Theoretical chemical characterization of phosphate-metal-humic complexes and relationships with their effects on both phosphorus soil fixation and phosphorus availability for plants.

BACKGROUND: Previous studies showed that phosphate can be complexed by humic acids (HA) through stable metal (M) bridges (PMHA). We studied the thermodynamic properties of PMHA and their relationships with the ability of PMHA to both decrease soil P fixation and increase P availability for plants. With this aim, we studied the theoretical stability of PFeHA, PAlHA and PCaHA by molecular modelling methods in relation to the degree and intensity of P absorption in soils and the ability of plants to take up complexed P. RESULTS: A density functional theory (DFT) quantum chemical study enabled us to obtain stable structures for the three PMHA complexes in water solution. The theoretical stabilities (ΔG°) were consistent with that for apparent stability obtained by Scatchard method, PFeHA ≥ PAlHA > PCaHA, though the differences were clearer by the DFT method. Also the reduction of soil P fixation and the release of P from PMHA in the presence of an anionic resin confirmed the stability order of the different PMHA. Plant studies confirmed the ability of diverse plant species to take up both P and metal complexed in PMHA. CONCLUSION: The results indicated the potential efficiency of PMHA-based fertilizers to optimize P fertilization for crops cultivated in soils with high P fixation ability.

The molecular conformation, but not disaggregation, of humic acid in water solution plays a crucial role in promoting plant development in the natural environment.

Many studies have shown the capacity of soil humic substances (HS) to improve plant growth in natural ecosystems. This effect involves the activation of different processes within the plant at different coordinated molecular, biochemical, and physiological levels. However, the first event triggered by plant root-HS interaction remains unclear. Some studies suggest the hypothesis that the interaction of HS with root exudates involves relevant modification of the molecular conformation of humic self-assembled aggregates, including disaggregation, which might be directly involved in the activation of root responses. To investigate this hypothesis, we have prepared two humic acids. A natural humic acid (HA) and a transformed humic acid obtained from the treatment of HA with fungal laccase (HA enz). We have tested the capacity of the two humic acids to affect plant growth (cucumber and Arabidopsis) and complex Cu. Laccase-treatment did not change the molecular size but increased hydrophobicity, molecular compactness and stability, and rigidity of HA enz. Laccase-treatment avoided the ability of HA to promote shoot- and root-growth in cucumber and Arabidopsis. However, it does not modify Cu complexation features. There is no molecular disaggregation upon the interaction of HA and HA enz with plant roots. The results indicate that the interaction with plant roots induced in both HA and laccase-treated HA (HA enz), changes in their structural features that showed higher compactness and rigidity. These events might result from the interaction of HA and HA enz with specific root exudates that can promote intermolecular crosslinking. In summary, the results indicate that the weakly bond stabilized aggregated conformation (supramolecular-like) of HA plays a crucial role in its ability to promote root and shoot growth. The results also indicate the presence of two main types of HS in the rhizosphere corresponding to those non-interacting with plant roots (forming aggregated molecular assemblies) and those produced after interacting with plant root exudates (forming stable macromolecules).

Genetic Diversity among Four Populations of Aedes aegypti (Diptera: Culicidae) from Honduras as Revealed by Mitochondrial DNA Cytochrome Oxidase I.

Aedes aegypti is a hematophagous and highly anthropophilic mosquito with a wide distribution, particularly in tropical and subtropical regions of the world. Ae. aegypti is the main vector of several febrile diseases called arboviruses (dengue, yellow fever, chikungunya, and zika viruses), which represent an important public health problem. Populations of this mosquito were nearly eliminated from the Americas in the mid-20th century; however, after the abandonment of control measures, mosquito populations have been recovering territory, have expanded by anthropogenic mechanisms, and have been joined by new populations reintroduced from other continents. The objective of this pilot study was to determine the genetic variability of Aedes aegypti collected in four cities located along the so-called logistics corridor of Honduras, which connects the Caribbean Sea to the Pacific Ocean. We studied the sequences of two molecular markers: the cytochrome c oxidase 1 (COI mtDNA) gene and the internal transcribed spacer 2 (ITS2 rDNA) of 40 mosquitoes. Phylogenetic analyzes show two separate clades with a low number of nucleotide differences per site, three haplotypes, and low haplotype diversity. These results suggest a low genetic diversity in the populations of Ae. aegypti in Honduras in relation to that reported in other countries of the Central American isthmus.

