1,720,963 research outputs found
Host-Guest Interactions between 2,4-Dichlorophenol and Humic Substances As Evaluated by H-1 NMR Relaxation and Diffusion Ordered Spectroscopy
No full textHigh-power gradient diffusion NMR spectroscopy for the rapid assessment of extra-virgin olive oil adulteration
No full textAggregation and disaggregation of humic supramolecular assemblies by NMR diffusion ordered spectroscopy (DOSY-NMR)
No full textDiffusion ordered nuclear magnetic resonance spectroscopy
(DOSY-NMR) was applied to a number of fulvic (FA) and humic
(HA) acids of different origin. Spectral separation achieved
by DOSY based on diffusion coefficients (D), and correlated to
molecular sizes by calibration standards, showed that
carbohydrates had the largest molecular size in FA, whereas
alkyl or aromatic components were the most slowly diffusing
moieties in HA. At increasing concentrations, these components
had invariably lower D values in DOSY spectra for all humic
samples, thereby indicating an aggregation into apparently larger
associations, whose increased hydrodynamic radius was
confirmed by viscosity measurements. When humic solutions
werebroughtfromalkalinetoacidicpH(3.6),componentsdiffusivity
detected by DOSY increased significantly, suggesting a
decreaseofaggregationandmolecularsize.Ageneralcomparison
of HA and FA molecular sizes was achieved by multivariate
statistical analysis. While a larger extent of aggregation and
disaggregation was observed for HA than for FA, no aggregation
wasdetected,undersimilar conditions, foratruemacropolymeric
standard. Such difference in diffusion between a polymeric
molecule and humic samples, is in line with the supramolecular
nature of humic matter. The possible formation of humic
micelles was also investigated by both changes of diffusivity
in DOSY spectra and shift of 1H NMR signals. Except for HA of
peat and soil origin, revealing a self-assembling in micellelike
structures at the 4 mg mL-1 concentration, no other humic
sample showed evidence of critical micelle concentration
(cmc) up to 20 mg mL-1. These results indicated that DOSYNMRspectroscopy
is a useful technique to evaluate components
of different molecular size in natural humic superstructures
Multivariate analysis of CPMAS (13)C-NMR spectra of soils and humic matter as a tool to evaluate organic carbon quality in natural systems
No full text
Spectroscopic and conformational properties of size-fractions separated from a lignite humic acid
No full textA lignite humic acid (HA) was fractionated by preparative high performance size-exclusion chromatography (HPSEC) in seven different size-fractions. The size-fractions were characterized by cross polarization (CP) magic angle spinning (MAS) 13C NMR spectroscopy and a further analytical HPSEC elution under UV and fluorescence detection. The alkyl hydrophobic components mainly distributed in the largest molecular-size-fraction, whereas the amount of oxidized carbons increased with decreasing size of fractions. Cross polarization time (TCH) and proton spin-lattice relaxation time in the rotating frame (T1ρ(H)) were measured from variable contact time (VCT) experiments. The bulk HA was characterized by the shortest TCH values and the longest T1ρ(H) values which suggested, respectively, one. an aggregation of components in a large conformation that favored a fast H–C cross polarization, and, two. consequent steric hindrances that prevented fast local molecular motions and decreased proton relaxation rates. Conversely, the separated size-fractions showed longer TCH values and shorter T1ρ(H) values than the bulk HA, thereby indicating that they were constituted by a larger number of mobile molecular conformations. The UV and fluorescence absorptions were both low in the large size-fractions that mainly contained alkyl carbons, whereas they increased in the olephinic- and aromatic-rich fractions with intermediate molecular-size, and decreased again in the smaller fractions which were predominantly composed by oxidized carbons. These results support the supramolecular structure of humic substances and indicate that the observed variation in conformational distribution in humic association may be used to explain environmental processes with additional precision
Structural Characterization of Isomeric Dimers from the Oxidative Oligomerization of Catechol with a Biomimetic Catalyst
No full textIn a previous paper, it has been reported about the formation of catechol di-, tri- and tetramers within an oxidative polymerization catalyzed by synthetic water-soluble iron-porphyrin as an efficient alternative to bio-labile natural peroxidase. It has also been demonstrated the occurrence of both C-C and C-O-C coupling mechanisms. However, since the coupling products were determined by mass spectroscopy, the exact bonding position could not have been precisely ascertained for the C-C bonded isomeric dimers that are the dominant products of catechol oligomerization. Therefore, here isolation and characterization of catechol isomeric dimers, obtained by oxidative coupling under the catalysis of a synthetic iron-porphyrin, have been done by NMR spectroscop
Reduced Toxicity of Olive Mill Waste Waters by Oxidative Coupling with Biomimetic Catalysis
No full textLarge quantities of environmentally toxic olive mill waste waters (OMWW) result from olive oil production worldwide. A synthetic water-soluble meso-tetra(2,6-dichloro-3-sulfonatophenyl) porphyrinate of iron(III) chloride (FePha) was used as biomimetic catalyst to oxidatively couple toxic phenols inOMWW fractions obtained by micro-, ultra-, and nanofiltration, and reverse osmosis.Theoccurrence of oxidative coupling in different OMWW size-fractions was assessed by high performance size exclusion chromatography (HPSEC), before and after conformational disruption with acetic acid, and measurements
of proton spin-lattice relaxation time in the rotating frame (T1FH) through 13C-CPMAS-NMR spectroscopy. The concurrent reduction in toxicity of OMWW size-fractions brought about by the FePha treatment was monitored by an algal bioassay. HPSECchromatogramsofOMWWsamplessubjectedtocatalyzed
coupling showed apparent weight-average molecular weight (Mwa) values varying from 18 to 185% larger than for control. Moreover, when such FePha-treated fractions were added to acetic acid prior to HPSEC, the Mwa values still ranged from 14 to 162% larger than for control fractions similarly treated
with acetic acid. This evidence of polymerization among toxic phenols was confirmed by T1F(H) values which were significantly enhanced by the FePha treatment, thereby indicating an increased conformational rigidity of OMWW materials. These molecular changes were reflected in a significantly reduced toxicity exerted on microalgae by the OMWWsize-fractions subjected to catalyzed oxidative couplings. Our results suggest that OMWW can be effectively treated with a biomimetic catalyst to induce oxidative phenol polymerization and reduce their toxicity before amendments to soils or other disposal means
- …
