Nuclear Polarization Measurements Research Articles

Dynamic nuclear polarization studies of nitroxyl spin probes in agarose gel using Overhauser-enhanced magnetic resonance imaging.

Agarose is a tissue-equivalent material and its imaging characteristics similar to those of real tissues. Hence, the dynamic nuclear polarization studies of 3-carboxy-2,2,5,5-tetramethyl-pyrrolidine-1-oxyl (carboxy-PROXYL) in agarose gel were carried out. The dynamic nuclear polarization parameters such as spin lattice relaxation time, longitudinal relaxivity, leakage factor, saturation parameter and coupling parameter were estimated for 2mM carboxy-PROXYL in phosphate-buffered saline solution and water/agarose mixture (99:1). From these results, the spin probe concentration was optimized as 2mM, and the reduction in enhancement was observed for carboxy-PROXYL in water/agarose mixture (99:1) compared with phosphate-buffered saline solution. Phantom imaging was also performed with 2mM concentration of carboxy-PROXYL in various concentrations of agarose gel at various radio frequency power levels. The results from the dynamic nuclear polarization measurements agree well with the phantom imaging results. These results pave the way for designing model system for human tissues suited to the biological applications of electron spin resonance/Overhauser-enhanced magnetic resonance imaging.

A high-field 3He metastability exchange optical pumping polarizer operating in a 1.5 T medical scanner for lung magnetic resonance imaging

After being hyperpolarized using the technique of Metastability Exchange Optical Pumping (MEOP), 3He can be used as a contrast agent for lung magnetic resonance imaging (MRI). MEOP is usually performed at low magnetic field (∼1 mT) and low pressure (∼1 mbar), which results in a low magnetization production rate. Polarization preserving compression with a compression ratio of order 1000 is also required. It was demonstrated in sealed cells that high nuclear polarization values can be obtained at higher pressures with MEOP, if performed at high magnetic field (non-standard conditions). In this work, the feasibility of building a high-field polarizer that operates within a commercial 1.5 T scanner was evaluated. Preliminary measurements of nuclear polarization with sealed cells filled at different 3He gas pressures (1.33 to 267 mbar) were performed. The use of an annular shape for the laser beam increased by 25% the achievable nuclear polarization equilibrium value (Meq) at 32 and 67 mbar as compared to a Gaussian beam shape. Meq values of 66.4% and 31% were obtained at 32 and 267 mbar, respectively, and the magnetization production rate was increased by a factor of 10 compared to the best results obtained under standard conditions. To study the reproducibility of the method in a polarizing system, the same experiments were performed with small cells connected to a gas handling system. Despite careful cleaning procedure, the purity of the 3He gas could not be matched to that of the sealed cells. Consequently, the polarization build-up times were approximately 3 times longer in the 20–30 mbar range of pressure than those obtained for the 32 mbar sealed cell. However, reasonable Meq values of 40%–60% were achieved in a 90 ml open cell. Based on these findings, a novel compact polarizing system was designed and built. Its typical output is a 3He gas flow rate of 15 sccm with a polarization of 33%. In-vivo lung MRI ventilation images (Signal to Noise Ratio (SNR) of approximately 55 for a voxel size of 50 mm × 3 mm × 3 mm) were acquired to demonstrate the polarizer's application.

Structural Insight into Proteorhodopsin Oligomers

Oligomerization has important functional implications for many membrane proteins. However, obtaining structural insight into oligomeric assemblies is challenging, as they are large and resist crystallization. We focus on proteorhodopsin (PR), a protein with seven transmembrane α-helices that was found to assemble to hexamers in densely packed lipid membrane, or detergent-solubilized environments. Yet, the structural organization and the subunit interface of these PR oligomers were unknown. We used site-directed spin-labeling together with electron spin-resonance lineshape and Overhauser dynamic nuclear polarization analysis to construct a model for the specific orientation of PR subunits within the hexameric complex. We found intersubunit distances to average 16 Å between neighboring 55 residues and that residues 177 are >20 Å apart from each other. These distance constraints show that PR has a defined and radial orientation within a hexamer, with the 55-site of the A-B loop facing the hexamer core and the 177-site of the E-F loop facing the hexamer exterior. Dynamic nuclear polarization measurements of the local solvent dynamics complement the electron spin-resonance-based distance analysis, by resolving whether protein surfaces at positions 55, 58, and 177 are exposed to solvent, or covered by protein-protein or protein-detergent contacts.

