Ladislau Martin Neto

Effects Of Cultivation On Esr Spectra Of Organic Matter From Soil Size Fractions Of A Mollisol

Organic matter from different soil size fractions (<2 μm; 2–20 μm; 20–50 μm; 50–150 μm, and > 150 μ) was obtained, using a combination of physical separation and light chemical treatment, in an Argentine Mollisol. Quantification of semiquinone free radicals was done using electron spin resonance spectroscopy, taking into account the organic carbon content of each sample. Values obtained for fractions less than 50 μm in size were significantly higher than values for fractions greater than 50μm. Comparison of this data with C/N ratios suggests that there is an association between fractions where more humified organic matter occurs and a higher level of semiquinone is detected. A 2 to 20-μm size fraction was identified as that with the highest humification degree and one which undergoes only minor changes as a result of agricultural practices. Alterations associated with soil compaction, principally in an A12 horizon in an area cultivated for 80 years, were accompanied by a reduction in the level of semiquinone free radicals compared with a native grassland area. Otherwise, in an area with only 10 years of soybean monoculture and where soil compaction effects are not yet extensive, a strong increase of free radicals was observed, principally in the Ap horizon. This increase of free radicals is associated with higher oxidative conditions of a relatively new cultivation area and also with probable rapid decomposition of soybean residues and incorporation as humified organic matter.

EPR OF MICRONUTRIENTS-HUMIC SUBSTANCES COMPLEXES EXTRACTED FROM A BRAZILIAN SOIL

The technique of electron paramagnetic resonance was used to examine the formation of micronutrients-humic substances complexes extracted from a Brazilian soil. Results indicate that there are two types of copper (II) complexes, one with oxygen and another with nitrogen atoms, in humic acids while in fulvic acids only the copper complexed with oxygen was detected. The iron (III) complex signal with g = 4.3 was observed in both humic acid and fulvic acid. The concentration of free radicals (semiquinone) detected in humic acids (1015 spins/g) was bigger than in fulvic acids (1014 spins/g) due to a greater humification degree confirmed by elementary analysis and E4/E6 ratio measurements.

The mechanism of reaction of nitrosyl with met- and oxymyoglobin: an ESR study

In this ESR work we have studied the pentacoordinate symmetry in horse, whale and sperm-whale myoglobin (Mb) in different physical states such as solution and powder. Experiments were performed in which the following parameters were varied: the sample temperature, pH, reaction time with NO, and NO concentration. The results enabled us to explain the NO reaction mechanism in the oxy and met forms of myoglobin. The study of powder samples at different degrees of hydration allowed us to identify the diamagnetic intermediate species existent in the reaction of NO with met-Mb proposed in the literature. The results presented explain adequately the pH effect and temperature dependence observed in the ESR spectra obtained using the met-Mb sample solutions from Sigma Chemical Co., which consist of a mixture (13%) of Mb-O2.

Formas químicas, disponibilidade de manganês e produtividade de soja em solos sob semeadura direta

This field study had the following objectives: to determine the most appropriate micronutrient extractor to evaluate manganese bioavailability in soils under no-tillage; to evaluate the interaction of soil manganese with humic acids in soils under no-tillage with and without manganese deficiency through electronic paramagnetic resonance (EPR); and to evaluate manganese fractions (exchangeable, organic, oxides and residual) after manganese addition in soils under no-tillage with and without previous manganese deficiency for soybean. Four sites (I to IV) were selected to study the response curve to manganese (0 to 48 kg ha-1) fertilization in soybean. Sites I to III were located in Tibagi, state of Parana (PR), Brazil and site IV was in Castro (PR). Manganese rates affected the manganese concentrations in the soil, but not the manganese concentrations in leaves and the amount of manganese taken up by plants. Exchangeable, organic and oxide forms of manganese increased with the increasing rates applied at site I, with the largest increase being observed for the organic fraction (changed from 5.4 mg kg-1 manganese in the control to 35.1 mg kg-1 of manganese in the highest manganese dose). Manganese was not detected by EPR in the solid samples of humic acid - pH 3.0. Manganese quantification in the same samples after nitric-perchloric digestion, along clear signals of manganese detected by EPR in samples of humic acid - pH 2.4, indicated that manganese in the samples was strongly linked to the functional (probably carboxylic) groups of humic acid. The fact that there was no effect of manganese on soybean yield might be due to the complexation of the nutrient by soil organic matter. In general, DTPA-TEA was the most reliable manganese extractor to evaluate manganese availability to soybean under no-tillage.

