Magnetic field dependence of13C photo-CIDNP MAS NMR in plant photosystems I and II

Photochemically induced dynamic nuclear polarization is observed in the two photosynthetic reaction centers of plants, photosystem I (PSI) and photosystem II (PSII) by¹³C magic-angle spinning nuclear magnetic resonance (NMR) at three different magnetic fields 17.6, 9.4, and 4.7 T. There is a significant difference in field dependence detected in the light-induced signal pattern of the two photosystems. For PSII the optimal NMR enhancement factor of about 5000 is observed at 4.7 T. On the other hand, the maximal light-induced signals of PSI are observed at 9.4 T.     

Temperature dependence of low field CIDNP and time resolved SNP in cyclic ketones

¹H CIDNP and time resolved Stimulated Nuclear Polarization (SNP) in the photolysis of 2,2,12,12-tetramethylcyclododecanone have been investigated at low magnetic field in the temperature range from ?70 to +90°C. Considerable differences for protons of different reaction products are seen in the CIDNP field dependences and SNP decay times at high temperatures. For disproportion products of acyl-alkyl biradicals the width of the CIDNP field dependence and the position of the emission maximum exhibit a pronounced temperature dependence with the largest shift towards low field around ?30°C. At lower temperatures a significant slowing down of the SNP decay is observed. The results are discussed in terms of a superposition of contributions from acyl-alkyl and bis(alkyl) biradicals to the nuclear spin polarization.     

Distinctive field dependence of the longitudinal muon polarization for Mu* in polycrystalline silicon

A distinctive longitudinal magnetic field dependence of the muon polarization for anomalous muonium in polycrystalline semiconductor targets has been predicted. The polarization exhibits a cusp,i.e., a discontinuous jump in the slope from negative to positive. Measurements of the longitudinal polarization for polycrystalline silicon in fields up to 0.5 T, and temperatures 53 and 200 K have been made at LAMPF. A cusp in the field dependence indeed occurs at 0.345 T, in excellent agreement with the prediction. No cusp is observed at 200 K because Mu^* has been thermally ionized.     

Time resolved CIDNP study of electron transfer reactions in proteins and model compounds

Intramolecular electron transfer (IET) from tyrosine to tryptophan cation radicals is investigated using time resolved chemically induced dynamic nuclear polarization (CIDNP) spectroscopy in combination with laser flash photolysis. In both the tryptophan-tyrosine dipeptide and the denatured state of hen lysozyme in aqueous solution, the transformation TrpH⁺ → TyrO by IET leads to an increase in the tyrosine radical concentration, growth in the tyrosine CIDNP signal, fast decay of the tryptophan CIDNP, and inversion of the phase of the CIDNP of the photosensitizing dye, 2,2′-dipyridyl. IET effects are not observed for mixtures of the amino acid or for the native state of lysozyme. The steady state CIDNP effects seen for denatured lysozyme thus depend not only on the accessibility of the amino acid residues on the surface of the protein but also on the reactivity of the radical intermediates.     

Probing four orders of magnitude of the diffusion time in porous silica glass with unconventional NMR techniques

The combined use of two unconventional NMR diffusometry techniques permits measurements of the self-diffusion coefficient of fluids confined in porous media in the time range from 100 microseconds to seconds. The fringe field stimulated echo technique (FFStE) exploits the strong steady gradient in the fringe field of a superconducting magnet. Using a standard 9.4 T (400 MHz) wide-bore magnet, for example, the gradient is 22 T/m at 375 MHz proton resonance and reaches 60 T/m at 200 MHz. Extremely short diffusion times can be probed on this basis. The magnetization grid rotating frame imaging technique (MAGROFI) is based on gradients of the radio frequency (RF) field. The RF gradients not necessarily need be constant since the data are acquired with spatial resolution along the RF gradient direction. MAGROFI is also well suited for unilateral NMR applications where all fields are intrinsically inhomogeneous. The RF gradients reached depend largely on the RF coil diameter and geometry. Using a conic shape, a value of at least 0.3 T/m can be reached which is suitable for long-time diffusion measurements. Both techniques do not require any special hardware and can be implemented on standard high RF power NMR spectrometers. As an application, the influence of the tortuosity increasing with the diffusion time is examined in a saturated porous silica glass.     

