorbed by both WTRs were tightly bound and not extractable, and this was particularly apparent for Cu(II). The FT-IR spectra indicated the formation of inner-sphere complexes between the Fe(Al)-O nucleus and Pb(II) and Cu(II). Moreover, the FT-IR spectra also suggested that the humic fraction of WTRs interacted, through the carboxylate groups, with Cu(II) and Pb(II) by forming mainly monodentate and bidentate complexes, respectively.

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 absorbing 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 survey which deals with the oxidation of CAF by the Fe(III) ions bound to a polygalacturonate network (Fe(III)-PGA network). The interaction between Fe(III) and CAF was studied by using Fe(III)-PGA networks equilibrated in the 2.4-7.0 pH range by means of kinetic and spectroscopic methods. 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 was inserted in the Fe(III) coordination sphere. The iron complexed in the network was protected from hydrolysis reactions, as shown by the high pH values at which its reduction occurred. Two different fractions of Fe(II) produced were identified, one diffusible and another exchangeable with CaCl2 6.0 mM. The existence of the exchangeable form was attributed to the electrostatic interaction of the Fe(II) ions with the carboxylate groups of the fibrils and with the degradation products of CAF. The arrangement of the fibrils was altered following the substitution of Ca(II) by Fe(III) ions and was restored following the reduction of Fe (III) by CAF.

Spectroscopic studies on the binding of Iron(II) and (III) ions to polygalacturonic acid

Abstract The iron(II) and (III) complexes formed by polygalaturonic acid have been investigated by means of Mossbauer and ESR spectra. It is observed that the binding of iron(II) is independent on the water content of the samples. In fact, different hexacoordinated species are revealed by Mossbauer spectra in fully hydrated, air-dried and anhydrous samples. On the other hand, iron(III) ions give rise to polynuclear structures, which are not destroyed upon salvation and are stable over a wide pH range. The results provide plausible explanation of the mechanisms governing the iron uptake and transport in plants.

Zinc(II) adsorption on aluminium hydroxide

Abstract The adsorption of zinc(II) on aluminium hydroxide from ZnSO4 or ZnCl2 solutions in the presence of anions such as chloride, perchlorate or sulphate has been investigated. The results substantiate that the adsorption process depends on several interactive processes occurring in solution (e.g., zinc speciation) and at the oxide—solution interface (specific anion binding). It is suggested that chloride promotes the sorption of chloro-complexes, sulphate enhances the cationic adsorption by making the surface potential more negative, and perchlorate behaves as an indifferent anion. Specific adsorption of cationic species promotes the binding of adsorbing anions.

Interaction of transition-metal ions with polygalacturonic acid: A potentiometric study

Abstract The complexes formed by polygalacturonic acid (PGA) with Mn2+, Co2+ and Ni2+ were investigated by potentiometric titration of the polyelectrolyte in presence of varying amounts of metal ions. Formation functions, evaluated from the titration data and analyzed according to the Bjerrum plot method, were interpreted as evidence of: 1) the involvement of two carboxylate groups in the complex formation; 2) very little difference in preference for the binding of all the three ions by PGA. By taking into account Potentiometric and spectroscopic results (the latter reported elsewhere), an outer-sphere electrostatic structure is suggested for all the complexes. The relevance of this type of interaction in soil biochemistry is discussed.