Efficiency of urease and nitrification inhibitors in reducing ammonia volatilization from diverse nitrogen fertilizers applied to different soil types and wheat straw mulching.

BACKGROUND: Some authors suggest that the absence of tillage in agricultural soils might have an influence on the efficiency of nitrogen applied in the soil surface. In this study we investigate the influence of no-tillage and soil characteristics on the efficiency of a urease inhibitor (N-(n-butyl)thiophosphoric triamide, NBPT) and a nitrification inhibitor (diciandiamide, DCD) in decreasing ammonia volatilization from urea and ammonium nitrate (AN), respectively. RESULTS: The results indicate that ammonia volatilization in soils amended with urea was significantly higher than in those fertilized with AN. Likewise, the main soil factors affecting ammonia volatilization from urea are clay and sand soil contents. While clay impedes ammonia volatilization, sand favours it. The presence of organic residues on soil surface (no-tillage) tends to increase ammonia volatilization from urea, although this fact depended on soil type. The presence of NBPT in urea fertilizer significantly reduced soil ammonia volatilization. This action of NBPT was negatively affected by acid soil pH and favoured by soil clay content. CONCLUSION: The presence of organic residues on soil surface amended with urea increased ammonia volatilization, and was particularly high in sandy compared with clay soils. Application of NBPT reduced ammonia volatilization although its efficiency is reduced in acid soils. Concerning AN fertilization, there were no differences in ammonia volatilization with or without DCD in no-tillage soils.

Arsenite provides a selective signal that coordinates arsenate uptake and detoxification through the regulation of PHR1 stability in Arabidopsis.

In nature, plants acquire nutrients from soils to sustain growth, and at the same time, they need to avoid the uptake of toxic compounds and/or possess tolerance systems to cope with them. This is particularly challenging when the toxic compound and the nutrient are chemically similar, as in the case of phosphate and arsenate. In this study, we demonstrated that regulatory elements of the phosphate starvation response (PSR) coordinate the arsenate detoxification machinery in the cell. We showed that arsenate repression of the phosphate transporter PHT1;1 is associated with the degradation of the PSR master regulator PHR1. Once arsenic is sequestered into the vacuole, PHR1 stability is restored and PHT1;1 expression is recovered. Furthermore, we identified an arsenite responsive SKP1-like protein and a PHR1 interactor F-box (PHIF1) as constituents of the SCF complex responsible for PHR1 degradation.We found that arsenite, the form to which arsenate is reduced for compartmentalization in vacuoles, represses PHT1;1 expression, providing a highly selective signal versus phosphate to control PHT1;1 expression in response to arsenate. Collectively, our results provide molecular insights into a sensing mechanism that regulates arsenate/phosphate uptake depending on the plant's detoxification capacity.

Both chemical and crystalline phase configuration influence the molecular features of humic acids in humic-calcium-phosphates fertilizers.

Phosphate-metal-humic complexes are very relevant in nature due to their crucial role in phosphate availability for plants and microorganisms. Synthetic phosphate-calcium-humic acid (HA) complexes have proven to be efficient sources of available phosphorus for crops. However, the current knowledge about their structure and molecular features is very poor. The structural implications of phosphate interaction with humic binding sites through calcium bridges, in both monocalcium phosphate and dicalcium phosphate is investigated by using molecular modeling, 31P-NMR, 1H-NMR and X-ray diffractometry. The conformational changes in the molecular configuration of the humic acid involved in the interaction resulting from the synthetic process is also studied by using HPSEC and synchronous fluorescence. The results obtained allow us to identify the phosphate type in the crystalline phase that is involved in the interaction of humic acid binding sites and the different forms of calcium phosphate. Synchronous fluorescence also shows that whereas the conformational configuration of the HA binding site is only partially affected in the monocalcium phosphate interaction, it changes in the case of dicalcium phosphate showing simpler molecular arrangements. These changes in the molecular conformation of the binding site in HA in solution may influence the biological activity of the humic acid. On the other hand, HPSEC studies show that the humic-calcium-phosphate interaction is accompanied by increases in the humic acid apparent size distribution. This effect is more intense in the case of monocalcium phosphate system probably due the influence of pH.

