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ABSTRACT: Many approaches are now available for achieving high levels of nuclear spin polarization. One of these methods is based on the notion that as the temperature is reduced, the equilibrium nuclear polarization will increase, according to the Boltzmann distribution. The main problem with this approach is the length of time it may take to approach thermal equilibrium at low temperatures, since nuclear relaxation times (characterized by the spin-lattice relaxation time T1) can become very long. Here, we show, by means of relaxation time measurements of frozen solutions, that selected lanthanide ions, in the form of their chelates with DTPA, can act as effective relaxation agents at low temperatures. Differential effects are seen with the different lanthanides that were tested, holmium and dysprosium showing highest relaxivity, while gadolinium is ineffective at temperatures of 20 K and below. These observations are consistent with the known electron-spin relaxation time characteristics of these lanthanides. The maximum relaxivity occurs at around 10 K for Ho-DTPA and 20 K for Dy-DTPA. Moreover, these two agents show only modest relaxivity at room temperature, and can thus be regarded as relaxation switches. We conclude that these agents can speed up solid state NMR experiments by reducing the T1 values of the relevant nuclei, and hence increasing the rate at which data can be acquired. They could also be of value in the context of a simple low-cost method of achieving several-hundred-fold improvements in polarization for experiments in which samples are pre-polarized at low temperatures, then rewarmed and dissolved immediately prior to analysis.
Physical Chemistry Chemical Physics 04/2013; · 3.57 Impact Factor
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ABSTRACT: Over the years, several strategies have been developed for generating highly polarized nuclear spin systems, including dynamic nuclear polarization, optical pumping, and methods exploiting parahydrogen. Here, we present an alternative strategy, using an enhanced 'brute-force' approach (i.e. exposure to low temperatures and high applied magnetic fields). The main problem with this approach is that it may take an excessively long time for the nuclear polarization to approach thermal equilibrium at low temperatures, since nuclear relaxation becomes exceedingly slow due to the loss of molecular motion. We show that low-field thermal mixing can alleviate the problem by increasing the rate at which slowly-relaxing nuclei reach equilibrium. More specifically, we show that polarization can be transferred from a relatively rapidly relaxing (1)H reservoir to more slowly relaxing (13)C and (31)P nuclei. The effects are particularly dramatic for the (31)P nuclei, which in experiments at a temperature of 4.2 K and a field of 2 T show a 75-fold enhancement in their effective rate of approach to equilibrium, and an even greater (150-fold) enhancement in the presence of a relaxation agent. The mixing step is also very effective in terms of the amount of polarization transferred-70-90% of the maximum theoretical value in the experiments reported here. These findings have important implications for brute-force polarization, for the problem becomes one of how to relax the solvent protons rather than individual more slowly-relaxing nuclei of interest. This should be a much more tractable proposition, and offers the additional attraction that a wide range of nuclear species can be polarized simultaneously. We further show that the (1)H reservoir can be tapped repeatedly through a number of consecutive thermal mixing steps, and that this could provide additional sensitivity enhancement in solid-state NMR.
Physical Chemistry Chemical Physics 03/2012; 14(16):5397-402. · 3.57 Impact Factor
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ABSTRACT: Strongly enhanced spin polarization in the form of longitudinal spin order can be generated on target molecules by using parahydrogen in a catalyzed hydrogenation reaction. An optimal control algorithm was used to generate radiofrequency pulse sequences which convert the arising longitudinal two-spin order into single-spin Zeeman order with high efficiency and distribute it evenly between three coupled spins within the same molecule. The pulses are designed to be very robust towards variations in the B(0) and B(1) fields. Furthermore, this strategy is applied to enhance the NMR signal in an ultrafast gradient assisted single excitation two-dimensional spectroscopy experiment.
The Journal of chemical physics 03/2012; 136(9):094201. · 3.09 Impact Factor
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ABSTRACT: A strategy is described for simulations of solid effect dynamic nuclear polarisation that reduces substantially the dimension of the quantum mechanical problem. Averaging the Hamiltonian in the doubly rotating frame is used to confine the active space to the zero quantum coherence subspace. A further restriction of the Liouville space is made by truncating higher spin order states, which are weakly populated due to the presence of relaxation processes. Based on a dissipative transport equation, which is used to estimate the transport of the magnetisation starting from single spin order to higher spin order states, a minimal spin order for the states is calculated that needs to be taken into account for the spin dynamics simulation. The strategy accelerates individual spin calculations by orders of magnitude, thus making it possible to simulate the polarisation dynamics of systems with up to 25 nuclear spins.