Multiple response optimization of laser-induced breakdown spectroscopy parameters for multi-element analysis of soil samples.

Laser-induced breakdown spectroscopy (LIBS) is an emerging analytical technique to perform elemental analysis in natural samples independent of their physical state (solid, liquid, or gaseous). Due to its instrumental features, LIBS shows promising potential to perform analysis in situ and in environments at risk. Since the analytical performance of LIBS strongly depends on the choice of experimental conditions, each particular application needs a specific instrumental adjustment. The present study evaluated three LIBS instrumental parameters regarding their influences on signal-to-noise ratio (SNR) of seven elements in soil samples: laser pulse energy, delay time, and integration time gate. A multivariate technique was used due to the significant interaction among the evaluated parameters. Subsequently, to optimize LIBS parameters for each individual element response, a method for multiple response optimization was used. With only one simple screening design, it was possible to obtain a good combination among the studied parameters in order to simultaneously increase the SNR for all analytes. Moreover, the analysis of individual response for elements is helpful to understand their physical behavior in the plasma and also how they are embedded in the sample matrix.

Effect of hydration in metHb: Reversible changes monitored by ESR of iron

The dehydration of human and bovine methemoglobins was monitored using ESR spectroscopy of the iron signal. The interconversion of the Fe(III) signal between the high spin form (at g approximately 6) in solution and low spin form (at g approximately 2) was quantitatively studied as a function of hydration. The dehydration process leads also to a loss of paramagnetism resulting in the appearance of about 40% Fe(II) below 0.40 grH2O/grHb. The remaining 60% of Fe(III) ESR signal is distributed as the residual high spin form (at g approximately 6, 5%) and low spin form (hemichromes H and P, 55%). The formation of hemichrome P was explained as resulting from the coordination of the cysteine residue at beta 93 with the iron atom which follows the rupture of the proximal histidine bond. Experiments with hemoglobins where the sulphur atom of cysteine beta 93 was blocked (N-ethylmaleimide) did not showed the hemichrome P, confirming the involvement of the sulphur atom. This implies that the dehydration process induces displacements and torsion of the F helix, drastically changing the iron coordination at proximal site. In agreement with this proposition the Fe(II) symmetry is pentacoordinated with the disrupted bond to the proximal histidine at fifth coordination. This is also supported by ESR experiments with nitrosyl complex at low hydrations. All conformational changes were reversibly modulated by hydration degree and partially by lyophilization rate. A one-cycle dehydration of bovine hemoglobin followed by solubilization shows 100% reversibility of hemichrome P. Increasing the number of cycles of dehydration-hydration reduces the reversibility degree. With three cycles a reversibility of 70%-75% is observed. The level of 0.40 grH2O/grHb was the critical hydration for the molecules to return to aquo met form and correspond also to a minimal water content necessary to cover all protein surface as obtained from other techniques.

Relações entre matéria orgânica do solo e declividade de vertentes em toposseqüência de Latossolos do Sul de Minas Gerais

The accumulation and stability of soil organic matter are related to slope steepness. The relationships between steepness and the humification degree of humic acids, the charcoal fragment content and its radiocarbon age were quantitatively evaluated. Four Latosols (Oxisols) of the summit, shoulder, back slope and back/foot slope of a toposequence under pasture in a cratonic area in the southern Minas Gerais state, southwest Brazil, were used. The parent material of the Latosols was gneiss of the Precambrian complex, with an udic hydric regime and isothermic thermic regime. Samples were collected from every horizon and subhorizon of the four Latosols for chemical and physical characterization, humic acid extraction and determination of the semiquinone free radical content. Likewise, charcoal fragments were collected for gravimetric, nitrogen and carbon content quantification and radiocarbon dating. Our results show the close relationship of the accumulation and humification degree of soil organic matter in Latosol landscapes to slope steepness, whereas the quantity and radiocarbon age of charcoal fragments were related to the processes of soil genesis and soil position in the slope.

Tunneling within localized states in nitrosyl myoglobin

The temperature (T) dependence of the spin lattice relaxation time of nitrosyl myoglobin (MbNO) was measured in the range from 9 to 310 K. Both powdered and solution samples exhibit an unexpected dependence. Experimental data is well fitted by a Tn function with n=2.2±0.3 in this large temperature range. This result is interpreted within the tunneling localized states model associated with the existence of conformational equilibrium in MbNO.