Some structural and electronic features of the interaction of phosphate with metal-humic complexes.

Previous studies demonstrated the formation of stable phosphate-metal-humic complexes in solution. These studies, however, indicated that the proportion of complexed metal that intervenes in phosphate fixation is rather low. In this study we investigate the possible structural and electronic features of the binding site involved in phosphate fixation in metal-humic complexes that could explain this fact. To this end, we have studied phosphate-metal-humic complexes involving Fe(III), Al(III), and Zn(II) using three complementary techniques: infrared spectroscopy (FTIR), fluorescence, and molecular modeling. The FTIR study indicated that, in the case of those complexes involving Fe and Zn phosphate, fixation is associated with a stabilization of the metal-carboxylate bond. In the case of Al this effect is less clear. This effect of phosphate fixation on the characteristics of the metal-humic binding site was also supported by the results obtained in the Fluorescence study, which showed significant changes in the quenching effect normally associated with metal complexation in humic substances upon phosphate fixation. Finally, the molecular modeling study revealed that the stability of phosphate-metal-humic complexes is inversely related to the stability of the metal-humic interaction. This result could explain why only a relatively low proportion of humic complexed metal is involved in phosphate fixation.

Design, synthesis, and biological evaluation of phosphoramide derivatives as urease inhibitors.

The design, synthesis, and biological evaluation of phosphoramide derivatives as urease inhibitors to reduce the loss of ammonia has been carried out. Forty phosphorus derivatives were synthesized and their inhibitory activities evaluated against that of jack bean urease. In addition, in vivo assays have been carried out. All of the compounds were characterized by IR, (1)H NMR, MS, and elemental microanalysis. In some cases, detailed molecular modeling studies were carried out, and these highlighted the interaction between the enzyme active center and the compounds and also the characteristics related to their activity as urease inhibitors. According to the IC(50) values for in vitro inhibitory activity, 12 compounds showed values below 1 microM and 8 of them represent improvements of activity in comparison to the commercial urease inhibitor N-n-butylthiophosphorictriamide (NBPT) (100 nM) (AGROTAIN). On the basis of the activity results and the conclusions of the molecular modeling study, a structural model for new potential inhibitors has been defined.

New Amphiphilic Composite for Preparing Efficient Coated Potassium-Fertilizers for Top-Dressing Fertilization of Annual Crops.

This study describes the efficiency of a new coating material for preparing granulated potassium-fertilizers with a potassium release to the soil solution sensitive to rainfall intensity. The composite is prepared by reaction of an alkyd-resin with cement in the absence of water. The complementary use of diverse analytical techniques showed that the presence of the cement fraction induced alkyd resin reticulation and gradual cement-resin hardening. Scanning electron microscopy revealed the formation of micro and nanopores within cement-clusters, whose water permeability is affected by the resin reticulation and amphiphilic character. Potassium release was evaluated in water, soil-columns, and in soil-plant trials in pots and open-field. Agronomic results were consistent with potassium release rates obtained in water solution and soil columns. The composite-coated potassium fertilizer was more efficient than the noncoated one in providing plant available potassium, with this effect being dependent on water presence in soil.

Size distribution, complexing capacity, and stability of phosphate-metal-humic complexes.

