In this study, Overhauser effect (OE) type of dynamic nuclear polarization (DNP) experiments were performed to study suspensions of MC800 asphaltene in bromopentafluorobenzene, chloropentafluorobenzene and hexafluorobenzene aromatic solvents. The experiments were performed at a low field of 1.53 mT in a double-resonance nuclear magnetic resonance (NMR) spectrometer. In this technique the nuclei of diffusing solvent molecules and the unpaired electron existing on the asphaltene micelles interact magnetically. The DNP parameters were determined. Additionally, the interactions between ¹⁹F nuclei of the solvent and the electrons delocalized on the asphaltene are interpreted. The highest enhancement factor value (5.90) was obtained for the hexafluorobenzene solvent medium, because between these, hexafluorobenzene has the highest fluorine atom number. The solvent molecules attach to the colloidal asphaltene particles for a very short time forming complexes and making scalar interaction. Morphologies of asphaltene surfaces depending on the solvent effects were observed by using scanning electron microscopy (SEM).
This article reviews recent theoretical treatments of field dependent relaxation processes in complex systems containing mutually coupled dipolar, quadrupole, and electron spins. The presented approaches are based on an analogy between the Hamiltonian formalisms for quadrupole and zero field splitting interactions. Limitations of the presented treatments, resulting from the validity conditions of the second-order perturbation theory are discussed in detail.
The main objective of the presented study was to characterize the high (HAS) and low affinity (LAS) binding sites of ketoprofen (KP) in human serum albumin (HSA) structure with the use of spectrofluorescence and proton nuclear magnetic resonance spectroscopy. In vitro fluorescence analysis was used to estimate the effect of KP on the HSA fluorescence. The association constants K_{a} [M^{-1}] of KP-HSA complex in the HAS were determined with the use of Scatchard, Klotz, and Hill analysis. The quenching K_{Q} [M^{-1}] constants were determined on the basis of the Stern-Volmer equation. Binding of ketoprofen to plasma protein was also studied with the use of 8-anilinonapthalene-1-sulfonic acid (ANS) and 5-dimethylaminonaphthalene-1-sulfonic acid (DNSA) as the fluorescence probes in IIIA and IIA subdomains of HSA, respectively. To estimate the cooperativeness in proteins Hill's coefficient n_{H} was used. The analysis of proton nuclear magnetic resonance spectra of KP in the presence of HSA allows us to observe the interactions between aromatic rings of the drug and the rings of amino acids located in the hydrophobic subdomains of the protein on the basis of the changes of chemical shifts Δ σ [ppm] of drug protons resonances. Moreover the K_{a} constants [M^{-1}] of KP-HSA complex in the LAS were determined.
The ^1H NMR relaxometry in combination with water proton spin-spin relaxation time measurements and rheometry have been applied to study the ionic gelation of 1% w/w aqueous low methoxyl pectin solution induced by divalent Ca^{2+} cations from a calcium chloride solution. The model-free approach to the analysis of ^1H NMR relaxometry data has been used to separate the information on the static (β) and dynamic (〈τ_c〉) behaviour of the systems tested. The ^1H NMR results confirm that the average mobility of both water and the pectin molecules is largely dependent on the concentration of the cross-linking agent. The character of this dependency (β,〈τ_c〉 and T_2 vs. CaCl_2 concentration) is consistent with the two-stage gelation process of low methoxyl pectin, in which the formation of strongly linked dimer associations (in the range of 0-2.5 mM CaCl_2) is followed by the appearance of weak inter-dimer aggregations (for CaCl_2 ≥ 3.5 mM). The presence of the weak gel structure for the sample with 3.5 mM CaCl_2 has been confirmed by rheological measurements. Apart from that, the T_1 and T_2 relaxation times have been found to be highly sensitive to the syneresis phenomenon, which can be useful to monitor the low methoxyl pectin gel network stability.
An origin of narrow 1H NMR signals in pyridine-N-oxide (PyO)...HCl crystal has been investigated by means of MAS, SPEDAS, NOESY and COSY techniques. Spectra of crystalline samples are compared with those of solid phase obtained from liquid PyO...HCl solutions (in acetonitile/H2O) after the heterogeneous phase separation. It has been concluded that partially resolved peaks in 1H NMR spectra of solids are related with heterogeneity of spin system and presence of different H-bond clusters of water molecules. NOESY spectra show no cross-peaks even at very long mixing time (500 ms). This indicates there is no exchange process between spins causing different peaks, and thus the corresponding molecular aggregates are captured in “islands of mobility8 without any channels sufficient for exchange. Appearance of MAS side bands as “pseudo8 cross-peaks in 2D NMR spectra using MAS/COSY technique is reported. In the case of accidental coincidence of spinning frequency (ωMAS) with spectral distances between some diagonal signals, intensive non-diagonal peaks are observed at the corresponding cross-positions. A misleading conclusion concerning spin coupling is easy to avoid using various ωMAS.
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