Magnetic Isotope Research Articles

Metabolic Reprogramming Is an Initial Step in Pancreatic Carcinogenesis That Can Be Targeted to Inhibit Acinar-to-Ductal Metaplasia.

Metabolic reprogramming is a hallmark of cancer and is crucial for cancer progression, making it an attractive therapeutic target. Understanding the role of metabolic reprogramming in cancer initiation could help identify prevention strategies. To address this, we investigated metabolism during acinar-to-ductal metaplasia (ADM), the first step of pancreatic carcinogenesis. Glycolytic markers were elevated in ADM lesions compared with normal tissue from human samples. Comprehensive metabolic assessment in three mouse models with pancreas-specific activation of KRAS, PI3K, or MEK1 using Seahorse measurements, nuclear magnetic resonance metabolome analysis, mass spectrometry, isotope tracing, and RNA sequencing analysis revealed a switch from oxidative phosphorylation to glycolysis in ADM. Blocking the metabolic switch attenuated ADM formation. Furthermore, mitochondrial metabolism was required for de novo synthesis of serine and glutathione (GSH) but not for ATP production. MYC mediated the increase in GSH intermediates in ADM, and inhibition of GSH synthesis suppressed ADM development. This study thus identifies metabolic changes and vulnerabilities in the early stages of pancreatic carcinogenesis. Significance: Metabolic reprogramming from oxidative phosphorylation to glycolysis mediated by MYC plays a crucial role in the development of pancreatic cancer, revealing a mechanism driving tumorigenesis and potential therapeutic targets. See related commentary by Storz, p. 2225.

Hyperfine interactions of paramagnetic radiation-induced defect centres in advanced ceramic breeder pebbles

Advanced ceramic breeder (ACB) pebbles consisting of 65 mol% lithium orthosilicate (Li4SiO4) and 35 mol% lithium metatitanate (Li2TiO3) are developed for tritium breeding in the European Union’s (EU) helium cooled pebble bed (HCPB) breeder blanket concept of the demonstration fusion power plant (DEMO). Electron-nuclear double resonance (ENDOR) spectroscopy is a powerful and widely used magnetic resonance technique that combines aspects of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). In the present work, ENDOR spectroscopy was applied to investigate hyperfine (HF) interactions of paramagnetic radiation-induced defect centres (containing unpaired electrons) in the biphasic ACB pebbles after irradiation with 10 MeV accelerated electrons. To separate individual EPR signals of the formed and accumulated paramagnetic centres, isochronal annealing of the irradiated ACB pebbles followed by EPR spectra measurements at X- and Q-microwave frequency bands at room and low temperature were performed. Afterwards, X-band ENDOR spectra at low temperature were recorded for selected EPR signals to reveal HF interactions between the unpaired electron spins with the spins of magnetic 7Li and 1H isotope nuclei. The g-factor and HF coupling values determined from EPR and ENDOR spectra simulations provide novel insights into the local structure of paramagnetic hole- and electron-type centres in the irradiated ACB pebbles.

Silicate weathering linked with global climate change along the Mid-Pleistocene transition: A record from the Himalayan foreland Basin, India

Tectonic uplift and climate change play a pivotal role in controlling silicate weathering in the source region. Being adjacent to the hinterland, the sediments of the Himalayan foreland basin (HFB) hold an excellent archive to elucidate weathering link with climate and uplift. In this study, we present geochemical records of major, trace and rare earth elements, clay mineral compositions as well as the environmental magnetic and stable carbon isotope composition (δ13C) of siliciclastic sediments of the HFB over the past 2.0 Ma. These records have been used to link silicate weathering with global climate change along the Mid-Pleistocene Transition (MPT) period. The source rock compositions are characterized by selected elemental ratios, i.e., Cr/Th, Th/Sc, Th/Co and La/Sc as well as other geochemical proxies such as Al2O3/TiO2 and TiO2/Zr, suggesting a felsic source rock composition. The enriched LREE pattern of Chondrite-normalised REE with a negative Europium anomaly also attributes to felsic source rock composition. All these inferences suggest that most of the studied sediments were derived from mixed sources of the Higher Himalayan rocks and the Lesser Himalayan crystalline regions. A declining trend in kaolinite/(illite + chlorite) ratios, decreasing values of geochemical weathering indices (such as Rb/Sr and CIA), a decrease in magnetic susceptibility and gradual depleted value of δ13C indicate a decrease in chemical weathering of the siliciclastic rocks in the source region, which is most probably associated with the global climate change from warm-humid to cold-dry condition along the MPT. This suggests that variation in silicate weathering in the Himalayan region are mainly controlled by global climate and monsoon conditions.

