Thermo-FTIR spectroscopic study of the siderophore ferrioxamine B: spectral analysis and stereochemical implications of iron chelation, pH, and temperature

FTIR spectra of the microbial siderophore, ferrioxamine B, and its nonchelated form (iron free; desferrioxamine B) were studied to facilitate in-depth investigation on the undisrupted structure of the siderophore and its interactions with the environment. Effects of iron chelation as well as those of various levels of pH and temperature on the stereochemical structure of the free ligand and the ferric complex were examined. The presence of a number of functional groups in these compounds and the mutual interaction between them resulted in significant shifts and overlapping of their characteristic absorption bands. Thermal and pH treatments combined with a comprehensive use of curve-fitting analysis facilitated bands resolution. Absorption bands of all functional groups were identified. The results imply that the compact and rigid structure of the ferric complex (ferrioxamine B) is sustained by intense and specific intramolecular hydrogen bonds. Dehydration was the main process observed at low temperature (25-60 degrees C). At 105 degrees C the free ligand form (desferrioxamine B) had already begun to decompose, whereas ferrioxamine B exhibited stability. The thermal destruction became acute at the 170 degrees C treatment for both molecules. The secondary amide groups and the hydroxamate groups, which comprise the binding site for the Fe atom in the complex, were found to be the most sensitive to the thermal degradation. Significant pH effects were observed only for desferrioxamine B samples at pH 9, accompanied by partial decomposition, similar to that observed at 105 degrees C. Deprotonation of desferrioxamine B was found to begin with the deprotonation of the NH(3+) group. Characteristics of the rigid conformational structure of the ferric complex and the state of the NH(3+) group, both assumed to play an important role in the recognition and uptake of the siderophore by membranal receptors, were elaborated by means of FTIR and are discussed in detail.     

Influence of polysaccharide structure on dextran adsorption by montmorillonite

Two dextrans of similar molecular weight (?2 × 10⁶) but containing different structural linkages (B-215F: 95% α-1→ 6 and 5% α-1→ 3 and Polytran: 75% β-1→ 3 and 25% β-1→ 6) were adsorbed on Na-montmorillonite. Adsorption isotherms showed strong, H-2-type (high-affinity. Langmuir mono-layer adsorption) clay-dextran interactions for both polymers. Maximum adsorption of the Polytran dextran (60 $\text{mg}\text{100mg}$ clay) was 33 per cent greater than that of the B-512F dextran (44.5 $\text{mg}\text{100mg}$ clay) for comparative equilibrium systems. Stable clay-Polytran complexes containing up to 47% dextran were prepared. Excess Na₂SO₄ (0.1 m) did not affect the quantity of dextran adsorption by montmorillonite at the 15% dextran concentration. Acid hydrolysis and modified Pregl method-C analyses did not quantitatively recover adsorbed dextran from complexes containing more than 13 mg of dextran adsorbed per 100 mg of clay. Loss on ignition determinations were in good agreement with the difference measurements of dextran adsorption, suggesting that part of the C was expelled as something other than CO₂ in the ignition determinations. The maximum adsorption segment for Polytran was much smaller than the individual molecule. In contrast, the maximum adsorption segment for the B-512F dextran appeared to be the same magnitude as the individual polymer molecule. Adsorption segment length was regarded as a manifestation of the relative proportions of primary and secondary alcohol groups of the molecules.     

Fractionation of humic acids upon adsorption on montmorillonite and palygorskite

The adsorption of three humic acid (HA) preparations by clays—montmorillonite (Wyoming, USA) and palygorskite (Kolomenskoe district, Moscow oblast)—has been studied. The HA preparations were isolated from samples of the humus-accumulative horizons of a leached chernozem (Voronezh) and a chestnut soil (Volgograd), and a commercial preparation of sodium humate (Aldrich) was also used. The solid-state ¹³C NMR spectroscopy and IR spectroscopy revealed the selective adsorption of structural HA fragments (alkyls, O-alkyls (carbohydrates), and acetal groups) on these minerals. As a result, the aromaticity of the organic matter (OM) in the organic-mineral complexes (OMCs) and the degree of its humification have been found to be lower compared to the original HA preparations. The fractionation of HAs is controlled by the properties of the mineral surfaces. The predominant enrichment of OMCs with alkyls has been observed for montmorillonite, as well as an enrichment with O-alkyls (carbohydrates) for palygorskite. A decrease in the C : N ratio has been noted in the elemental composition of the OM in complexes, which reflected its more aromatic nature and (or) predominant sorption of N-containing structural components of HA molecules. The adsorption of HA preparations by montmorillonite predominantly occurs on the external surface of mineral particles, and the interaction of nonpolar alkyl groups of HAs with this mineral belongs to weak (van der Waals, hydrophobic) interactions. The adsorption of HA preparations by palygorskite is at least partly of chemical nature: Si-OH groups of minerals are involved in the adsorption process. The formation of strong bonds between the OM and palygorskite explains the long-term (over 300 million years) retention of fossil fulvate-type OM in its complex with palygorskite, which we revealed previously.     