Exploring Photoreactions between Polyazaaromatic Ru(II) Complexes and Biomolecules by Chemically Induced Dynamic Nuclear Polarization Measurements

Steady-state (1)H photo-chemically induced dynamic nuclear polarization (CIDNP) experiments were conducted at 14.1 T on deoxygenated (buffered pH 7) aqueous solutions of [Ru(phen)(3)](2+), [Ru(tap)(2)(phen)](2+), and [Ru(tap)(3)](2+) (tap = 1,4,5,8-tetraazaphenanthrene; phen = 1,10-phenanthroline) in the presence of guanosine-5'-monophosphate or N-acetyltyrosine. For the first time, CIDNP arising from photo-oxidation by polyazaaromatic Ru(II) complexes is reported. In agreement with the occurrence of a photo-electron-transfer process, photo-CIDNP effects are observed with [Ru(tap)(3)](2+) and [Ru(tap)(2)(phen)](2+) but not with [Ru(phen)(3)](2+). With [Ru(tap)(2)(phen)](2+), no significant photo-CIDNP is observed for the (1)H nuclei of the phen ligand, consistent with the fact that the metal-to-ligand charge-transfer triplet excited states responsible for the photo-oxidation involve a tap ligand. Successive experiments with [Ru(tap)(3)](2+) highlight the accumulation of long-lived radical species in solution that cause (1)H NMR signal broadening and photo-CIDNP extinction. The (1)H photo-CIDNP observed for the biomolecules is rather weak, less than about 30% of the equilibrium magnetization. However, up to 60% polarization enhancement is observed for H-2 and H-7 of the tap ligands, which indicates high unpaired electron density in the vicinity of these atoms in the transient radical pair. This is consistent with the structure of known photoadducts formed, for instance, between the metallic compounds and the guanine base of mono- and polynucleotides. Indeed, in these adducts the covalent bond involves carbon C-2 or C-7 of a tap ligand. The occurrence of photo-CIDNP with polyazaaromatic Ru(II) complexes opens new perspectives for the study of this type of compound.

AB-type lectin (toxin/agglutinin) from mistletoe: differences in affinity of the two galactoside-binding Trp/Tyr-sites and regulation of their functionality by monomer/dimer equilibrium

Viscumin of mistletoe (Viscum album L.) has a concentration-dependent activity profile unique to plant AB-toxins. It starts with lectin-dependent mitogenicity and then covers toxicity and cell agglutination, associated with shifts in the monomer/dimer equilibrium. Each lectin subunit harbors two sections for ligand contact. In the dimer, the B-chain sites in subdomain 2 gamma (designated as the Tyr-sites) appear fully accessible, whereas Trp-sites in subdomain 1 alpha are close to the dimer interface. It is unclear whether both types of sites operate similarly in binding glycoligands in solution. By systematically covering a broad range of lactose/lectin ratio in isothermal titration calorimetry, we obtained evidence for two sites showing dissimilar binding affinity. Intriguingly, the site with higher affinity was only partially occupied. To assign the observed properties to the Trp/Tyr-sites, we next performed chemically induced dynamic nuclear polarization measurements of Trp and Tyr accessibility. A Tyr signal, but not distinct Trp peaks, was recorded when testing the dimer. Lactose-quenchable Trp peaks became visible on the destabilization of the dimer by citraconylation, intimating Trp involvement in ligand contact in the monomer. Fittingly, Tyr acetylation but not mild Trp oxidation reduced the dimer hemagglutination activity and the extent of binding to asialofetuin-Sepharose 4B. Altogether, the results attribute lectin activity in the dimer primarily to Tyr-sites. Full access to Trp-sites is gained on dimer dissociation. Thus, the monomer/dimer equilibrium of viscumin regulates the operativity of these sites. Their structural divergence affords the possibility for differences in ligand selection when comparing monomers (Tyr- and Trp-sites) with dimers (primarily Tyr-sites).

Laser flash photolysis and time-resolved CIDNP study of photochemical reactions between aqueous tryptophan and nucleotides

Triplet tryptophanTTrpH quenching the by nucleotides adenosine-5′-monophosphate (AMP), cytidine-5′-monophosphate (CMP), guanosine-5′-monophosphate (GMP), and uridine-5′-monophosphate (UMP) is reported for the first time. The quenching rate constants for AMP, CMP, and UMP are of the same orderkq=(3–6)·108 M−1s−1, for GMP this value is 5.6·107 M−1s−1. This difference correlates with the reduction potentials of the nucleotides under study, pointing at the involvement of the electron transfer process.TTrpH quenching by AMP, CMP, and GMP does not result in the radical formation, whereas in the reaction ofTTrpH with UMP radicals are formed with the quantum yield Φ≈0.13, the, radical formation is confirmed by both laser flash photolysis and time-resolved chemically induced dynamic nuclear polarization measurements.