Variation in the relationship between odd isotopes of tin in mass-independent fractionation induced by the magnetic isotope effect

Experimental results are presented showing the variation in the relationship between odd isotopes of tin (Sn) in mass-independent fractionation caused by the magnetic isotope effect (MIE), which has previously only been observed for mercury. These results are consistent with the trend predicted from the difference between the magnitudes of nuclear magnetic moments of odd isotopes with a nuclear spin. However, the correlation between odd isotopes in fractionation induced by the MIE for the reaction system used in this study (solvent extraction using a crown ether) was different from that reported for the photochemical reaction of methyltin. This difference between the two reaction systems is consistent with a theoretical prediction that the correlation between odd isotopes in fractionation induced by the MIE is controlled by the relationship between the spin conversion time and radical lifetime. The characteristic changes in the correlation between odd isotopes in fractionation induced by the MIE observed for Sn in this study provide a guideline for quantitatively determining fractionation patterns caused by the MIE for elements that have multiple isotopes with a nuclear spin. These results improve our understanding of the potential impact of the MIE on mass-independent fractionation observed in natural samples, such as meteorites, and analytical artifacts of high-precision isotope analysis for heavy elements.

Magnetic isotope effects: a potential testing ground for quantum biology.

One possible explanation for magnetosensing in biology, such as avian magnetoreception, is based on the spin dynamics of certain chemical reactions that involve radical pairs. Radical pairs have been suggested to also play a role in anesthesia, hyperactivity, neurogenesis, circadian clock rhythm, microtubule assembly, etc. It thus seems critical to probe the credibility of such models. One way to do so is through isotope effects with different nuclear spins. Here we briefly review the papers involving spin-related isotope effects in biology. We suggest studying isotope effects can be an interesting avenue for quantum biology.

Magnetic isotope effects and nuclear spin catalysis in living cells and biomolecular motors: recent advances and future outlooks: Nuclear spin catalysis.

Biomolecular nanoreactors are constructed from chemical elements many of which have magnetic and nonmagnetic stable isotopes. The magnetic isotope effects (MIE) were discovered in experiments with the cells enriched with different isotopes of magnesium, magnetic or nonmagnetic ones. The striking catalytic effect of the magnetic isotope, 25Mg, was revealed in the reaction of ATP hydrolysis driven by myosin, the biomolecular motor utilizing the chemical energy of ATP to perform the mechanical work. The rate of the enzymatic ATP hydrolysis with 25Mg as the enzyme cofactor is twice higher than the rates of the reactions with nonmagnetic 24Mg or 26Mg. A similar effect of the nuclear spin catalysis was revealed in the experiments with zinc as the myosin cofactor. MIE unambiguously indicate that, in the chemo-mechanical process catalyzed by the molecular motor, there is a limiting step which depends on the electron spin state of the reagents, and this step is accelerated by the nuclear spin of the magnetic isotope. The recent developments in this field highlight promising venues for future research of MIE in biophysics with possible applications of the magnetic isotopes in medical physics including radiation medicine and biomedical effects of electromagnetic fields.

