Cotton Defoliation Guide in Texas.

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Direct enhancement of nitrogen-15 targets at high-field by fast ADAPT-SABRE

Signal Amplification by Reversible Exchange (SABRE) is an attractive nuclear spin hyperpolarization technique capable of huge sensitivity enhancement in nuclear magnetic resonance (NMR) detection. The resonance condition of SABRE hyperpolarization depends on coherent spin mixing, which can be achieved naturally at a low magnetic field. The optimum transfer field to spin-1/2 heteronuclei is technically demanding, as it requires field strengths weaker than the earth's magnetic field for efficient spin mixing. In this paper, we illustrate an approach to achieve strong 15N SABRE hyperpolarization at high magnetic field by a radio frequency (RF) driven coherent transfer mechanism based on alternate pulsing and delay to achieve polarization transfer. The presented scheme is found to be highly robust and much faster than existing related methods, producing ∼3 orders of magnitude 15N signal enhancement within 2 s of RF pulsing

Doping and phase segregation in Mn2+- and Co2+-doped lead halide perovskites from Cs-133 and H-1 NMR relaxation enhancement

Lead halide perovskites belong to a broad class of compounds with appealing optoelectronic and photovoltaic properties. Doping with transition metal ions such as Mn2+ and Co2+ has recently been reported to substantially enhance luminescence and stability of these materials. However, so far atomic-level evidence for incorporation of the dopants into perovskite phases has been missing. Here, we introduce a general and straightforward method for confirming the substitutional doping of bulk perovskite phases with paramagnetic dopants. Using Cs-133 and H-1 solid-state MAS NMR relaxation measurements we provide for the first time direct evidence that, consistent with current understanding, Mn2+ is incorporated into the perovskite lattice of CsPbCl3 and CsPbBr3 and does not form clusters. We also show that, contrary to current conviction, Co2+ is not incorporated into the perovskite lattice of MAPbI(3)

Sub-minute kinetics of human red cell fumarase: H-1 spin-echo NMR spectroscopy and C-13 rapid-dissolution dynamic nuclear polarization

Fumarate is an important probe of metabolism in hyperpolarized magnetic resonance imaging and spectroscopy. It is used to detect the release of fumarase in cancer tissues, which is associated with necrosis and drug treatment. Nevertheless, there are limited reports describing the detailed kinetic studies of this enzyme in various cells and tissues. Thus, we aimed to evaluate the sub-minute kinetics of human red blood cell fumarase using nuclear magnetic resonance (NMR) spectroscopy, and to provide a quantitative description of the enzyme that is relevant to the use of fumarate as a probe of cell rupture. The fumarase reaction was studied using time courses of H-1 spin-echo and C-13-NMR spectra. H-1-NMR experiments showed that the fumarase reaction in hemolysates is sufficiently rapid to make its kinetics amenable to study in a period of approximately 3 min, a timescale characteristic of hyperpolarized C-13-NMR spectroscopy. The rapid-dissolution dynamic nuclear polarization (RD-DNP) technique was used to hyperpolarize [1,4-C-13]fumarate, which was injected into concentrated hemolysates. The kinetic data were analyzed using recently developed FmR analysis and modeling of the enzymatic reaction using Michaelis-Menten equations. In RD-DNP experiments, the decline in the C-13-NMR signal from fumarate, and the concurrent rise and fall of that from malate, were captured with high spectral resolution and signal-to-noise ratio, which allowed the robust quantification of fumarase kinetics. The kinetic parameters obtained indicate the potential contribution of hemolysis to the overall rate of the fumarase reaction when C-13-NMR RD-DNP is used to detect necrosis in animal models of implanted tumors. The analytical procedures developed will be applicable to studies of other rapid enzymatic reactions using conventional and hyperpolarized substrate NMR spectroscopy.Cancer Research UK‐Engineering and Physical Sciences Research Council (CRUK/EPSRC) Imaging Centre in Cambridge and Manchester, Grant/Award Number: 16465; Cancer Research UK Programme, Grant/Award Number: 17242; European Research Council (ERC); Australian Research Council, Grant/Award Number: DP14010259

Sub-minute kinetics of human red cell fumarase: 1 H spin-echo NMR spectroscopy and 13 C rapid-dissolution dynamic nuclear polarization.