Influence of size effects on the Knight shift of NMR lines in the gallium-indium alloy

A study is reported of the Knight shift of ⁷¹Ga, ⁶⁹Ga, and ¹¹⁵In NMR lines in a liquid gallium-indium alloy with a composition of 90 at % Ga and 10 at % In, introduced into porous glasses with pore sizes of 5 and 200 nm, relative to the corresponding shifts in the bulk alloy. The measurements have been performed at room temperature. The study has revealed a size-dependent decrease in the Knight shift. A sample with a 5-nm pore size has demonstrated a noticeable difference in the magnitudes of the Knight shift of both gallium isotopes measured in magnetic fields of 9.4 and 17.6 T, which implies a dependence of the electronic susceptibility of the melt on the magnetic field under the nanoconfinement conditions.     

H DNP at 1.4 T of Water Doped with a Triarylmethyl-Based Radical

Recently a triarylmethyl-based (TAM) radical has been developed for research in biological and other aqueous systems, and in low magnetic fields, 10 mT or less, large ¹H dynamic nuclear polarization (DNP) enhancements have been reported. In this paper the DNP properties of this radical have been investigated in a considerably larger field of 1.4 T, corresponding to proton and electron Larmor frequencies of 60 MHz and 40 GHz, respectively. To avoid excessive microwave heating of the sample, an existing DNP NMR probe was modified with a screening coil, wound around the sample capillary and with its axis perpendicular to the electric component of the microwave field. It was found that with this probe the temperature increase in the sample after 4 s of microwave irradiation with an incident power of 10 W was only 16°C. For the investigations, 10 mM of the TAM radical was dissolved in deionized, but not degassed, water and put into a 1-mm i.d. and 6-mm long capillary tube. At 26°C the following results were obtained: (I) The relaxivity of the radical is 0.07 (mMs)⁻¹, in accordance with the value extrapolated from low-field results; (II) The leakage factor is 0.63, the saturation factor at maximum power is 0.85, and the coupling factor is −0.0187. It is shown that these results agree very well with an analysis where the electron–dipolar interactions are the dominant DNP mechanism, and where the relaxation transitions resulting from these interactions are governed by translational diffusion of the water molecules. Finally, the possibilities of combining DNP with magnetic resonance microscopy (MRM) are discussed. It is shown that at 26°C the overall DNP-enhanced proton polarization should become maximal in an external field of 0.3 T and become comparable to the thermal equilibrium polarization in a field of 30 T, considerably larger than the largest high-resolution magnet available to date. It is concluded that DNP MRM in this field, which corresponds to a standard microwave frequency of 9 GHz, has the potential to significantly increase the sensitivity in NMR and MRI experiments of small aqueous samples doped with the TAM radical.     

Field dependence of CIDNP in photoreactions in plastic crystalline matrix

Photo-CIDNP (chemically induced dynamic nuclear polarization) of three ketones (cycloheptanone, cyclodecanone, and acetone) in cyclohexane-d₁₂ and its dependence on the external magnetic field (0-7 T) is measured in liquid, plastic crystalline and rigid crystalline phases by temperature variation between 185 and 300 K. Spin polarization is seen only in liquid and plastic crystalline phases, but not in the rigid crystal giving evidence that molecular mobility is a requisite for the formation of CIDNP. From the characteristics of the field dependence the mechanisms of polarization are identified and properties of paramagnetic reaction intermediates determined. The factors causing the observed differences in polarization between the liquid and the solid phase are discussed in detail. *** DIRECT SUPPORT *** A04RK026 00003     

Dynamic nuclear polarization at 9T using a novel 250 GHz gyrotron microwave source. 2003

In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9 T (250 GHz for g = 2 electrons, 380 MHz for ¹H). In these experiments, ¹H enhancements of up to 170 ± 50 have been observed in 1-¹³C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20 K; in addition, we have observed significant enhancements in ¹⁵N spectra of unoriented pf1-bacteriophage. Finally, enhancements of ∼17 have been obtained in two-dimensional ¹³C–¹³C chemical shift correlation spectra of the amino acid U–¹³C, ¹⁵N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250 GHz gyrotron oscillator used in these experiments.     

The transient EPR spectra and spin dynamics of coupled three-spin systems in photosynthetic reaction centres

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an “observer”. The spin polarized transient EPR spectra are investigated using the coupled three-spin system P⁺I⁻Q⁻ _A in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of P⁺I⁻Q⁻ _A and Q⁻ _A following recombination of the radical pair (P = primary donor, I = primary acceptor, Q_A = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state. The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, J_PI, in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of J_PI.     