Magnetic stimulation of chemical reactions in coal

Purpose. To identify a mechanism in terms of which a signal of weak magnetic field is transformed into response of some components of nanostructure of a carbonaceous matter with further formation of chemical bonds. Methodology. Physical and mathematical modelling procedures of elementary chemical acts have been applied. Regularities of quantum mechanics and dynamics of chemical bonds were also used in addition to a magnetic isotope theory, a diffusion theory of recombination of radical pairs taking into consideration triplet-singlet transit, and nuclear-spin selectivity of chemical reactions. Findings. The physical mechanism of the magnetic scenario of interradial reactions is considered from the viewpoint of their energy stimulation through magnetic fields, i.e. the idea has been implemented according to which the number of radical pairs, able to be recombined into stable molecules, increases significantly if the weak magnetic field exercises certain influence. In addition to stimulation of interradial reactions, the magnetic field impact on organic coal mass-radicals results in stabilization of carbonaceous structures with regular atomic arrangement (being two-dimensional matrices, chains etc.) and their increase. Originality. A physical model of structural and phase changes in coal, depending upon the effect by external weak magnetic field, has been developed. It has been shown that the weak magnetic field signal, being incomparably smaller energetically to compare with the energy of thermal molecular motion, can initiate triplet-singlet transitions, i. e. transform radicals into reactive state. A mechanism has been proposed to form chemical bonds between movable radicals and non-complete bonds of atoms at the surfaces of solid phases of carbonaceous matter. Practical value. Implementation of the obtained results, concerning magnetic coal processing, may be connected with the development of a new procedure to avoid explosive conditions in coal seams. Progress in the field of magnetic processing should involve studies concerning calculation and selection of coal processing parameters (i.e. magnetic field density, frequency, energy, and a processing period) to control efficiently the chemical reactions in the carbonaceous matter. A substantiation degree of coal processing will influence greatly both efficiency of chemical processes and expediency of practical use of the results.

Study of the possibility of using data on the content of stable isotopes of magnesium to predict the region of apple production

Relevance. Reliable criteria for determining the region of origin of fruit products attract the constant attention of researchers in various countries. A number of approaches aimed at solving this problem are already known, and some progress has been made in interpreting experimental data for fruits on the content of stable isotopes of hydrogen, carbon, oxygen, nitrogen, and sulfur. It is possible that information about the fractionation of stable isotopes of magnesium will allow us to obtain equally important and reliable information to achieve the same goals.Methods. A mass spectrometer with inductively coupled plasma, standard samples of stable isotopes of magnesium produced in Russia were used to perform the work. The total concentration of magnesium cations was determined by atomic absorption in the flame and on the capillary electrophoresis system.Results. During the research, the mass and percentage ratio of three stable isotopes (24Mg, 25Mg, 26Mg) in the studied samples of apples grown in different soil and climatic zones was established. The data obtained reflect the relative prevalence of isotopes of this element, expressed in the ratio of the least common isotope to the most common. The deviation of the isotopic composition of Mg prototypes from the conventional standard was determined, significant deviations were obtained for the magnetic isotope 25Mg and non-magnetic 26Mg. The initial data on the fractionation of stable isotopes of magnesium in raw materials growing in different regions were obtained, with the aim of possible subsequent use as indicators in biochemical and physiological studies. A combination of the obtained information on Mg isotopes is assumed, taking into account the content of isotopes of other metals, which will make it possible to specify the region of growth of raw materials with greater reliability.

Mass-independent Sn isotope fractionation and radiogenic 115Sn in chondrites and terrestrial rocks

