Macromolecular changes of humic substances induced by interaction with organic acids

Two different humic materials, one from a forest soil and the other from wormcasts, were used to study the influence of mineral and organic acids on the conformational properties of humic substances. The macromolecular changes were followed by low pressure gel permeation chromatography after titrating humic material to low pHs with acids. All organic acids (mono-, di-, tri-carboxylic, and oxy-acids), added to humic solution prior to a gel permeation in an alkaline buffer, were able to shift the totality of absorbance of the humic chromatographic peak from high to low molecular sizes. Mineral acids, phenol, alcohols, dipolar aprotic solvents, could not produce the same shift and gave total absorbance at the column void volume as in the case of humic substances alone. The chromatographic peak shifted back to elution volumes proper of higher size molecules when the humic-organic acid mixture was back-titrated to high _pHs before gel permeation. Elution in a much stronger alkaline buffer did not change the overall macromolecular behaviour. These results suggest that humic substances behave as micelles in solution and that hydrophobic bondings play an important role in holding humic molecules together. The organic acids enter the interior of the humic micelle-like aggregates and alter the stereochemical hydrophobic arrangement of the humic material. In alkaline conditions the negative charges developed disrupt the apparent high molecular size configuration and disperse the humic aggregates into small micelles. Such conformational properties of humic substances appear to be a function of pH and of the concentration of organic acid.     

Effects of some dicarboxylic acids on the association of dissolved humic substances

Changes in molecular size distributions of four different humic materials were evaluated through high performance size-exclusion chromatography (HPSEC), before and after treating humic solutions with naturally occurring dicarboxylic (oxalic, malonic, succinic and glutaric) acids. Chromatograms of dissolved humic substances showed a decrease in peak absorbance as well as an increase in peak elution volume when the solution pH was lowered from 7 to 3.5 by addition of dicarboxylic acids before HPSEC analysis. The resulting reduction in molecular size is explained by the disruption of unstable humic superstructures into smaller-sized associations stabilized by the formation of strong intra- and inter-molecular hydrogen bonds among humic molecules. The extent of humic size variation was in the order: oxalic相似文献   

Fate of humic acids isolated from natural humic substances

Composition of humic acids (HA) is a function of plant-derived inputs, degradation processes regulated by microorganisms, organo-mineral interactions and age. Characterization of different origin humic substances is important for evaluation of their contribution to stabile and labile carbon pool in the environment. The relative abundance of chemical components in HA isolated from soils, compost, commercial lignohumates, alginite, acadiane and lignite was studied with aim to quantify content of important biomarkers such as amino acid, lipids and polyphenols. HA were considered as a heterogeneous complex and high concentration of peptides, polyphenols and lipids was determined in acadian-HA to compare with soil-HA. Compost-HA contained much more amino acids to compare with soil-HA samples. Alginite-HA and lignite-HA were similar in biomarkers content to soil-HA. Fourier transform infrared spectroscopy confirmed that chemical composition and functional groups content differs with the origin, humification degree and the age of studied samples. Soil-HA are typically composed of a variety of ?OH, COOH?, C–O, C–H₂, (aliphatic and aromatic) groups, quinines, lignin fragments, polysaccharide, monosaccharide and proteins fragments, which are linked together by ?O?, ?NH?, ?H=, >C=O, metal ions and –S? groups. ¹³C NMR spectroscopy showed that aromatic carbon content was the highest in lignite-HA and soil-HA.     

A comparative study of elemental composition of water-soluble humic substances,humic acids,and fulvic acids extracted from sod-podzolic soils

Elemental analysis of water-soluble humic substances extracted from three sod-podzolic soils was carried out. Data on elemental composition were compared to those of humic and fulvic acids extracted from the same soils.     

A comparative study of molecular weight distribution of water-soluble humic substances,humic acids,and fulvic acids extracted from sod-podzolic soils

The alteration of organic matter from three sod-podzolic soils in the course of the extraction of water-soluble humic substances was studied by means of size-exclusion chromatography using a combined UV-DOC detector. A comparison of water-soluble humic substances with humic and fulvic acids extracted from the same soils was carried out.     