Nuclear tensor polarization in a laser-driven polarized deuterium internal target

We present the results of the first direct measurement of nuclear polarization in an optically pumped spin-exchange polarized deuterium target. Measurements of the nuclear tensor polarization p zz are made in a storage cell as a function of flow of deuterium nuclei. For these measurements, the optical pumping was performed on atoms in magnetic fields of 600 and 3600 G. These measurements verify that the spin-exchange method is capable of producing polarized deuterium nuclei without the use of rf transitions. The electronic polarization, P e, is also measured. The P e and p zz measurements at 600 G indicate that the system is in spin-temperature equilibrium, whereas the 3600 G measurements show signatures of non-equilibrium effects. P e is also measured with the system operated as a source of polarized atoms, with no storage cell. These measurements are compared with previous measurements using a similar source.

Influence of strong axial quadrupole interactions on the measurement of nuclear polarization

We solve for the exact eigenvalues and eigenstates of the Hamiltonian for spin I=1 including axially symmetric quadrupolar interactions and also calculate approximate eigenenergies and eigenstates up to second order in perturbation theory. The results are compared for a case where the quadrupolar splitting is a large fraction of the dipolar splitting. The relationship between polarization and the integral of an NMR signal under the influence of the quadrupolar interaction is discussed using nitrogen in solid ammonia as an example.

Structure and dynamics of the DNA binding protein HU from Bacillus stearothermophilus by NMR spectroscopy.

The DNA-binding protein HU from Bacillus stearothermophilus (HUBst) is a dimer with a molecular weight of 195 kDa that is capable of bending DNA. An x-ray structure has been determined previously [Tanaka et al. 1984) Nature, vol. 310, pp. 376-381], but no structure could be established for a large part of the supposed DNA-binding beta-arms. Distance geometry and restrained molecular dynamics using nmr restraints were used to generate a set of 25 structures. These structures display a backbone rms deviation (RMSD) of 0.36 A for the well-defined region (residues 2-54 and 75-90). The structure of the core is very similar to that observed in the x-ray structure, with a pairwise RMSD of 1.06 A. The structure of the beta-hairpin arm contains a double flip-over at the prolines in the two strands of the beta-arm. Heteronuclear 15N relaxation measurements indicate that the beta-arm and the tip of the beta-arm is flexible. This explains the disorder observed in the solution and x-ray structures of the beta-arm with respect to the core of the protein. Overlayed onto itself the beta-arm is better defined, with a backbone RMSD of 1.0 A calculated for residues 54-59 and 69-74. The tip of the arm adopts a well-defined 4 : 6 beta-hairpin conformation. Changes in amide 15N and 1H chemical shifts upon titrating DNA are most pronounced for the residues in the beta-hairpin arm and for the residues in the second half of the third alpha-helix. Heteronuclear 15N relaxation data for free and complexed HUBst show that that the arms become structured upon DNA binding. Together with chemically induced nuclear polarization measurements on a mutant HUBst (M69Y; V76Y) this shows that the beta-hairpin arm is involved in direct DNA interaction.

Performance of a polarized-hydrogen storage cell target

A storage cell has been constructed for use as an internal polarized gas target in the storage ring at the Indiana University Cyclotron Facility. The storage cell has thin teflon walls which allow for detection of low energy recoil particles. We report on nuclear polarization measurements of hydrogen atoms, produced by an atomic beam source, in this storage cell. The results indicate that a target polarization in excess of 0.70 is achieved. We discuss the design and construction details of a storage cell target and the polarization results which are based on low-energy pp spin correlation measurements using the University of Wisconsin tandem accelerator.

NMR calibration of optical measurement of nuclear polarization in 3He.

We have performed precise NMR measurements in order to calibrate a standard optical technique for measuring the polarization of $^{3}\mathrm{He}$ nuclei. This optical technique requires knowledge of the pressure-dependent relationship between the nuclear polarization and the degree of circular polarization of the 668-nm light emitted from a $^{3}\mathrm{He}$ discharge. We determined the nuclear polarization by comparing the adiabatic fast-passage NMR signal from samples of polarized $^{3}\mathrm{He}$ at pressures between 0.1 and 5 torr to an identical sample of water. For water, the polarization is known from statistical mechanics. The $^{3}\mathrm{He}$ is polarized using the metastability exchange technique for optical pumping of $^{3}\mathrm{He}$. The accuracy of the calibration is \ifmmode\pm\else\textpm\fi{}2%, which is required for applications in nuclear physics.