Tin has ten stable isotopes, providing the opportunity to investigate and discriminate nucleosynthetic isotope anomalies from mass-dependent and mass-independent isotope fractionation. Novel protocols for chemical separation (based on TBP-resin) and MC-ICP-MS analyses are reported here for high precision Sn isotope measurements on terrestrial rocks and chondrites. Relative to the Sn reference standard (NIST SRM 3161a), terrestrial basalts and chondrites show isotope patterns that are consistent with mass-dependent and mass-independent isotope fractionation processes as well as with 115Sn radiogenic ingrowth from 115In.Two different mass-independent isotope effects are identified, namely the nuclear volume (or nuclear field shift) and the magnetic isotope effect. The magnetic isotope effect dominates in the two measured ordinary chondrites, while repeated analyses of the carbonaceous chondrite Murchison (CM2) display a pattern consistent with a nuclear volume effect. Terrestrial basalts show patterns that are compatible with a mixture of nuclear volume and magnetic isotope effects. The ultimate origin of the isotope fractionation is unclear but a fractionation induced during sample preparation seems unlikely because different groups of chondrites show distinctly different patterns, hence pointing towards natural geo/cosmochemical processes. Only the carbonaceous chondrite Murchison (CM2) shows a Sn isotope pattern similar to what expected for nucleosynthetic variations. However, this pattern is better reproduced by nuclear volume effects. Thus, after considering mass-independent and mass-dependent effects, we find no evidence of residual nucleosynthetic anomalies, in agreement with observations for most other elements with similar half-mass condensation temperatures.Most chondrites show a deficit in 115Sn/120Sn (typically −150 to −200 ppm) relative to terrestrial samples, with the exception of one ordinary chondrite that displays an excess of about +250 ppm. The 115Sn/120Sn data correlate with In/Sn, being consistent with the β− decay of 115In over the age of the solar system. This represents the first evidence of the 115In-115Sn decay system in natural samples. The radiogenic 115Sn signature of the BSE derives from a suprachondritic In/SnBSE, which reflects preferential partitioning of Sn into the Earth’s core.

Controversial Issues of Hydrocarbon Field Formation and the Role of Geomagnetic Fields

This review paper presents controversial issues on the formation of hydrocarbon deposits. We look into the geological contradictions of the abiogenic and biogenic theories of petroleum origin, indicating the connection between hydrocarbon deposits and disjunctive dislocations, as well as present disputes about the geological period over which hydrocarbon deposits have been formed. We further overviewed the radical chain mechanism of hydrocarbon generation from organic matter as proposed by Prof. Nesterov. It is noted that the petroleum generation process in reservoir conditions occurs almost instantly in the presence of discrete geomagnetic fields and does not require a long geological time. This is explained by spin magnetic effects (spin catalysis, magnetic isotope properties). We briefly highlight the effect of magnetic fields on chemical reactions involving organic compounds and the use of magnetic fields to enhance oil recovery. We also present the leading causes of discrete magnetic fields in the sedimentary cover: Earth’s geomagnetic reversals, generation of ferromagnetic minerals in oil deposits, electromechanical effects of rock friction near faults, and intermixing of reservoir waters with different mineralization (spontaneous ion polarization). Based on the material reported, we conclude that the radical chain mechanism of petroleum generation processes explains some contradictions of the abiogenic and biogenic theories of petroleum origin. Elaborating this research area has excellent prospects for developing new criteria for hydrocarbon prospecting and devising innovative methods to enhance the oil recovery for shale oil production.

A rapid high‐precision analytical method for triple oxygen isotope analysis of CO2 gas using tunable infrared laser direct absorption spectroscopy

RationaleThe simultaneous analysis of the three stable isotopes of oxygen—triple oxygen isotope analysis—has become an important analytical technique in natural sciences. Determination of the abundance of the rare 17O isotope in CO2 gas using magnetic sector isotope ratio mass spectrometry is complicated by the isobaric interference of 17O by 13C (13C16O16O and 12C16O17O, both have mass 45 amu). A number of analytical techniques have been used to measure the 17O/16O ratio of CO2 gas. They either are time consuming and technically challenging or have limited precision. A rapid and precise alternative to the available analytical methods is desirable.MethodsWe present the results of triple oxygen isotope analyses using an Aerodyne tunable infrared laser direct absorption spectroscopy (TILDAS) CO2 analyzer configured for 16O, 17O, and 18O combined with a custom gas inlet system. We evaluate the sensitivity of our results to a number of parameters. CO2 samples with a wide range of δ18O values (from −9.28‰ to 39.56‰) were measured and compared to results using the well‐established fluorination‐gas source mass spectrometry method.ResultsThe TILDAS system has a precision (standard error, 2σ) of better than ±0.03‰ for δ18O and ±10 per meg for Δ′17O values, equivalent to the precision of previous analytical methods. Samples as small as 3 μmol CO2 (equivalent to 300 μg CaCO3) can be analyzed with a total analysis time of ~30 min.ConclusionsWe have successfully developed an analytical technique for the simultaneous determination of the δ17O and δ18O values of CO2 gas. The precision is equal to or better than that of existing techniques, with no additional chemical treatments required. Analysis time is rapid, and the system is easily automated so that large numbers of samples can be analyzed with minimal effort.