Enzymatic transformation of humic substances by NDO

Enzymatic transformation of humic acids (HA), fulvic acids (FA) and indole was examined using naphthalene 1,2-dioxygenase (NDO). NDO was used as a model for dioxygenase enzymes found in various microbial species. Indole was used as a model substrate for NDO-catalyzed reactions resulting in condensation products. Although NDO is not classified as a soil enzyme, all HA and FA tested were susceptible to NDO-induced transformation. The extent of NDO-specific NADH oxidation in solutions containing HA and FA paralleled the percent aromaticity of the HA and FA. Furthermore, the UV–Vis absorptive properties of NDO-treated HA and FA were altered in a manner suggesting condensation reactions similar to the formation of indigo from indole. Condensation reactions were enhanced in NDO-treated mixtures containing indole and an FA. NDO retained activity for 2 weeks under ambient conditions, and retained some enzymatic activity for 9 days based on detection of specific metabolites by HPLC, suggesting prolonged extracellular activity. Humic substances have not previously been known to be substrates for dioxygenases; even more significant was that dioxygenase enzymes can facilitate condensation reactions between indole-like functional groups well-known to be present in HA and FA. These results illustrate how dioxygenases can be potential humic-modifying enzymes when released into the environment upon microbial death and concurrent cell lysis which could alter the bioavailability of organic contaminants associated with dissolved organic matter through specific modulation of enzyme activity involving substrate competition.     

Effects of mineral and monocarboxylic acids on the molecular association of dissolved humic substances

The conformational structure of dissolved humic substances is an important property that controls the reactivity of humus in the soil solution. High performance size-exclusion chromatography was used here to study the changes in molecular size of different humic substances brought about by addition of mineral (HCl) and monocarboxylic (formic, acetic, propionic, and butyric) acids. The CPMAS-NMR spectra showed that humic substances had varying chemical composition and that the ratio of hydrophilic to hydrophobic carbon (HI/HB) was greater for a humic acid from soil than for ones from oxidized coal and lignite. All humic substances showed a decrease in UV absorbance of chromatographic peaks when treated with either HCl or monocarboxylic acids. This was due to the hypochromic effect by which the absorptivity of associated molecules is decreased when they are separated. We attributed the molecular separation upon acid treatment to the formation of intermolecular hydrogen bonding that alters the original conformation stabilized mainly by weaker hydrophobic interactions. Addition of organic acids not only further decreased peak absorbances of humic acids but also caused their shift to larger elution volumes, indicating a larger conformational disruption than with HCl. The extent of the molecular size changes showed a relation to the number of carbons of monocarboxylic acids and to the HI/HB ratios of humic materials. The larger the carbon content of organic acids and the smaller the HI/HB ratio of humic materials, the larger was the decrease of the average molecular size of humic acids. These results suggest that dissolved humic substances associate predominantly by hydrophobic forces and that the apolar components of humic substances largely control their aggregation and reactivity in the environment.     

Systems of solutions,minerals, and humic substances in soils

The equilibrium composition of solutions, minerals, and humic substances of the humus-clay plasma from the humus horizon of a leached chernozem was calculated using the method of computerized physicochemical simulation based on Gibbs’ principle of minimum free energy in the equilibrium state of the system. It was shown that the plasma from the humus horizons of chernozems and soddy-deeply podzolic soil mainly consists of oxides, hydroxides, humic substances, kaolinite, and 2: 1 illite-smectite minerals. The stoichiometry of the humic substances corresponds to calcium-saturated humus acids with appreciable contents of iron and aluminum. The crystallochemical composition of the 2: 1 layered minerals classifies them among ferruginated mixed-layered illite-montmorillonites. Simulation at an increased concentration of carbon dioxide indicated the degradation of the humus-clay plasma in a leached chernozem under extensive soil use, which can be restored by application of potassium at rates 1-to ?1.5-fold its removal.     

Inhibition of malate dehydrogenase by humic acids

Malate dehydrogenase (from pig heart mitochondria) is markedly inhibited by a number of humic acids extracted from different types of soil or prepared synthetically. The inhibitors were found to be competitive for both substrates of the forward reaction, whereas inhibition is of the mixed type with NADH in the reverse reaction. In both directions, inhibition is strictly dependent on the number of phenolic hydroxyl groups on the humic acids. The affinity of the inhibitors for the enzyme is increased in such a way that it is readily reversible in the forward reaction but becomes more irreversible in the opposite direction.Studies on complex formation between humic acids and NAD⁺ led to the conclusion that different effects on both reactions are due to the formation of such a complex. Enzyme activity could be recovered in the presence of calcium ions. This is assumed to be due to changes in the electron structure of the humic acid particles.     