On the mechanism of specific phosphate adsorption by hydroxylated mineral surfaces: A review

Abstract

The mechanism of specific phosphate adsorption by hydroxy‐lated mineral surfaces comprises two aspects: the phosphate‐hydroxyl surface reaction and the configuration of the adsorbed phosphate ion. Evidence pointing to ligand exchange as the mechanism of the phosphate‐surface hydroxyl reaction include kinetics of adsorption and desorption; hydroxyl ion release; infrared spectroscopy, and stereochemical calculations. Data pertaining to the coordination of adsorbed phosphate on hydroxy‐lated mineral surfaces have not been conclusive overall. Isotopic exchange experiments and studies of desorption kinetics do not provide definitive information on surface coordination. Measurements of hydroxyl ion release and crystallographic calculations provide support for the existence of both monodentate and bidentate surface complexes of phosphate ions. Infrared spectroscopic investigations suggest a binuclear complex on dried, phosphated goethite. However, these studies cannot be extrapolated automatically to soil minerals, since the addition of water favors formation of a monodentate surface complex. Further research is needed to establish the configuration of adsorbed phosphate ions.     

A study of phosphate adsorption by four Bangladesh soils

The adsorption curves for four soils were divided into segments I, II and III. Segment I ($\text{< 2}\text{μg P/ml equil. concn.}$) and segment II (2–150 μg P/ml equil. concentr.) conformed to the Langmuir equation similar to that of pure materials like gibbsite, pseudoboehmite and kaolinite, whereas segment III $\text{(> 75–150 μ}\text{g P/ml equil. concn.}\text{)}$ conformed to the Freundlich equation and was unlike the previous finding of linear relationship for pure materials.     

Comparison of the adsorption of maize root mucilage and polygalacturonic acid on montmorillonite homoionic to divalent lead and cadmium

Summary Root mucilage material (RM) was isolated from maize plants grown in the field, and its affinity to montmorillonite (M) homoionic to Pb²⁺ and Cd²⁺ was compared with that of a commercial polygalacturonic acid (PGA). Adsorption isotherms of the commercial and natural materials on the two clay systems were compared in unbuffered systems at pH 3 and pH 6. Adsorption of PGA occurred only at pH 3, and was higher on M-Pb than on M-Cd. In contrast, the adsorption of RM was higher on M-Cd than on M-Pb. Total amounts of RM adsorbed at pH 3 were about 3 times lower on M-Cd and 20 times lower on M-Pb than the respective amounts of PGA adsorbed at the same pH. Polygalacturonic acid had a high content of relatively well dissociated (pKa = 3.5) carboxylic groups, and adsorbed on the clay surface at pH values lower than its pKa. At pH 6, the dissociation of the acid groups favoured its solubility, and the metal cations were then probably displaced by ion exchange. The lower affinity of RM to the clay materials was related to its average molecular weight, which was lower than that of PGA, and to its water solubility, which was higher than that of PGA. The low pH dependence of the adsorption of RM was related to its lower carboxylic acidity and higher content in hydroxyl and amino groups.     

Use of chrome azurol S reagents to evaluate siderophore production by rhizosphere bacteria

Summary Siderophores produced by rhizosphere bacteria may enhance plant growth by increasing the availability of Fe near the root or by inhibiting the colonization of roots by plant pathogens or other harmful bacteria. To examine the populations of siderophore-producing bacteria colonizing the roots of two grass species that differed in their susceptibility to Fe deficiency, we inoculated serial dilutions of root samples onto chrome azurol S (CAS) agar and several other selective and non-selective culture meida. CAS agar effectively differentiated bacteria that were capable of excreting large amounts of siderophore, but the composition of the medium limited its usefulness for ecological studies. A large proportion (71–79%) of the bacterial population that grew on a non-selective medium (tryptic soy agar) failed to grow on CAS agar, and several isolates that showed no sign of siderophore production on CAS agar produced siderophore in liquid culture. Similar populations of siderophore-producing bacteria were observed on roots of St. Augustine grass, which frequently exhibits Fe chlorosis, and bermuda grass, which does not. Roots of both grasses were colonized by bacteria that produced siderophore in vitro at concentrations ranging from 100 to 230 M. The CAS assay solution was also used to compare siderophore production by Pseudomonas fluorescens Q6, an isolate from bermuda grass, and by P. putida B 10, a plant growth-promoting pseudomonad. P. fluorescens Q6 produced 2.4 times more siderophore in vitro than P. putida B 10.     