Oligomerization of β-Like DNA Polymerases in the Presence of Fe2+ Ions

This work presents data on the relationship between the oligomeric state and catalytic activity of β-like DNA polymerases isolated from three cancer cell lines upon replacement of magnesium ions with iron. It was shown that the suppression of the catalytic activity of β-like DNA polymerases in the presence of divalent iron ions is related to enzyme oligomerization that can be responsible for the suppression of the magnetic isotope effect.

Construction and commissioning of the collinear laser spectroscopy system at BRIF

We have constructed a collinear laser spectroscopy (CLS) system installed at the Beijing Radioactive Ion-beam Facility (BRIF), aiming to investigate the nuclear properties of unstable nuclei. The first on-line commissioning experiment of this system was performed using the continuous stable (39K) and unstable (38K) ion beams produced by impinging a 100-MeV proton beam on a CaO target. Hyperfine structure spectra of these two isotopes are reasonably reproduced, and the extracted magnetic dipole hyperfine parameters and isotope shift agree with the literature values. The on-line experiment demonstrates the overall functioning of this CLS system, opening new opportunities for laser spectroscopy measurement of unstable isotopes at BRIF and other radioactive ion beam facilities in China.

A simple and high-sensitive fluorescence polarization method for screening small molecule agonists of glucagon-like peptide-1 receptor

Glucagon-like peptide-1 receptor (GLP-1R) is an important therapeutic target for the treatment of type-2 diabetes. Currently-available GLP-1R agonists in clinic are peptides that require injection. It is necessary to develop oral small molecule drugs to provide conventional therapeutics for this chronic disease. At present, nuclear magnetic resonance, ELISA method and isotope tracing, have been used to screen small molecule drugs for anti-type 2 diabetes, but these methods have the disadvantages of either operational cumbersome or high risk.

Spin Photochemistry: Electron Spin Multiplicity as a Tool for Reactivity and Selectivity Control.

Spin chemistry involving small organic molecules without heavy atoms is highly sensitive to spin-orbit-coupling (SOC) modulating biradical conformation as well as hyperfine coupling (HFC) modulating magnetic isotope interactions. Several easily available reaction properties such as chemo-, regio-, and diastereoselectivity as well as quantum yields serve as analytical tools to follow intersystem crossing (ISC) dynamics and allow titrating spin selectivities.

Mass-Independent Fractionation of Mercury Isotopes during Photoreduction of Soot Particle Bound Hg(II).

Soot and mercury (Hg) are two notorious air pollutants, and the fate and transport of Hg may be affected by soot at various scales in the environment as soot may be both a carrier and a reactant for active Hg species. This study was designed to quantify photoreduction of Hg(II) in the presence of soot and the associated Hg isotope fractionation under both atmospheric aerosol and aqueous conditions (water-saturated). Photoreduction experiments were conducted with diesel soot particulate matter under controlled temperature and relative humidity (RH) conditions using a flow-through semibatch reactor system. Mass-dependent fractionation resulted in the enrichment of heavier Hg isotopes in the remaining Hg(II) with enrichment factors (ε202Hg) of 1.48 ± 0.02‰ (±2 standard deviation) to 1.75 ± 0.05‰ for aerosol-phase reactions (RH 28-68%) and from 1.26 ± 0.11 to 1.50 ± 0.04‰ for aqueous-phase reactions. Positive odd mass-independent fractionation (MIF) was observed in aqueous-phase reactions, resulting in Δ199Hg values for reactant Hg(II) as high as 5.29‰, but negative odd-MIF occurred in aerosol-phase reactions, in which Δ199Hg values of reactant Hg(II) varied from -1.02 to 0‰. The average ratio of Δ199Hg/Δ201Hg (1.1) indicated that under all conditions, MIF was dominated by magnetic isotope effects during photoreduction of Hg(II). Increasing RH resulted in higher reduction rates but lower extents of negative MIF in the aerosol-phase experiments, suggesting that the reduction of soot particle-bound Hg(II) was responsible for the observed negative odd-MIF. Our results suggest that mass-independent Hg isotope fractionation during Hg(II) photoreduction varies with soot aerosol water content and that Hg-stable isotope ratios may be used to understand the transformational histories of aerosol-bound Hg(II) in the environment.