腐殖酸对磷在红壤中有效性的影响

An investigation was conducted to study the effect of humic substance （HS） on the phosphorus （P） solubility in acidic soil. The soil （2.5 g）, HS （0, 0.5, and 2.5 g）, and P as monocalcium phosphate （0.31 and 1.25 g P kg^-1 soil） were mixed with 50 mL distilled water and two different sequences of adding HS and P were used. The results indicated that the P concentration in water and 0.01 mol L-1 CaCl2 solution increased with increasing amounts of humic substance. The concentrations of Fe and Al were also increased. However, Olsen P decreased with increasing amount of humic substance. Water-soluble P concentrations from P rates at 0.31 and 1.25 g P kg^-1 soil in the treatment with 0.5 g （2.5 g） humic substance addition were 360% and 70% （500% and 90%） higher, respectively, than those in the treatment with no humic substance addition. P extracted by 0.01 mol L^-1 CaCl2 in the treatments with 0.5 and 2.5 g humic substance addition was increased by 400% and 540%, respectively, compared with that in the treatment without humic substance at the rate of 0.31 g P kg^-1 soil, while the corresponding P concentrations were increased by 80% and 90% at the rate of 1.25 g P kg^-1 soil. The order of mixing humic substance and phosphate did not significantly affect desorbed P and labile P extracted with CaCl2.     

Cleaning heavy metal contaminated soil with soluble humic substances instead of synthetic polycarboxylic acids

Abstract

Soils contaminated with heavy metals constitute a serious and widespread ecological problem but to clean such soils requires strong chemicals such as polycarboxylates; frequently ethylenediaminetetraacetic acid and nitrilotriacetic acid are used. However, these compounds are synthetic and toxic and their replacement by natural products such as soluble humic substances as washing agents for cleaning heavy metal polluted soils would be environmentally very attractive. In fact, such a replacement seems possible at least on cadmium and copper contaminated soil inasmuch as humic substances, depending on the concentration, were found to extract up to 45% and 54% of total cadmium and copper from a highly contaminated calcareous soil. Even though higher amounts of the two metals were extracted by ethylenediaminetetraacetic acid and nitrilotriacetic acid, the humic substances undoubtedly extracted the most reactive fractions. However, the humic substances extracted only 4% of total lead and 17% of total nickel, whereas the percentages for the synthetic polycarboxylates were about 30% for nickel and lead. Ethylenediaminetetraacetic acid and nitrilotriacetic acid may therefore be replaced by humic substances as washing agents for cadmium, copper and maybe nickel contaminated soils, whereas they seem unsuited for cleaning lead contaminated soils, at least if the soils are as calcareous as the soil tested.     

Dissolution of aluminum and iron phosphate by humic acids

Abstract

An investigation was conducted to study the effect of humic (HA) and fulvic acid (FA) on the dissolution of aluminum phosphate (AlPO₄) and iron phosphate (FePO₄), to analyze the dissolution products, and assess their availability to plants. The rate of dissolution was determined by shaking 10 mg of Al‐ or FePO₄ with 0 to 800 mg L^‐1 of HA or FA solutions at pH 7.0 for 0 to 192 hours. The phosphorus (P) concentration was measured in the extracts by spectrophotometry, whereas the nature of P‐humic acid complexes was determined by ³¹P NMR analysis. Availability of dissolution products was studied by growing corn plants in aerated hydroponic solutions receiving treatments of 50 mg Al‐ or FePO₄ and 0 to 800 mg L^‐1 of HA or FA at pH 5.0. The results indicated that the amount of P released by HA or FA increased with time. Humic acid was more effective than FA in dissolving the metal phosphates. The ³¹P NMR analysis showed that the dissolution products contained free orthophosphates and minor amounts of P‐humic acid complexes. This confirms the role of HA as a powerful chelator of Al and Fe, liberating in this way the orthophosphate anions. Corn plants grown in hydroponics, with AlPO₄ or FePO₄ as the source of P, exhibited better growth performance when HA or FA are present.     