钙缓解植物盐害的作用机制研究进展

介绍了盐胁迫对植物的伤害，及钙缓解盐害的作用效果，综合评述钙缓解盐害的作用机制。     

Studies on the thermodynamics of zinc exchange in montmorillonite

The ion exchange equilibria and mechanism of Zn exchange with Na montmorillonite has been studied with the help of adsorption isotherms and thermodynamic parameters. The exchange isotherms, K and △G° values indicated a spontaneity of reaction and a higher preference of Zn for the montmorillonite surface. A stronger binding of Zn and changes in the hydration rates of Zn and Na with increase in order were suggested by enthalpy an entropy effects respectively. The surface phase activity coefficients and the excess thermodynamic functions were indicative of a nonideal heterogeneous exchange in which the mixture of ions on the montmorillonite surface was more stable and more tightly bound with significant differences in the hydration rates of the ions in the mixture with respect to the homo-ionic forms.     

Aspects of the adsorption of pirimicarb by smectites and soils

A study was conducted to examine the adsorption and interaction mechanism of pirimicarb with smectites and soils. Studies were carried out in pesticide-smectite or soil-water systems and in pesticide-smectite-organic solvent systems, using homoionic samples of montmorillonite, smectites of different layer charge and soils with smectite in their clay fraction. The adsorption isotherms obtained at 30° and 45°C follow the empirical equation of Freundlich. In the adsorption of the pesticide by soils a highly significant correlation was seen between adsorption (distribution coefficient) and the content of smectite in them. In the studies in organic medium the formation of a smectite-pirimicarb bilayer complex with a basal d₀₀₁ spacing of 18.55 Å was observed; the formation of this is also related to the nature of the interlayer cation and the layer charge of the smectite. According to the I.R. results, the interlayer cations of the silicate and the oxygen atom of the C–O group of the pesticide molecule are involved in the interaction mechanism.     

Competitive adsorption of potassium and calcium on manganese oxide as studied with two ion-selective electrodes

Competitive adsorption of K and Ca in a Mn oxide (birnessite) system was investigated using a double indicator electrode technique. The data showed that Ca was more competitive than K for adsorption sites on the Mn oxide. The preference of the oxide for Ca over K was partially attributed to the specific adsorption of Ca on the oxide. The extent of the preference did not consistently increase as the amount of K and Ca was continuously added to the system, possibly due to a decrease in the availability of the specific adsorption sites as these sites were increasingly occupied by the added Ca. The pK-0.5pCa value for the system significantly increased when the solution to solid ratio was above 16 (mL g^?1). Raising pH caused an increase in the solution K/Ca equivalent activity ratio and a linear decrease in the pK-0.5pCa value, though the influence of the pH on the K/Ca equivalent activity ratio was not substantial until the pH was above 5.23. These results demonstrate an unique interaction of K and Ca on the surface of the Mn oxide, which cannot be solely interpreted by the classical ratio law. This study is of significance in the chemistry of K in soils which have properties of variable charge and specific adsorption of the competitive cations.     

Studies on the adsorption and release of copper by soils

In a previous communication we¹⁾ reported the adsorption and subsequent release of adsorbed Cu⁺⁺ by two soils and a compost sample using fairly high concentrations of Cu⁺⁺ ions in the form of copper sulphate solution.     

Selenite adsorption by a variety of oxides

Abstract

Selenite adsorption by a variety of oxides consisting of iron (Fe), aluminum (Al), titanium (Ti), manganes (Mn), and silicon (Si), and by two humic acids were investigated in order to grasp selenite behavior and fixation mechanisms in soil. It was found that selenite was apparently adsorbed even by the Mn oxides on which surface negative charge was dominant in normal pH range (pH <4). No selenite adsorption was observed in the silicon dioxide (SiO₂) and the two humic acids. A sequential extraction of adsorbed selenite with competitive anions showed the differences of binding force or stability of adsorbed selenite among the minerals. While the goethite fixed selenite strongly, selenite adsorbed on the Mn oxide was easily released to the liquid phase with other anions, such as phosphate. Each mineral had its inherent characteristic in ligand exchange reactions accompanied with selenite sorption. Selenite sorption by the Mn and the Ti oxides resulted in large increase of surface negative charge, while only a little increase in the Fe and Al oxides. Proton consumption with selenite sorption was extremely smaller for the Mn oxide than for the others.     