Mass-independent fractionation of oxygen isotopes during thermal decomposition of divalent metal carbonates: Crystallographic influence, potential mechanism and cosmochemical significance

Few physical or chemical processes defy well-established laws of mass-dependent isotopic fractionation. A surprising example, discovered two decades ago, is that thermal decomposition of calcium and magnesium carbonate minerals (conducted in vacuo, to minimise back-reaction and isotopic exchange) causes the oxygen triple-isotope compositions of the resulting solid oxide and CO2 to fit on parallel mass-dependent fractionation lines in ln(1 + δ17O) versus ln(1 + δ18O) space, with anomalous depletion of 17O in the solid and equivalent enrichment of 17O in the CO2. By investigating the thermal decomposition of other natural divalent metal carbonates and one synthetic example, under similar conditions, we find that the unusual isotope effect occurs in all cases and that the magnitude of the anomaly (Δ′17O) seems to depend on the room temperature crystallographic structure of the carbonate. A lower cation coordination number (as associated with smaller cation radius) correlates with a Δ′17O value closer to zero. Local symmetry considerations may therefore be influential. Relative to a reference fractionation line of slope 0.524 and passing through VSMOW, solid oxides produced by thermal decomposition of orthorhombic carbonates were characterised by Δ′17O = −0.367 ± 0.004‰ (standard error). The comparable figure from rhombohedral examples was −0.317 ± 0.010‰, whereas from the sole monoclinic (synthesised) specimen it was −0.219 ± 0.011‰. The numerical values are, to some extent, dependent on details of the experimental procedure. We discuss potential origins of the isotopic anomaly, including the possibility of hyperfine coupling between 17O nuclei and unpaired electrons of transient radicals (the ‘magnetic isotope effect’). A new mechanism based on the latter process is proposed. The associated transition state is compatible with that suggested by recent quantum chemical and kinetic studies of the thermal decompositions of calcite and magnesite. An earlier suggestion based on the magnetic isotope effect is shown to be incompatible with the generation of a 17O anomaly, regardless of the identity of the carbonate. We cannot exclude the possibility that a Fermi resonance between states leading to dissociation may additionally affect the magnitude of Δ′17O in some cases. Our findings have cosmochemical implications, with thermal processing of carbonates providing a potential mechanism for the mass-independent fractionation of oxygen isotopes in protoplanetary systems.

Mass-Independent Fractionation of Even and Odd Mercury Isotopes during Atmospheric Mercury Redox Reactions.

Mass-independent fractionation (MIF) of stable even mass number mercury (Hg) isotopes is observed in rainfall and gaseous elemental Hg0 globally and is used to quantify atmospheric Hg deposition pathways. The chemical reaction and underlying even-Hg MIF mechanism are unknown however and speculated to be caused by Hg photo-oxidation on aerosols at the tropopause. Here, we investigate the Hg isotope composition of free tropospheric Hg0 and oxidized HgII forms at the high-altitude Pic du Midi Observatory. We find that gaseous oxidized Hg has positive Δ199Hg, Δ201Hg, and Δ200Hg and negative Δ204Hg signatures, similar to rainfall Hg, and we document rainfall Hg Δ196Hg to be near zero. Cloud water and rainfall Hg show an enhanced odd-Hg MIF of 0.3‰ compared to gaseous oxidized HgII, potentially indicating the occurrence of in-cloud aqueous HgII photoreduction. Diurnal MIF observations of free tropospheric Hg0 show how net Hg0 oxidation in high-altitude air masses leads to opposite even- and odd-MIF in Hg0 and oxidized HgII. We speculate that even-Hg MIF takes place by a molecular magnetic isotope effect during HgII photoreduction on aerosols that involves magnetic halogen nuclei. A Δ200Hg mass balance suggests that global Hg deposition pathways in models are likely biased toward HgII deposition. We propose that Hg cycling models could accommodate the Hg-isotope constraints on emission and deposition fluxes.