Physical Chemistry Chemical Physics 02/2012; 14(8):2658-68. · 3.57 Impact Factor
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ABSTRACT: We present an algebraic foundation for the state space restriction approximation in spin dynamics simulations and derive applicability criteria as well as minimal basis set requirements for practically encountered simulation tasks. The results are illustrated with nuclear magnetic resonance (NMR), electron spin resonance (ESR), dynamic nuclear polarization (DNP), and spin chemistry simulations. It is demonstrated that state space restriction yields accurate results in systems where the time scale of spin relaxation processes approximately matches the time scale of the experiment. Rigorous error bounds and basis set requirements are derived.
The Journal of chemical physics 08/2011; 135(8):084106. · 3.09 Impact Factor
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ABSTRACT: Nuclear magnetic resonance (NMR) in combination with pulsed magnetic field gradients has proven very successful for measuring molecular diffusion, where the correlation time of the motion is much shorter than the timescale of the experiment. In this article, it is demonstrated that a single-scan NMR technique to measure molecular diffusion can be employed to also study incoherent random motions over macroscopic length scales that show correlation times similar to the timescale of the experiment. Such motions are observed, for example, after the mixing of two components or after transferring a sample from one container into another. To measure the fluid settling, a series of magnetization helices were encoded onto a sample. Stimulated gradient echo trains were then generated after different mixing times, which enabled the determination of an effective dispersion coefficient for the fluid. This technique was used to optimize the timing of NMR experiments combined with dissolution dynamic nuclear polarization, where a sample was shuttled between two magnets. In addition to the decay of fluid turbulences, the presence of microbubbles in the sample tube at the end of the shuttling step was identified as another contribution to the NMR linewidth. Microbubbles could be indirectly observed through the line broadening effect on the NMR signal due to their different susceptibility compared to the solvent, which induced field gradients near the interfaces. Using these data, the signal attenuation caused by sample motion in single-scan two-dimensional correlation spectroscopy NMR experiments could be predicted with reasonable accuracy.
The Journal of chemical physics 06/2010; 132(24):244507. · 3.09 Impact Factor
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ABSTRACT: Short acquisition time and single scan capability of gradient-assisted ultrafast multidimensional spectroscopy makes it possible to record 2D spectra of highly polarised spin systems in the liquid state using dynamic nuclear polarization (DNP) in conjunction with fast dissolution. We present a slice selective experiment, suitable for back-to-back acquisition of two independent single-scan 2D experiments from different sample volumes. This scheme maximizes the amount of information obtainable from a sample that is prepolarised with a non-repeatable DNP technique. It is particularly suitable for samples with the short longitudinal relaxation times common to proton NMR spectroscopy. This technique is demonstrated by applying two filtered proton 2D COSY experiments on a DNP-polarised mixture of glutamine and glutamate to selectively amplify the correlation pattern of the protons connected to the beta and gamma carbons of either one of the two amino acids. Particular emphasis was put on the reproducibility of the experiments, especially the polarisation enhancement. Data for the liquid-state proton enhancement from amino acids and small proteins was assembled in a map that allowed the prediction of signal levels in liquid-state NMR experiments employing dissolution DNP.
Physical Chemistry Chemical Physics 06/2010; 12(22):5771-8. · 3.57 Impact Factor
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ABSTRACT: Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.
Physical Chemistry Chemical Physics 06/2010; 12(22):5883-92. · 3.57 Impact Factor
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ABSTRACT: In terms of the master equation of relaxation, long-lived states in systems of n interacting spin-1/2 nuclei are described as those with eigenvalues much less than the characteristic T(1) rate. It is demonstrated that degeneracies of correlations between interacting dipole-dipole pairs are responsible for their presence. The case n=3 is considered in full detail.
The Journal of chemical physics 11/2009; 131(20):204105. · 3.09 Impact Factor
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ABSTRACT: The implementation of electron paramagnetic resonance (EPR) detection in a low-temperature dissolution dynamic nuclear polarization (DNP) setup is presented. Using a coil oriented parallel to the static magnetic field, the change of the longitudinal magnetization of free radicals is measured upon resonant irradiation of an amplitude or frequency modulated microwave (mw) field. The absorption EPR spectrum is measured if the amplitude of the mw field is modulated, whilst the first derivative of the spectrum is obtained with frequency modulation. Using a burst of pulses, it is also possible to perform pump-probe experiments such as saturation-recovery or electron-electron double resonance experiments. Furthermore, the magnetization could be monitored in a time-resolved manner during amplitude modulation, thus making it possible to record its transient as it is approaching an equilibrium value. Experimental examples are shown with frozen solutions of trityl radical and TEMPO, two commonly used radicals for dissolution DNP experiments.