Dissolution of alkaline earth sulfates in the presence of montmorillonite

In a study of the effect of montmorillonite on the dissolution of BaSO₄ (barite), SrSO₄ (celestite), and²²⁶Ra from U mill tailings, it was found that: (1) More of these substances dissolve in an aqueous system that contains montmorillonite than dissolve in a similar system without clay, due to the ion exchange properties of the clay; (2) Na-montmorillonite is more effective in aiding dissolution than is Ca-montmorillonite; (3) the amount of Ra that moves from mill tailings to an exchanger increases as solution sulfate activity decreases. Leaching experiments suggest that²²⁶Ra from H₂SO₄-Circuit U mill tailings from Edgemont, South Dakota, is not present as pure Ra sulfate or as an impurity in anhydrite or gypsum; it is less soluble, and probably occurs as a trace constituent in barite.     

Effect of lake acidification on the adsorption of phosphorus by sediments

The retention properties of acidic and non-acidic lake sediments were determined in order to assess the effects of lake acidification on the immobilization of P from solution by sediments. The adsorption of P by solids was described by the Langmuir model which was used to determine the sorption parameters, e.g. sorption maxima and equilibrium constant of adsorption. The pH of solution and the chemical and mineralogical characteristics of sediments affect mainly the magnitude of adsorption maxima. The binding strength of the adsorbed complex is similar for all the investigated sediments (Δ=?25.3 to ?28.5 kJ mol^?1) and it is affected little by variation in pH or by chemical and mineralogical composition of sediments. The results indicate that the magnitude of P removal is determined more by sediment chemistry and mineralogy (amorphous Al/Fe oxy-hydroxides, carbonate content) than by pH of the water.     

Zinc adsorption by andepts from the central plateau of Costa rica

Abstract

The adsorption of Zn by soil and subsoil samples from the slopes of the Irazu and Poas volcanoes in Costa Rica was correlated with soil properties and represented by Langmuir isotherms.

Soil pH, texture and free aluminum and sesquioxide content correlated significantly with Zn adsorption. No correlation with organic matter content was found probably due to the variable nature and properties of its compounds. The adsorption maxima ("b”; values) calculated by the Langmuir equation are higher for the soils developed on the more weathered Poas than for the more recent Irazu ash. The bonding energy constants ("K") followed a similar trend.     

A mathematical model for the adsorption of water vapor by soils

A two-parameter mathematical model based on some physical assumptions was developed for the adsorption of water vapor by soils: W = W _mh[(p/p ₀) − (p/p ₀)³ + (p/p ₀)⁶] + W _res. It was shown that one of the model’s parameters is close to a conventional soil-hydrological constant, namely, the maximum hygroscopic moisture, or maximum hygroscopicity W _mh. The second parameter reflects the residual water content W _res as the content of immobilized water, which is bound to the most active part of the adsorbing surface, is adsorbed at the initial stage of adsorption (0 ≤ p/p ₀ ≤ 0.05), and later does not participate in the adsorption processes. Methods were proposed for the differential calculation of singular points and parameters of the model corresponding to the characteristic physical phenomena of water vapor adsorption in soils. The model was tested for the quantitative assessment of the interaction between the soil solid phase and the water vapor in different soils (a soddy-strongly podzolic soil, an ordinary chernozem, a chestnut soil, and a medium-columnar solonetz). A method was proposed for calculating the integral adsorption energy E _max of the soil solid phase-water vapor interaction. It was shown that the E _max values are determined by the physicochemical properties of the soils and characterize the capacity of the separate soil horizons for adsorbing water vapor. The relationship of the integral adsorption energy of the soils with the relative pressure of the water vapor and the water content was studied.     

Mechanism of sulphate adsorption by two tropical SoilS

The mechanism of sulphate (SO₄) adsorption was investigated using two highly-weathered soils with contrasting surface-charge characteristics (one with a net negative charge and the other with a net positive charge). The addition of SO₄ caused a release of OH and increased the adsorption of cations, in keeping with the widely-held view that SO₄ is chemisorbed by replacing -OH and -OH₂ ligands. On the other hand, adsorbed SO₄ was completely recovered by washing with an indifferent electrolyte solution (1 M KC1), an extractant which is usually taken to be specific for ‘exchangeable’ ions. The amounts of SO, adsorbed were several times higher than those of non-specifically adsorbed Cl. It is suggested that the experimental observations can be interpreted in terms of a model in which SO₄ adsorbs in a plane closer to the surface than monovalent anions (e.g. in the Stern layer). The consequence of higher counter-ion valence and proximity to the surface is that additional positive charge is induced onto the surface by OH release. Charge reversal in the diffuse layer allows more cations to be adsorbed. This type of adsorption may involve forces other than the purely electrostatic but, in contrast to chemisorption, the adsorbed anion does not become chemically coordinated to the surface metal atoms.