Fumarate is an important probe of metabolism in hyperpolarized magnetic resonance imaging and spectroscopy. It is used to detect the release of fumarase in cancer tissues, which is associated with necrosis and drug treatment. Nevertheless, there are limited reports describing the detailed kinetic studies of this enzyme in various cells and tissues. Thus, we aimed to evaluate the sub-minute kinetics of human red blood cell fumarase using nuclear magnetic resonance (NMR) spectroscopy, and to provide a quantitative description of the enzyme that is relevant to the use of fumarate as a probe of cell rupture. The fumarase reaction was studied using time courses of 1 H spin-echo and 13 C-NMR spectra. 1 H-NMR experiments showed that the fumarase reaction in hemolysates is sufficiently rapid to make its kinetics amenable to study in a period of approximately 3 min, a timescale characteristic of hyperpolarized 13 C-NMR spectroscopy. The rapid-dissolution dynamic nuclear polarization (RD-DNP) technique was used to hyperpolarize [1,4-13 C]fumarate, which was injected into concentrated hemolysates. The kinetic data were analyzed using recently developed FmRα analysis and modeling of the enzymatic reaction using Michaelis-Menten equations. In RD-DNP experiments, the decline in the 13 C-NMR signal from fumarate, and the concurrent rise and fall of that from malate, were captured with high spectral resolution and signal-to-noise ratio, which allowed the robust quantification of fumarase kinetics. The kinetic parameters obtained indicate the potential contribution of hemolysis to the overall rate of the fumarase reaction when 13 C-NMR RD-DNP is used to detect necrosis in animal models of implanted tumors. The analytical procedures developed will be applicable to studies of other rapid enzymatic reactions using conventional and hyperpolarized substrate NMR spectroscopy

Enhanced Intersystem Crossing and Transient Electron Spin Polarization in a Photoexcited Pentacene-Trityl Radical

Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics and magnetic resonance spectroscopy. The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics. After photoexcitation of the chromophore, pentacene-radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing. Under the right conditions, the triplet spin polarization, arising from mS-selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited state energetics, molecular geometry, and spin-spin coupling. Here we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical, but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a two orders of magnitude difference in the timescale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals

Long-lived states in multi-spin systems

Long-lived states are nuclear spin configurations that, in suitable circumstances, decay very slowly towards thermal equilibrium. The first paper on the subject reported a long-lived order in 2,3-dibromothiophene between a pair of inequivalent proton nuclei of about 100 s at about 20 mT. The lifetime exceeded the relaxation time of longitudinal magnetization T1 by more than one order of magnitude. Currently many systems cansurvive T1 even at magnetic fields of several Tesla. Long lifetimes may benefit different methodologies used to investigate for example nuclear spin diffusion, chemical reactivity and metabolic processes. In addition hyperpolarization methods may profit from long-lived states in order to enhance both sensitivity and temporal resolution. Although this research field is relatively young, the first publication being 11 years old, about 150 investigations so far have been published on peer-reviewed scientific journals on this subject. In this work the main focus is to extend the analysis to multiple spin systems. The structure of the thesis is composed of a theoretical and an experimental part. We propose a model based on nuclear spin permutations that uses the formalism of discrete group theory. This approach allows the classification of nuclear wave functions and internal Hamiltonian operators, according to nuclear spin permutation symmetry, in order to predict the number of long-lived orders and their analytical expression. The mathematical structure can also be applied to investigate fundamental bounds on spin conversion in the presence of symmetry. The theoretical model is grounded on a set of approximations used to define the symmetry operations and the corresponding permutation symmetry groups. The experimental section includes examples of long-lived orders occurring under different magnetic, geometric and dynamic conditions. This large variety of regimes shows on one hand the ubiquitous character of long-lived species. On the other hand a common trait is identified in the formal characterisation that uses permutation and rotational symmetry concepts. For this reason a permutation symmetry characterisation is presented alongside the experimental description. The role of local geometry in a rigid spin system is highlighted by comparing two isomers with a different local arrangement of spin nuclei, and showing how a long-lived order is predicted and detected only in one case. Non rigid molecules can also display long-lived character. This is demonstrated by considering the methyl group 13CH3 in ?-picoline. Interestingly, as proton nuclei are magnetically equivalent, the long-lived order accessibility cannot employ coherent mechanisms. Finally a derivative of naphthalene is shown to possess an exceptionally long lifetime in solution and at room temperature. The accessibility of a very long lifetime opens up the possibility to store (hyper)polarization into singlet order and retrieve it later in time. A set of preliminary dissolution dynamic nuclear polarization experiments are also presented as a first attempt in this sense

Homonuclear ADAPT: A general preparation route to long-lived nuclear singlet order

We introduce a simple strategy to access and readout nuclear singlet order based on the alternate repetition of hard pulses and delays. We demonstrate the general applicability of the method by accessing nuclear singlet order in spin systems characterized by diverse coupling regimes. We show that the method is highly efficient in the strong-coupling and chemical equivalence regimes, and can overcome some limitations of other well-established and more elaborated pulse sequences. A simulation package is provided which allows the determination of pulse sequence parameters.</p