Binding of methylmercury compounds by humic and fulvic acids

Humic and fulvic acids were isolated from Fawn Lake in Ontario and investigated by membrane dialysis for their ability to bind CH₃Hg⁺. By measuring the distribution of methylmercury compounds in and outside of the dialysis membrane, for the first time, binding capacities and conditional stability constants for CH₃Hg⁺ and humic acids could be calculated. Equilibrium between inner and outer dialysis solution was reached within 24 hours without losses of CH₃Hg⁺ in humic solution. The ratio of bound to unbound CH₃Hg⁺ increased with decreasing total CH₃Hg⁺ levels in the system. The percentage of free methylmercury compounds increased with decreasing pH. Fawn Lake humic acid showed two different binding sites. The conditional stability constant of the stronger site was calculated as 1.3 × 10¹². This site was able to bind 0.2 ng CH₃Hg⁺ per mg of humic acid.     

Solubilization of iron by water‐extractable humic substances

The capability of water‐extractable humic substances (WEHS) to solubilize Fe from sparingly soluble Fe‐hydroxide was studied. Addition of WEHS (1.7 mmol organic C l^—1) to a dialysis tube containing labeled insoluble Fe‐hydroxide caused an increase in the amount of ⁵⁹Fe measured in the external solution. The humic fraction was also able to solubilize Fe from soil samples, with levels comparable to those obtained using a solution containing 100�μM DTPA. By measuring the amount of ⁵⁹Fe eluted from soil columns pre‐loaded with ⁵⁹Fe‐WEHS it was possible to evaluate the mobility of Fe complexed to the humic molecules. The recovery of ⁵⁹Fe varied from 2% to 25% in respect to the soil type used. The ability of Fe‐WEHS to serve as an Fe source for the phytosiderophore hydroxy‐mugineic acid (HMA) was also analyzed. The removal of ⁵⁹Fe from the Fe‐WEHS complex by HMA was demonstrated by adding the phytosiderophore to a dialysis tube containing the ⁵⁹Fe‐WEHS complex. The observations suggested a ligand exchange between the phytosiderophore and the humic fraction. The results indicate that WEHS is able to increase the amount of Fe present in the soil solution, possibly by forming mobile complexes with the micronutrient. These complexes could act as easily available Fe sources in Fe acquisition processes by both monocot and dicot plants, playing an important role particularly in soils with low available Fe.     

Copper(II) binding by free and kaolinite-sorbed humic substances

Humic preparations isolated from different sources—soils (a soddy-podzolic soil and a typical chernozem), high-moor peat, and brown coal—have been used. To analyze the binding of copper ions by humic substances (HSs), the preparations were obtained in two forms: solutions and humic-clay complexes (HSs irreversibly sorbed on kaolinite). With this approach, the binding of copper(II) ions by HSs has been studied in different systems: (1) Cu(II)-HSs irreversibly sorbed on kaolinite, (2) Cu(II)-dissolved HSs, and (3) Cu(II)-dissolved HSs-HSs irreversibly sorbed on kaolinite. In the systems containing both dissolved HSs and humic-clay complexes, HSs of similar structure isolated from the same source were used. The quantitative estimation of the copper binding was based on the constant of sorption (K) for HSs in humic-kaolinite complexes and the stability constant (β) of complexes for free (dissolved) substances. Both parameters were expressed in similar units: L/kg. The values of logK = 3.31—3.33 are independent of the quantity and quality of the HSs in the sorption complexes but reliably exceed the K value for pure kaolinite (2.92). The value of β is not affected by the presence of insoluble HSs together with their soluble forms, but it depends on the source of HSs. The value of logβ varies in the range from 5.62 to 6.93, which significantly exceeds K and indicates a significantly higher affinity of dissolved HSs for copper ions than that of irreversibly sorbed HSs. The revealed regularities have shown that the content of HSs in the soil solution can significantly affect the mobility of a heavy metal bound to the soil organic matter.     

Photodegradation of the surfactants Na-dodecylbenzenesulfonate and dodecylpyridinium-chloride as affected by humic substances

Surfactants account for a great proportion of organic xenobiotica released into the environment by anthropogenic action. In natural waters and sewage sludge they can be removed from solution by different reactions, including adsorption to solid phases, biodegradation and abiotic photolytic degradation. In the present investigation we found that photodegradation of Na- dodecylbenzenesulfonate (LAS) was reduced by humic substances by a factor of 2 or more. In contrast, photodegradation of dodecylpyridinium-chloride (LPC) was increased in the presence of humic substances by a factor of 1.16–1.43. These differences were explained by the strong bonding of LPC but not of LAS to the humics. Photolytic degradation of LAS begins at the aliphatic side chains followed by aromatic ring cleavage. Photodegradation of LPC mainly associated with the humics showed similarity to that of pyridine. The experimental data demonstrate strong effects of photodegradation on the persistence of surfactants and the relevance of humic-surfactants associations for photolytic processes.     