Size distribution, maximum complexing ability, and stability constants for phosphate-metal-humic (PO43--M-HA) complexes involving two trivalent (Fe and Al) and five divalent metal (M) bridges (Zn, Cu, Mn, Ca, and Mg) were investigated at the pH values 4, 6, and 8. Results highlighted the existing competition between metal-humic acid (M-HA) aggregation and the formation of PO43--M-HA complexes. However, the fact that only a very low fraction of complexed metal is involved in PO43- fixation seems to be related to the existence of specific electronic and/or steric requirements in the binding site in the metal-humic complex. The importance of the ionic form of phosphate (H2PO4- or HPO42-) and the involvement of phenolic and especially carboxylic groups in the phosphate binding are discussed. Finally, the order of stability obtained for PO43--M-HA complexes was similar to that of M-HA complexes. This result suggests that PO43--M-HA might play a significant role in the dynamics of phosphorus in certain soil types.

Development and agronomical validation of new fertilizer compositions of high bioavailability and reduced potential nutrient losses.

To optimize the economical cost of each unit of fertilizer applied and to reduce the environmental contamination caused by nutrient losses, the development of highly efficient granulated fertilizers is of great importance. This study proposes a strategy that consists of developing specific fertilizers having nutrient release patterns that are dependent on plant activity in the rhizosphere. This type of fertilizer is named "rhizosphere-controlled fertilizer" (RCF fertilizer). This fertilizer is based on the introduction of an organomineral matrix composed of metal [Mg (Ca is also possible), Zn (Fe and other metals are also possible)]-humic phosphates. The presence of this matrix modifies the nutrient release pattern of the fertilizer. In this way there are two main nutrient fractions: (i) a water-soluble fraction or "starter" fraction and (ii) a "rhizosphere-controlled" fraction insoluble in water but soluble by the action of the rhizospheric acids released by plants and microorganisms. This study shows the chemical and structural characterization of the organomineral matrix, as well as its efficiency in slowing the nutrient release rate of the RCF fertilizer, principally with respect to P and N. It is demonstrated how these properties of the matrix were also reflected in the significant reduction in both ammonia volatilization and N leaching in a pot system consisting of wheat plants cultivated in a calcareous soil and fertilized with a RCF fertilizer.

Methodological approach to the study of the formation and physicochemical properties of phosphate-metal-humic complexes in solution.

The aim of this work is to study the suitability of the complementary use of ultrafiltration (UF) and the interaction with an anion-exchange resin (AR) to characterize of phosphate-metal-humic complexes in solution. The results indicate that a methodological approach consisting of the validation and calibration of the AR method by the UF method and the further use of the AR method is suitable for characterizing phosphate-metal complexes. Such an approach has proven to be useful for calculating the phosphate maximum binding capacity of iron-humic complexes and stability constants. It might also be used to obtain valuable purified phosphate-metal-humic complexes for further structural characterization.

Imaginarios sociales, expresiones simbólicas, prácticas alimenticias y estados de salud asociados, en estudiantes de la Universidad Salvadoreña Alberto Masferrer. 2017 - 2018 / Social imaginaries, symbolic expressions, eating practices and associated health states, in university students Salvadoran Alberto Masferrer. 2017 - 2018

Esta investigación presenta la descripción de los imaginarios, las prácticas alimentarias y el estado de salud observado en la población universitaria activa del ciclo 02 -2017 de la Universidad Salvadoreña Alberto Masferrer, USAM. La descripción permitió la identificación de expresiones simbólicas y la construcción de categorías imaginarias significativas que nos dan la posibilidad de comprender cómo los sujetos de estudio legitiman sus prácticas de alimentación a partir de dichos imaginarios. Permitió además la elaboración del perfil de las dietas de los estudiantes de la muestra, lo que mostró prácticas poco saludables. Los factores que los afectan no son precisamente económicos, sino socioculturales. Como por ejemplo: costumbres personales y familiares; actitudes y hábitos mal estructurados en la responsabilidad por la dieta y la salud; educación del gusto por alimentos cargados de sal y azúcar y saborizantes artificiales; abandono de la dieta tradicional en favor de la comida industrial; e imaginarios en cuanto a qué llenar es la función fundamental de la acción de comer. Finalmente, fue posible comprender que existe una negociación entre la dieta tradicional y la novedad de los alimentos industrializados, identificando el riesgo potencial para la salud, en el mediano plazo, lo que implica la práctica de la dieta. El estudio de tipo descriptivo utilizó un método complementario que se justifica en el modelo de arco hermenéutico propuesto por Paul Ricoeur, y de los pasos fundamentales de la antropología médica. El grupo estudiado se observó desde dos puntos de vista complementarios: 1) Una muestra ampliada, compuesta por 433 participantes a los que se les aplicó un cuestionario de preguntas abiertas para recopilar la información, y 2) Una muestra cualitativa, compuesta por 45 sujetos, con quienes se hicieron cuatro grupos de enfoques y 40 entrevistas en profundidad. Estos temas fueron relacionados principalmente con su estado general de salud, en el período de septiembre a octubre de 2017.