Accurate optical measurement of nuclear polarization in optically pumped 3He gas

Large nuclear polarizations M (over 80 %) can now be achieved in gaseous 3He by optical pumping. The gas is excited by an RF discharge and is oriented using a high power LNA laser which is lamp pumped and tuned to the 2 3S-2 3P transition at 1.08 μm. In this paper we describe an experiment in which we measure M with high absolute precision. Our method is based on a change as a function of M in the ratio of σ or π polarized light absorbed from a weak probe beam by the 2 3S metastable atoms. The probe was delivered by a diode pumped LNA laser and propagated perpendicular to the direction of the magnetization. Simultaneous measurement of M was made by monitoring the degree of circular polarization $\cal{P}$ of the optical line at 668 nm emitted by the discharge. Our measurements show a linear relationship between M and $\cal{P}$ for all accessible M values and for a wide range of experimental conditions (sample pressure, magnetic field, RF discharge level, etc.). This provides a second method of measurement of the 3He nuclear polarization which is simple to operate and is calibrated and is calibrated over a pressure range of 0.15 to 6.5 torr.

Polarization measurements following the beam-foil interaction

Induced nuclear spin polarization P I by hyperfine interaction following the passage of 0.5 μA, 300 keV beams of 15N and 13C through single tilted carbon foils yields P I=(5.7±0.9)% and (3.2±0.6)%, respectively. The nuclear polarizations were enhanced to (9.5±1.4)% and (4.1±0.8)% by passage through two tilted foils, and the sign of the polarization flipped by a simple flip of the foil direction with respect to the beam direction. Polarization transfer from the electrons to the nucleus was observed by a single frequency quantum beat pattern for the 5667 Å line in 15N downstream after a single tilted foil. This greatly simplifies determinations of the beat frequencies involved and in this case an experimental magnetic hyperfine constant A(5667)=(7.5±0.7)×10 −6 eV was deduced. The atomic and nuclear polarization measurements are discussed in detail.

Nuclear polarization measurements of oriented 3He gas by ’’frequency jump’’ spectrometry

An NMR technique capable of yielding measurements of polarization of highly oriented 3He gas without appreciably disturbing the state of nuclear orientation is described. The nuclear polarization is determined through a measurement of dispersive shifts in the frequency of the NMR oscillator as the magnetic field is swept through resonance, rather than from the magnitude of NMR signals.

Grundlagen der Polarisationsmessung in polarisierten Targets mit Hilfe des inneren Feldes

It is shown how the difficulties with the “Lorentz field” method of Abragamet al. for the measurement of nuclear polarization can be resolved. The observable field shift is derived as a function of nuclear polarization, target shape and some other parameters. For purposes of practical application numerical results are presented for polarized targets of lanthanum magnesium nitrate (1% Nd) having the shape of thin square slabs.

Dynamic Nuclear Polarization of Liquids in Very Weak Fields

The validity of the modified Bloch equations (MBE) for liquids in extremely weak dc magnetic fields and large values of electron spin saturation, using linear rf fields, has been checked for anthracene-negative-ion solutions (ANI). Strictly numerical solutions of the MBE were obtained by use of a computer to show the electron spin saturation as functions of rf field strength and frequency. These functions were strongly nonmonotonic and exceed unity in some cases. Dynamic nuclear polarizations were measured by a low-field transient method in which proton free-precession was observed in a weak field. Conventional epr measurements at 3000 and 18 G, cw dynamic nuclear polarization measurements at 18 G, and the low-field transient dynamic nuclear polarization measurements were all compatible with ${T}_{1}={T}_{2}=T=50$ nsec for the ANI at 29\ifmmode^\circ\else\textdegree\fi{}C. The ratio $k$ of nuclear polarization enhancement to the electron spin saturation was found to be very nearly constant, in agreement with the Solomon two-spin relaxation theory. By use of MBE with $T=50$ nsec and setting $k=\ensuremath{-}162$, the results of the dynamic nuclear polarization measurements were fit to very nearly within known experimental errors despite the wide range of low-field resonant and nonresonant conditions and the strongly nonmonotonic curves. No dynamic polarization of the ANI could be obtained with the rf field parallel to the dc field, in agreement with the MBE. However, a crude-oil-derived free radical with ${T}_{1}>{T}_{2}\ensuremath{\approx}10$ nsec yielded about one-third as large a dynamic nuclear polarization with the rf field parallel to the dc field as perpendicular. It was noticed for a deteriorated ANI sample that the time constant for the establishment of a dynamic nuclear polarization exceeded the proton relaxation time and also varied markedly with the strength of the rf field.