Nuclear Polarization Transfer Research Articles

Parahydrogen-Induced Hyperpolarization of Unsaturated Phosphoric Acid Derivatives

Parahydrogen-induced nuclear polarization offers a significant increase in the sensitivity of NMR spectroscopy to create new probes for medical diagnostics by magnetic resonance imaging. As precursors of the biocompatible hyperpolarized probes, unsaturated derivatives of phosphoric acid, propargyl and allyl phosphates, are proposed. The polarization transfer to 1H and 31P nuclei of the products of their hydrogenation by parahydrogen under the ALTADENA and PASADENA conditions, and by the PH-ECHO-INEPT+ pulse sequence of NMR spectroscopy, resulted in a very high signal amplification, which is among the largest for parahydrogen-induced nuclear polarization transfer to the 31P nucleus.

Neutron physics investigations of fundamental processes of statistical mechanics

Studies in spin dynamics of disordered media and multiple ultra-small angle neutron scattering are considered. The experiments were carried out on unique installations designed in ITEP laboratory of neutron physics: beta-NMR spectrometer and universal neutron diffractometer. The main attention is paid to random walks in disordered systems and ultra-small angle neutron scattering (USANS) on objects with space correlations in positions of scatterers. Synthesis of concentration expansion, semi-phenomenological theory and numerical simulations produced satisfactory description of the nuclear polarization transfer within disordered 8Li–6Li spin subsystem in LiF single crystal. The theory of USANS starts from eikonal approximation for the scattering amplitude, which (a) reproduces the phenomenological Moliere–Bethe theory for the observable neutron angular distributions for uncorrelated random positions of scatterers, and (b) gives a possibility to take into account their spatial correlations.

Direct and Indirect Dynamic Nuclear Polarization Transfer Observed in Mesoporous Materials Impregnated with Nonionic Surfactant Solutions of Polar Polarizing Agents

Mesoporous silica materials (SBA-15) with surfaces modified with aminopropyltriethoxysilane (APTES) of two different surface coverages were synthesized, and their structural pore characteristics were analyzed. These two mesoporous silica materials were impregnated with various solutions of radicals in a nonionic surfactant solvent. Differential scanning calorimetry (DSC) analysis of the impregnated mesoporous silica materials confirmed that the surfactant solutions were confined into the pores. Dynamic nuclear polarization (DNP)-enhanced solid-state 13C magic-angle spinning (MAS) NMR spectra recorded for these impregnated mesoporous silica materials showed the presence of superimposed spectra from direct and indirect channel polarization transfer processes not only for the confined surfactant solvent but also for the APTES surface modification. The observation of the indirect channel resonances implies that the surfactant solvents as well as the APTES exhibit molecular motions with correlation times on th...

Chirality-Induced Electron Spin Polarization and Enantiospecific Response in Solid-State Cross-Polarization Nuclear Magnetic Resonance.

NMR-based techniques are supposed to be incapable of distinguishing pure crystalline chemical enantiomers. However, through systematic studies of cross-polarization magic angle spinning (CP-MAS) NMR in a series of amino acids, we have found a rather unexpected behavior in the intensity pattern of optical isomers in hydrogen/nitrogen nuclear polarization transfer that would allow the use of CP NMR as a nondestructive enantioselective detection technique. In all molecules considered, the d isomer yields higher intensity than the l form, while the chemical shift for all nuclei involved remains unchanged. We attribute this striking result to the onset of electron spin polarization, accompanying bond charge polarization through a chiral center, a secondary mechanism for polarization transfer that is triggered only in the CP experimental setup. Electron spin polarization is due to the chiral-induced spin selectivity effect (CISS), which creates an enantioselective response, analogous to the one involved in molecular recognition and enantiospecific separation with achiral magnetic substrates. This polarization influences the molecular magnetic environment, modifying the longitudinal relaxation time T1 of 1H, and ultimately provoking the observed asymmetry in the enantiomeric response.

