Magnetic Resonance in Chemistry最新文献

Osteoarthritis (OA) is a highly common chronic disease that decreases functional capacity and can cause disability. The early detection of the disease could help to develop treatments that may reduce the progression if not cure the disease. Proteoglycan depletion is known to occur at an early state of OA and the delayed gadolinium-enhanced magnetic resonance imaging (MRI) of cartilage (dGEMRIC) is currently considered as one of the most accurate methods for analyzing the depletion in articular cartilage (AC) despite the toxicity-related issues with gadolinium contrast agents. The aim of this study was to investigate the usability of amorphous manganese oxide (MnOx) for assessing the proteoglycan content in AC. The relaxation times of MnOx were determined at various fields and compared with the effect of gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA) at 7.1 T. The diffusion of MnOx and Gd-DTPA into AC was analyzed ex vivo and followed for 24 h. Cartilage degeneration was evaluated with two histological scoring systems (OARSI and Mankin) to assess the relationship between OA severity and MnOx concentration. Relaxivity of MnOx was high and diffusion to the AC was faster than that of Gd-DTPA at 7.1 T. Using MnOx, T₁ followed histological optical density (OD) of stained proteoglycans and correspondingly the concentration profiles followed in reverse the OD profiles in each human sample in a similar manner to Gd-DTPA in dGEMRIC. This pilot study showed some preliminary superiority in relaxation and diffusion into AC of MnOx in relation to Gd-DTPA.

Compositions containing bitumen emulsions and solids have many applications, such as soil stabilization, cold mix asphalt preparation, dust binding, surface dressing and slurry sealing. Therefore, the interaction of bitumen with minerals is of great interest in science and applications. In the interaction of bitumen emulsion with a mineral surface, one of the processes that influence the properties of the bitumen–mineral composition is the formation of interface layers. Nuclear magnetic resonance can provide insights into the interactions of bitumen and minerals at the molecular level. However, the presence of magnetic constituents in most solids as well as the background magnetic field gradients at the interface does not allow the use of NMR to its full potential. In this work, we describe a ¹H NMR approach to study the interface layer formed by a specified bitumen emulsion in the presence of non-magnetic as well as magnetic minerals. This approach is based on the consecutive flushing off of “bulky” components of the bitumen emulsion and the following extraction of the surface layer material, which can then be analyzed by NMR spectroscopy. As a proof of concept, this technique was tested on samples prepared using a bitumen emulsion and four different silicate minerals. The ¹H NMR study showed that interfacial layers may accumulate asphaltenes adsorbed to mineral surfaces to a different extent depending on the specific elemental composition of minerals. More asphaltenes were detected in the interfacial layers on surfaces of studied minerals with a higher content of calcium.

For the structure elucidation of biflavones in which the subunits are linked by an ether bridge, two NMR spectroscopic approaches are shown to determine the position of the ether bridge, without derivatization of the compound. We show that the structures of the biflavones loniflavone and methylloniflavone, isolated from the leaves of Lonicera japonica Thunb. (Caprifoliaceae), have to be revised to ochnaflavone and methylochnaflavone, respectively.

We report, for the first time, the isolation of taraxerone from an isopropanol extract of chaya (Cnidoscolus aconitifolius), a traditional plant used in Mayan culture. Although this natural product has been previously identified in other plant species, the complete assignment of its NMR data had not been accomplished until now. To achieve this, we employed 2D NMR experiments to assign the ¹H and ¹³C chemical shifts of taraxerone. Several approaches were used to determine the spin–spin coupling constants (J_H,H), including GIAO calculations, spin simulation, and dihedral angle analysis via a Karplus-type HLA equation. Conformational analysis revealed the presence of two conformations of Ring A: a chair and a twisted boat. To better understand the behavior of these conformers, a variable-temperature ¹H NMR experiment was performed, in which Δδ/°C values for selected protons were monitored. These changes are attributed to stereoelectronic effects, such as the Perlin effect, as evidenced by ¹J_C,H values obtained from HSQC nondecoupled experiments. Isotopomeric shifts (¹Δ^12/13C(¹H)) were also investigated. All findings were further supported by natural bond orbital (NBO) calculations, which helped explain the observed hyperconjugative interactions.

The ¹H, ¹³C, ¹⁵N, and ¹⁷O chemical shifts of 91 nitro-1H-pyrazoles, as well as 1-amino-1H-pyrazole, were calculated using the Gauge-Invariant Atomic Orbital/Becke Three-parameter Lee–Yang–Parr [GIAO/B3LYP/6-311++G(d,p)] method. The majority of these compounds are polynitropyrazoles, including substituents such as N-nitro, C-nitro, and N-trinitromethyl groups. The calculated chemical shifts were compared with available experimental data using an empirical equation that relates absolute shieldings to chemical shifts. However, a new equation was required to accurately account for the oxygen atoms in the nitro groups. Additionally, some conformational studies were performed to better understand the chemical shifts, particularly those of the OH and NO₂ substituents at position 1. Different statistical methods were used to analyze the calculated ¹⁵N chemical shifts.