Journal of Magnetic Resonance 09/2007; 187(2):266-76. · 2.14 Impact Factor
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ABSTRACT: Conventional NMR spectroscopy techniques require long acquisition times due to the recovery time between the repeated excitations necessary for each increment of the evolution times in the indirectly detected dimensions. Here we outline a pulse sequence element for gradient-assisted ultrafast multidimensional NMR spectroscopy using frequency-modulated 'chirp' pulses to generate phase-modulated magnetization in an indirectly detected spectral dimension. The potential of this sequence element is demonstrated by acquiring a correlation spectroscopy (COSY) spectrum in 96 ms. This new pulse sequence element is an extension of ultrafast spectroscopy techniques based on the generation of amplitude modulation of the NMR signal in the indirectly detected spectral dimensions. The use of phase modulation instead of amplitude modulation helps broaden the applicability and may provide an increase of sensitivity in some experiments due to the ability to distinguish between positive and negative frequency offsets relative to the carrier frequency of the sequence element.
Magnetic Resonance in Chemistry 11/2005; 43(10):795-7. · 1.44 Impact Factor
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ABSTRACT: Cyclic J cross polarisation (CYCLCROP) is a sensitive method for the noninvasive monitoring of (13)C distributions and fluxes. The PRAWN rotating frame Hartmann-Hahn mixing sequence ameliorates problems associated with sensitivity to Hartmann-Hahn mismatch and reduces RF power deposition. The combination of CYCLCROP with echo planar imaging (EPI) for spatial encoding of the proton detected carbon signal allows efficient use of the available signal to be made, permitting a significant improvement in the temporal resolution of any study. We report here on some initial experiments to demonstrate the feasibility of echo planar proton detected (13)C imaging using CYCLCROP based upon the PRAWN module, including the application of the technique to the measurement of transport and accumulation of (13)C-labelled sucrose in a castor bean seedling. Two methods that can be used to eliminate the effect of the J-splitting in the EP images are presented. In addition, a fast, image-based B(1) field-mapping method which may be used to quantitatively map the low frequency RF field in a dual resonant ((13)C/(1)H) probe is presented. The technique utilises the above described imaging method, permitting fully quantitative, 64x64 axial field maps to be generated in about a minute.
Journal of Magnetic Resonance 04/2002; 155(1):64-71. · 2.14 Impact Factor
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ABSTRACT: Liquid state, rotating frame cross polarisation experiments are very sensitive to RF field inhomogeneity. In this work, we
present an easily fabricated, co-resident high- and low-pass linear birdcage resonator, optimised to perform liquid state
rotating frame polarisation transfer at1H and13C frequencies. Both the RF fields have been experimentally mapped, and used to validate the spatial signal dependence of a
proton detected,13C image. The predicted performance was then confirmed using PRAWN-based, cyclic J-cross polarisation (CYCLCROP) imaging. A
novel variant of a B1-field mapping approach is also presented, using the signal enhancement of the CYCLCROP sequence to generate proton detected,13C field maps.
MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 05/2000; 10(2):61-68. · 1.88 Impact Factor
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ABSTRACT: The use of a U-shaped high-pass birdcage coil for microscopic imaging at 11.7 T has been investigated. The study was motivated
by the requirement for a side access coil, permitting higher filling factors for the in-vivo imaging of plant petioles and
stems. The performance of a U-shaped coil (with a cross section consisting of a 16 mm diameter semi-circle plus two 12 mm
length straight sections) has been experimentally assessed, and compared both in terms of homogeneity and sensitivity to a
16 mm diameter conventional (linear) birdcage and a saddle coil of the same diameter. The U-shaped coil, which offers 12 mm
width side access, has a significantly better performance than the saddle coil, whilst providing 57% of the B1 sensitivity of the bird-cage.
MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 05/2000; 10(2):69-74. · 1.88 Impact Factor
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ABSTRACT: Cyclic J cross polarisation (CYCLCROP) is a sensitive method for the noninvasive monitoring of 13C distributions and fluxes. The PRAWN rotating frame Hartmann-Hahn mixing sequence ameliorates problems associated with sensitivity to Hartmann-Hahn mismatch and reduces RF power deposition. The combination of CYCLCROP with echo planar imaging (EPI) for spatial encoding of the proton detected carbon signal allows efficient use of the available signal to be made, permitting a significant improvement in the temporal resolution of any study. We report here on some initial experiments to demonstrate the feasibility of echo planar proton detected 13C imaging using CYCLCROP based upon the PRAWN module, including the application of the technique to the measurement of transport and accumulation of 13C-labelled sucrose in a castor bean seedling. Two methods that can be used to eliminate the effect of the J-splitting in the EP images are presented. In addition, a fast, image-based B1 field-mapping method which may be used to quantitatively map the low frequency RF field in a dual resonant (13C/1H) probe is presented. The technique utilises the above described imaging method, permitting fully quantitative, 64×64 axial field maps to be generated in about a minute.
Journal of Magnetic Resonance.