Improved NMR transfer of magnetization from protons to half-integer spin quadrupolar nuclei at moderate and high magic-angle spinning frequencies.

Half-integer spin quadrupolar nuclei are the only magnetic isotopes for the majority of the chemical elements. Therefore, the transfer of polarization from protons to these isotopes under magic-angle spinning (MAS) can provide precious insights into the interatomic proximities in hydrogen-containing solids, including organic, hybrid, nanostructured and biological solids. This transfer has recently been combined with dynamic nuclear polarization (DNP) in order to enhance the NMR signal of half-integer quadrupolar isotopes. However, the cross-polarization transfer lacks robustness in the case of quadrupolar nuclei, and we have recently introduced as an alternative technique a -RINEPT (through-space refocused insensitive nuclei enhancement by polarization transfer) scheme combining a heteronuclear dipolar recoupling built from adiabatic pulses and a continuous-wave decoupling. This technique has been demonstrated at 9.4 T with moderate MAS frequencies, -15 kHz, in order to transfer the DNP-enhanced H polarization to quadrupolar nuclei. Nevertheless, polarization transfers from protons to quadrupolar nuclei are also required at higher MAS frequencies in order to improve the H resolution. We investigate here how this transfer can be achieved at and 60 kHz. We demonstrate that the -RINEPT sequence using adiabatic pulses still produces efficient and robust transfers but requires large radio-frequency (rf) fields, which may not be compatible with the specifications of most MAS probes. As an alternative, we introduce robust and efficient variants of the -RINEPT and PRESTO (phase-shifted recoupling effects a smooth transfer of order) sequences using symmetry-based recoupling schemes built from single and composite pulses. Their performances are compared using the average Hamiltonian theory and experiments at T on -alumina and isopropylamine-templated microporous aluminophosphate (AlPO-14), featuring low and significant H-H dipolar interactions, respectively. These experiments demonstrate that the H magnetization can be efficiently transferred to Al nuclei using -RINEPT with (27090) recoupling and using PRESTO with (180) or (27090) schemes at or 62.5 kHz, respectively. The -RINEPT and PRESTO recoupling schemes complement each other since the latter is affected by dipolar truncation, whereas the former is not. We also analyze the losses during these recoupling schemes, and we show how these magnetization transfers can be used at kHz to acquire in 72 min 2D HETCOR (heteronuclear correlation) spectra between H and quadrupolar nuclei, with a non-uniform sampling (NUS).

Mechanistic Aspects Regarding the Ultraviolet Degradation of Polychlorinated Biphenyls in Different Media: Insights from Carbon and Chlorine Isotope Fractionation.

In this study, the carbon and chlorine isotope fractionation during ultraviolet-photolysis of polychlorinated biphenyls (PCBs, including PCB18, PCB77, PCB110, and PCB138) in n-hexane (Hex), methanol/water (MeOH/H2O), and silica gel was first investigated to explore their mechanistic processes. We observed a significant variation in ΛCl-C (εCl/εC) for the same PCBs in different photochemical systems, implying that PCB degradation processes in various photoreaction systems could differ. Although all substrates showed normal apparent carbon/chlorine kinetic isotope effects (C-/Cl-AKIE >1), the putative inverse C-AKIE of nondechlorinated pathways was suggested by 13C depletion of the average carbon isotope composition of PCB138 and corresponding dechlorinated products in MeOH/H2O, which might originate from the magnetic isotope effect. Significant negative correlations were found between C-AKIE and relative disappearance quantum yields ("Φ") of ortho-dechlorinated substrates (PCB18, PCB110, and PCB138) in Hex and MeOH/H2O. However, the C-AKIE and "Φ" of PCB77 (meta/para-dechlorinated congener) obviously deviated from the above correlations. Furthermore, significantly different product-related carbon isotope enrichment factors of PCB77 in Hex were found. These results demonstrated the existence of dechlorination position-specific and masking effects in carbon isotope fractionations.