Field cycling by fast NMR probe transfer: Design and application in field-dependent CIDNP experiments

A novel field-cycling unit with fast digital positioning of a high-resolution nuclear magnetic resonance probe in a spatially varying magnetic field is described and used to measure CIDNP spectra of the amino acid-dye (histidine-bipyridyl) photoreaction system in the range between 0 and 7 T. The pattern of nuclear polarization varies with the magnetic field. In particular, strong polarization with an emission/absorption pattern (multiplet effect) is found at low field for two histidine ringprotons with scalar coupling below 3 Hz visible only because of the high resolution made possible by the new field-cycling setup. Also for the CH₂ protons in the β-position a multiplet effect is observed having a pattern changing with magnetic field. By analysis of the spin nutation the non-Boltzmann population differences of the nuclear levels have been determined.     

The CIDNP kinetics in recombination of successive radical pairs

A theory of chemically induced dynamic nuclear polarization (CIDNP) formed in recombination of successive radical pairs (PRs) is developed. The theory is based on that of RP recombination with the spin Hamiltonian instantaneously changing in time. With kinematics approximation it is shown that general relations for CIDNP are fully expressed via the quadratures of Green functions, which characterize the molecular motion of reagents. Analytical formulae for the time dependence of CIDNP both of primary and secondary RPs have been derived in the strong magnetic field approximation (S-T₀ approximation); field dependences of stationary CIDNP effect for some model cases have been analyzed. For long-lived systems the sensitivity of secondary RP CIDNP to the singlet-triplet evolution of primary RP has been demonstrated. It is shown that sometimes the correct analysis of the effect calls for taking into account the reactivity anisotropy.     

Magnetic relaxation dispersion probe

The magnetic field dependence of nuclear spin-lattice relaxation rates provides a powerful approach to characterizing intra and intermolecular dynamics. NMR spectrometers that provide extensive magnetic relaxation dispersion profiles may switch magnetic field strengths rapidly by either moving the sample or by changing the current in an electromagnet. If the sample is moved, the polarization and detection fields may be very high, which provides both high sensitivity and resolution. This report summarizes the design of a pneumatic sample transport system for glass sample containers that may be used in either a dual magnet spectrometer or in a single magnet system that exploits the fringe field as the secondary magnetic field.     

Permanent magnet applications

Rare-earth permanent magnets are ideally suited to generate magnetic fields comparable to their spontaneous polarization J_S. Near-square hysteresis loops and large values of the coercivity and anisotropy fields greatly simplify magnet design, as each magnet block is effectively transparent to the magnetic fields produced elsewhere in the magnet assembly. The fields generated by compact and efficient magnet structures requiring no continuous expenditure of energy can be static or variable, uniform or nonuniform. Permanent magnets are fully competitive with electromagnets for fields up to 2 T, and fields as high as to 5 T can be produced in a small volume. When a field with a rapid spatial variation is required, permanent magnets may offer the only practicable solution. Both permanent magnet structures and the uses to which they are put are reviewed, classifying the magnet applications in terms of the nature of the field, the effect on the magnet and the physical effect exploited.     

X‐Treme beamline at SLS: X‐ray magnetic circular and linear dichroism at high field and low temperature

X‐Treme is a soft X‐ray beamline recently built in the Swiss Light Source at the Paul Scherrer Institut in collaboration with École Polytechnique Fédérale de Lausanne. The beamline is dedicated to polarization‐dependent X‐ray absorption spectroscopy at high magnetic fields and low temperature. The source is an elliptically polarizing undulator. The end‐station has a superconducting 7 T–2 T vector magnet, with sample temperature down to 2 K and is equipped with an in situ sample preparation system for surface science. The beamline commissioning measurements, which show a resolving power of 8000 and a maximum flux at the sample of 4.7 × 10¹² photons s^?1, are presented. Scientific examples showing X‐ray magnetic circular and X‐ray magnetic linear dichroism measurements are also presented.     