Tweezers for Parahydrogen: A Metal-Free Probe of Nonequilibrium Nuclear Spin States of H2 Molecules

To date, only metal-containing hydrogenation catalysts have been utilized for producing substantial NMR signal enhancements by means of parahydrogen-induced polarization (PHIP). Herein, we show that metal-free compounds known as molecular tweezers are useful in this respect. It is shown that ansa-aminoborane tweezers QCAT provided (20-30)-fold signal enhancements of parahydrogen-originating hydrogens in (1)H NMR spectra. Nuclear polarization transfer from the polarized hydrogens to (11)B nuclei leads to a 10-fold enhancement in the (11)B NMR spectrum. Moreover, our results indicate that dihydrogen activation by QCAT and CAT tweezers is carried out in a pairwise manner, and PHIP can be used for understanding the activation mechanism in metal-free catalytic systems in general.

Production of nuclear polarization of unstable nuclei via polarization transfer reactions

The polarized 20F(Iπ = 2 + , T1/2 = 11.163 s), 24mAl(Iπ = 1 + , T1/2 = 131.3 ms), 29P(Iπ = 1/2 + , T1/2 = 4.142 s) and 41Sc(Iπ = 7/2−, T1/2 = 596.3 ms) isotopes were produced by nuclear-polarization transfer reactions with polarized proton or deuteron beams. In the case of 29P isotope, we have produced the polarization by two reactions; 29Si(\(\overrightarrow{p}\),n)29P and 28Si(\(\overrightarrow{d}\),n)29P. The degree of nuclear polarization of those nuclei have been measured using the β-ray detected nuclear magnetic resonance method.

Optimization of Electron–Nuclear Polarization Transfer

Optimal-control-based numerical algorithms make it possible to explore the physical limits of dynamic nuclear polarization. Examples of time-optimal and relaxation-optimized electron–nuclear polarization transfer experiments are presented for simple model systems consisting of an isolated electron–nuclear spin pair in the absence and presence of relaxation. The optimized pulse sequences are compared with conventional transfer schemes, such as electron–nuclear cross polarization and selective population inversion.

PrF3Van Vleck paramagnet as a promising material for the nuclear dynamic polarization of 3He

We suggest using insulating Van Vleck paramagnet PrF3 as a solid substrate for the dynamic polarization of 3He nuclei at high magnetic fields. The exploring of solid effect for this purpose assumes the knowledge of the optimal conditions for transfer of polarization from the Pr3+ electron shell in PrF3 to 141Pr nuclear spins as well as the existence of effective channel for magnetization transfer from 141Pr nuclear spins to nuclear spins of liquid 3He (so-called magnetic coupling phenomenon). To study solid effect in PrF3 the magnetic field dependencies of the Stark energy levels of 3H4 ground state multiple were calculated for high magnetic fields up to 40 T using the set of crystal-field parameters obtained early from magnetization measurements and exchange charges model. Also the results of searching for a direct magnetic coupling between nuclei of the liquid 3He and 141Pr nuclei in the system "PrF3 powder – liquid 3He" by pulse NMR method are represented. Also the surface effects in the PrF3 crystal were studied by SQUID and optical microscopy measurements. Other aspects of the nuclear polarization transfer are discussed.

Experimental Investigation of Nuclear Polarization Transfer in Spatially Disordered Spin System by Means of β-NMR Spectroscopy

β-NMR studies of delocalization of polarization of 8Li β-active nuclei in spatially disordered 8Li–6Li nuclear system in LiF single crystals are presented. Unique peculiarity of the investigated 8Li–6Li nuclear spin system is that the g-factors of these isotopes are very close. So, the cross-relaxation between them dominates over all other depolarization mechanisms. Depolarization kinetics of 8Li nuclear probe in LiF crystals are measured in a wide range of 6Li concentration: 0.15, 3.28, 5.30, 8.81, and 10.06% and retraced up to (8–10) T 1/2(8Li). The obtained data show good agreement with theoretical descriptions.

Direct NMR observation and DFT calculations of a hydrogen bond at the active site of a 44 kDa enzyme.