Conformational and rheological changes in catfish myosin as affected by different acids during acid-induced unfolding and refolding

Changes in the conformation of catfish (Ictalurus punctatus) myosin due to (i) anions, (ii) acid pH, and (iii) salt addition were determined using tryptophan fluorescence, hydrophobicity measurements, differential scanning calorimetry, and circular dichroism. The relationship between conformation and storage modulus (G') of acid-treated myosin was studied. Three acids, HCl, H2SO4, and H3PO4, were used for unfolding myosin at three acidic pH conditions, 1.5, 2.0, and 2.5. Unfolded myosin was refolded to pH 7.3. Denaturation and unfolding of myosin was significantly (p < 0.05) lower when salt (0.6 M NaCl) was present during acid unfolding than in the absence of salt. When salt was added before unfolding, the alpha-helix content of myosin treated at pH 1.5 was significantly lower than that treated at pH 2.5. When salt was added after refolding, the alpha-helix content of myosin was unaffected by different pH treatments. The G' of myosin increased with an increase in myosin denaturation. The G' of myosin was significantly (p < 0.05) higher when salt was added to myosin after refolding than before acid unfolding. Among the different anion treatments, the G' of acid-treated myosin decreased in the order Cl- approximately SO42- > PO43-. Among the different pH treatments, the G' of myosin treated at pH 1.5 was significantly (p < 0.05) higher than myosin treated at pH 2.5. The conditions that would result in maximum myosin denaturation and maximum G' were unfolding of myosin at pH 1.5 using Cl- (from HCl) followed by refolding at pH 7.3 and subsequent addition of 0.6 M NaCl.     

Influence of land use on the characteristics of humic substances in some tropical soils of Nigeria

In highly weathered tropical conditions, soil organic matter is important for soil quality and productivity. We evaluated the effects of deforestation and subsequent arable cropping on the qualitative and quantitative transformation of the humic pool of the soil at three locations in Nigeria. Cultivation reduced the humic pool in the order: acetone‐soluble hydrophobic fraction (HE) > humic acid (HA) > humin (HU) > fulvic acid (FA), but not to the same degree at all three sites. The C and N contents, as well as the C/N ratios of humic extracts, were large and not substantially influenced by land use. The δ¹³C values of the humic extracts were invariably more negative in forested soils thereby showing a dilution of δ¹³C signature with cultivation from C3 to C4 plants. The δ¹³C values of apolar HE fractions were generally more negative, indicating a reduced sensitivity compared with other humic fractions to turnover of crop residues. The contents of hydrophobic constituents (alkyl and aromatic C), as revealed by cross‐polarization magic angle spinning (CPMAS) ¹³C‐NMR spectroscopy, in HA, FA and HU were generally < 50%, with the exception of larger hydrophobicity in HU in the forested soil at Nsukka and HA in that at Umudike. The HE fraction contained significantly more apolar constituents, and consequently had a larger intrinsic hydrophobicity than the other humic fractions. The larger reduction of apolar humic constituents than of the less hydrophobic humic fractions, when these soils were deforested for cultivation, indicates that at those sites the stability of accumulated organic matter is to be ascribed mainly to the selective preservation of hydrophobic compounds.     

Conformational changes and some functional characteristics of gelatin esterified with fatty acid.

This study investigated the effects of attachment of fatty acid chains to gelatin molecules on their conformation and some functional properties in order to determine the effectiveness of this procedure for improving the emulsifying properties of gelatin. The esterification conferred upon the gelatin molecules a folded configuration resulting in a lambda(max) of fluorescence emission shift to lower wavelengths and decreases in solubility, intrinsic viscosity, and gelling ability. In contrast, surface hydrophobicity increased with extent of esterification, and the esterified derivatives exhibited higher emulsion stabilities. These results indicated that the oligomeric structure and functional attributes of the gelatin were altered with the modification.