Search for a Magnetic-Field Dependence of the Interaction of the Nuclear Quadrupole Moment with the Electric-Field Gradient

It is argued that the nuclear quadrupole–electric field gradient (EFG) interaction is, in principle, dependent on the presence of a magnetic fieldB. A rough estimate of the size of this effect yields 10⁻⁴in fields up to 10 T. However, if the site symmetry of the nucleus in question includes time-reversal symmetry, the linear dependence of the EFG onBvanishes. In diamagnetic compounds, time-reversal symmetry is violated only by the presence of nuclear spins. In such compounds, the dominant dependence of the EFG onBshould be quadratic and should be described by a fourth-rank tensor. In ferro- and antiferromagnetic compounds time-reversal symmetry is strongly violated and a linear dependence of the EFG onB, described by a third-rank tensor, is expected. A search for a magnetic field dependence of the EFG was carried out by measuring the quadrupole coupling constants (QCCs) of the²⁷Al and¹⁴N nuclei in corundum and sodium nitroprusside (SNP) by pure NQR, and by NMR in fields of 6.3 and 11 T. These diamagnetic compounds were selected because previous measurements, done in different fields, yielded differing results for the QCCs. A new technique for measuring QCCs by NMR is introduced that circumvents the necessity of precisely orienting the sample crystals. For the QCCs of both the²⁷Al and¹⁴N nuclei in corundum and SNP, respectively, a precision of distinctly better than 10⁻⁴is reached. The results obtained in 0, 6.3, and 11 T fields fully agree with each other which means that, in fields up to 11 T, any possible field dependence of the QCCs is smaller than 10⁻⁴. These results confirm that in diamagnetic compounds a linear dependence of QCCs onBis largely suppressed.     

Deprotonation of Transient Guanosyl Cation Radical Catalyzed by Buffer in Aqueous Solution: TR-CIDNP Study

The reaction of deprotonation of the guanosyl cation radical formed in the photoinduced reaction of guanosine monophospate (GMP) with triplet 2,2??-dipyridyl-d₈ is studied in aqueous solution by time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP). In the course of the cyclic photoreaction, spin-polarized products are generated. Their polarization patterns that reflect the properties at the radical stage are analyzed using high-resolution nuclear magnetic resonance. The identification of transient radicals contributing to the polarization kinetics is based on its sensitivity to the degenerate electron exchange reaction of transient radicals with the parent diamagnetic molecules. Degenerate electron exchange is allowed only for the cation radical and manifests itself in the fast decay of the CIDNP signal in time with the rate of decay proportional to the concentration of parent GMP molecules. Because the formation of the neutral transient radical stops the exchange, the deprotonation changes the CIDNP kinetics from a decaying to a growing one. The rate constant of deprotonation, k _d, was obtained from modeling of CIDNP kinetics data with taking into consideration the difference of the CIDNP enhancement factors for neutral and cation guanosyl radicals. The value obtained at pH^* 5 for k _d?=?1?×?10⁶?s^?1 is consistent with the proton dissociation constant of the radical (pK _a?=?3.9). The linear dependence of the deprotonation rate on the buffer concentration is revealed for phosphate, formate, and acetate. Deprotonation is catalyzed by the buffer to a degree that depends on the difference in pK _a value of the buffer and the guanosyl cation radical in full accordance with Eigen??s model.     

Nuclear magnetic resonance investigation of metallic sodium nanoparticles in porous glass

Sodium nanoparticles embedded in porous glass have been studied by NMR. The measurements have been carried out on pulse spectrometers in magnetic fields of 9.4 and 17.6 T in a wide temperature range. Changes in the magnitude and temperature dependence of the ²³Na Knight shift with respect to those in bulk sodium have been discovered. An additional component of the NMR line shifted to high frequencies has been observed in the temperature range from 240 to 100 K. Investigation of the specific heat has revealed a considerable decrease in the melting and crystallization temperatures of sodium under nanoconfinement, which were not accompanied by abrupt changes in the Knight shift.     

High-field27Al mas NMR studies of the formation of metakaolinite by flash calcination of kaolinite

Flash calcination was used to dehydroxylate kaolinite and produced calcines kinetically frozen at various stages during metakaolinite formation. High speed magic-angle spinning²⁷Al NMR, in magnetic fields of 14.1 and 11.8 T, enabled new features to be observed in the metakaolinite spectrum. The dependence on magnetic field of simulated and experimental spectra is discussed, and the effects of a distribution of electric field gradients is examined. The evolution of the metakaolinite peaks is followed during dehydroxylation.