A hydrogen bond between the amide backbone of Arg7 and the remote imidazole side chain of HisIO6 has been directly observed by improved TROSY-NMR techniques in the 44 kDa trimeric enzyme chorismate mutase from Bacillus subtilis. The presence of this hydrogen bond in the free enzyme and its complexes with a transition state analog and the reaction product was demonstrated by measurement of 15N-15N and 1H-15N trans-hydrogen bond scalar couplings, (2h)J(NN) and (lh)J(HN), and by transfer of nuclear polarization across the hydrogen bond. The conformational dependences of these coupling constants were analyzed using sum-over-states density functional perturbation theory (SOS-DFPT). The observed hydrogen bond might stabilize the scaffold at the active site of BsCM. Because the Arg7-His 106 hydrogen bond has not been observed in any of the high resolution crystal structures of BsCM, the measured coupling constants provide unique information about the enzyme and its complexes that should prove useful for structural refinement of atomic models.

Mechanism of dynamic nuclear polarization in high magnetic fields

Solid-state NMR signal enhancements of about two orders of magnitude (100–400) have been observed in dynamic nuclear polarization (DNP) experiments performed at high magnetic field (5 T) and low temperature (10 K) using the nitroxide radical 4-amino TEMPO as the source of electron polarization. Since the breadth of the 4-amino TEMPO EPR spectrum is large compared to the nuclear Larmor frequency, it has been assumed that thermal mixing (TM) is the dominate mechanism by which polarization is transferred from electron to nuclear spins. However, theoretical explanations of TM generally assume a homogeneously broadened EPR line and, since the 4-amino TEMPO line at 5 T is inhomogeneously broadened, they do not explain the observed DNP enhancements. Accordingly, we have developed a treatment of DNP that explicitly uses electron–electron cross-relaxation to mediate electron–nuclear polarization transfer. The process proceeds via spin flip–flops between pairs of electronic spin packets whose Zeeman temperatures differ from one another. To confirm the essential features of the model we have studied the field dependence of electron–electron double resonance (ELDOR) data and DNP enhancement data. Both are well simulated using a simple model of electron cross-relaxation in the inhomogeneously broadened 4-amino TEMPO EPR line.

Dynamic Nuclear Polarization in Pulsed ENDOR Experiments

Properly prepared pulse sequences of microwave and radio frequency have been employed to investigate the effect of polarization transfer from the polarized photo excited triplet state of pentacene inp-terphenyl crystals to the surrounding protons in pulsed ENDOR experiments. The ENDOR signal, measured as the change of electron spin echo (ESE) amplitude, is affected by the mode of RF pulses. WhenB0∥x(the long molecular axis), the ESE amplitude of the high-field transition of the triplet state changes from the maximum positive to zero with a π RF pulse, and to the maximum negative with a 2π pulse, while that of the low-field transition changes from nearly zero to the maximum negative as the RF pulse width increases. The effect is attributed to the strong electron spin polarization produced in the creation of the photoexcited triplet state and the subsequent efficient electron– nuclear polarization transfer process.

Spin coherence effects in the electron—nuclear polarization transfer process

The nuclear spin polarization resulting from optical pumping of molecular triplet states, ONP, has been studied in a time-resolving experiment by synchronized irradiation of light and rf pulses. After laser flash excitation of T 1 triplet states of acridine doped into a fluorene crystal, an rf pulse of variable intensity and duration is applied near the resonance of an electronic spin transition. It leads to partial transfer of optically generated electronic polarization to the nuclear spin reservoir (rf-ONP). For sufficiently high rf-intensity, the polarization transfer shows an oscillatory behaviour when varying the pulse length in the submicrosecond range, which reflects the initial coherence among the spins. Critical tests for the analysis are provided by experiments under different rf excitation conditions and for various isotopic compositions. The transfer process is shown to involve two steps on different time scales, the first of which is closely related to nutations of electron spins about the rotating B 1 field.

Analysis of double cross-polarization rates in solid proteins

The double cross-polarization experiment involving the sequential transfer of nuclear polarization among 1H-, 15N-, and 13C-spin systems can be used to detect the occurrence of 13C- 15N bonds in solid proteins. However, the complexity of most proteins requires simplifying approximations to make possible the determination of the concentrations of those 13C- 15N bonds. This paper demonstrates that a model involving only two 13C- 15N cross-polarization rates (determined from model compounds) and simple statistics is adequate to treat data from heavily 13C- and 15N-labeled proteins in soybean leaves. In the course of examining the applicability of model compounds to this problem, we have used a moment analysis of 15N NMR lineshapes to determine 13C- 15N dipolar couplings in two doubly labeled amino acids, glycine and asparagine.