Structural characterization of anion-calcium-humate complexes in phosphate-based fertilizers.

Fertilizers based on phosphate-metal-humate complexes are a new family of compounds that represents a more sustainable and bioavailable phosphorus source. The characterization of this type of complex by using solid (31)P NMR in several fertilizers, based on single superphosphate (SSP) and triple superphosphate (TSP) matrices, yielded surprising and unexpected trends in the intensity and fine structure of the (31)P NMR peaks. Computational chemistry methods allowed the characterization of phosphate-calcium-humate complexes in both SSP and TSP matrices, but also predicted the formation of a stable sulfate-calcium-humate complex in the SSP fertilizers, which has not been described previously. The stability of this complex has been confirmed by using ultrafiltration techniques. Preference towards the humic substance for the sulfate-metal phase in SSP allowed the explanation of the opposing trends that were observed in the experimental (31)P NMR spectra of SSP and TSP samples. Additionally, computational chemistry has provided an assignment of the (31)P NMR signals to different phosphate ligands as well as valuable information about the relative strength of the phosphate-calcium interactions within the crystals.

Organic complexed superphosphates (CSP): physicochemical characterization and agronomical properties.

A new type of superphosphate (organic complexed superphosphate (CSP)) has been developed by the introduction of organic chelating agents, preferably a humic acid (HA), into the chemical reaction of single superphosphate (SSP) production. This modification yielded a product containing monocalcium phosphate complexed by the chelating organic agent through Ca bridges. Theoretically, the presence of these monocalcium-phosphate-humic complexes (MPHC) inhibits phosphate fixation in soil, thus increasing P fertilizer efficiency. This study investigateed the structural and functional features of CSP fertilizers produced employing diverse HA with different structural features. To this end were used complementary analytical techniques: solid-phase ³¹P NMR, ¹³C NMR, laser-confocal microscopy, X-ray diffraction, and molecular modeling. Finally, the agronomical efficiency of four CSP have been compared with that of SSP as P sources for wheat plants grown in both alkaline and acidic soils in greenhouse pot trials under controlled conditions. The results obtained from the diverse analytical studies showed the formation of MPHC in CSP. Plant-soil studies showed that CSP products were more efficient than SSP in providing available phosphate for wheat plants cultivated in various soils with different physicochemical features. This fact is probably associated with the ability of CSP complexes to inhibit phosphate fixation in soil.

Structural characteristics of phosphoramide derivatives as urease inhibitors. Requirements for activity.

Taking as a reference the structural characteristics of a set of compounds that act as jack bean ( Canavalia ensiformis) urease inhibitors, namely, phenylphosphorodiamidate (PPD), N- n-butylthiophosphorictriamide (NBPT), and N- n-butylphosphorictriamide (NBPTO), we have studied the structure-activity relationships of a series of phosphoramide derivatives for which the activity as urease inhibitors in both in vitro and in vivo assays is known. Molecular modeling studies were carried out, and the results highlighted the relevance of characteristics such as the presence of intramolecular hydrogen bonds, the volume of the fragment involved in the enzyme interaction, and the degree of conformational freedom as well as the HOMO orbital and atomic orbital contributions to the HOMO orbital, electron density, and PEM distributions on the activity of these compounds as urease inhibitors. These data, along with the preliminary docking study carried out, allow us to propose a union mode to the active site of the enzyme for these compounds.