Cross-relaxation Spectroscopy Research Articles

Decoherence of V{}_{{{{{{{{rm{B}}}}}}}}}^{-} spin defects in monoisotopic hexagonal boron nitride

Spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of flexible two-dimensional quantum sensing platforms. Here we rely on hBN crystals isotopically enriched with either 10B or 11B to investigate the isotope-dependent properties of a spin defect featuring a broadband photoluminescence signal in the near infrared. By analyzing the hyperfine structure of the spin defect while changing the boron isotope, we first confirm that it corresponds to the negatively charged boron-vacancy center ({{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}). We then show that its spin coherence properties are slightly improved in 10B-enriched samples. This is supported by numerical simulations employing cluster correlation expansion methods, which reveal the importance of the hyperfine Fermi contact term for calculating the coherence time of point defects in hBN. Using cross-relaxation spectroscopy, we finally identify dark electron spin impurities as an additional source of decoherence. This work provides new insights into the properties of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-} spin defects, which are valuable for the future development of hBN-based quantum sensing foils.

High precision single qubit tuning via thermo-magnetic field control

Precise control of the resonant frequency of a spin qubit is of fundamental importance to quantum sensing protocols. We demonstrate a control technique on a single nitrogen-vacancy (NV) centre in diamond where the applied magnetic field is modified by fine-tuning a permanent magnet's magnetisation via temperature control. Through this control mechanism, nanoscale cross-relaxation spectroscopy of both electron and nuclear spins in the vicinity of the NV centre is performed. We then show that through maintaining the magnet at a constant temperature, an order of magnitude improvement in the stability of the NV qubit frequency can be achieved. This improved stability is tested in the polarisation of a small ensemble of nearby 13C spins via resonant cross-relaxation, and the lifetime of this polarisation explored. The effectiveness and relative simplicity of this technique may find use in the realisation of portable spectroscopy and/or hyperpolarisation systems.

Spin properties of dense near-surface ensembles of nitrogen-vacancy centers in diamond

We present a study of the spin properties of dense layers of near-surface nitrogen-vacancy (NV) centres in diamond created by nitrogen ion implantation. The optically detected magnetic resonance contrast and linewidth, spin coherence time, and spin relaxation time, are measured as a function of implantation energy, dose, annealing temperature and surface treatment. To track the presence of damage and surface-related spin defects, we perform in situ electron spin resonance spectroscopy through both double electron-electron resonance and cross-relaxation spectroscopy on the NV centres. We find that, for the energy ($4-30$~keV) and dose ($5\times10^{11}-10^{13}$~ions/cm$^2$) ranges considered, the NV spin properties are mainly governed by the dose via residual implantation-induced paramagnetic defects, but that the resulting magnetic sensitivity is essentially independent of both dose and energy. We then show that the magnetic sensitivity is significantly improved by high-temperature annealing at $\geq1100^\circ$C. Moreover, the spin properties are not significantly affected by oxygen annealing, apart from the spin relaxation time, which is dramatically decreased. Finally, the average NV depth is determined by nuclear magnetic resonance measurements, giving $\approx10$-17~nm at 4-6 keV implantation energy. This study sheds light on the optimal conditions to create dense layers of near-surface NV centres for high-sensitivity sensing and imaging applications.

Optically detected cross-relaxation spectroscopy of electron spins in diamond.

The application of magnetic resonance spectroscopy at progressively smaller length scales may eventually permit 'chemical imaging' of spins at the surfaces of materials and biological complexes. In particular, the negatively charged nitrogen-vacancy (NV(-)) centre in diamond has been exploited as an optical transducer for nanoscale nuclear magnetic resonance. However, the spectra of detected spins are generally broadened by their interaction with proximate paramagnetic NV(-) centres through coherent and incoherent mechanisms. Here we demonstrate a detection technique that can resolve the spectra of electron spins coupled to NV(-) centres, in this case, substitutional nitrogen and neutral nitrogen-vacancy centres in diamond, through optically detected cross-relaxation. The hyperfine spectra of these spins are a unique chemical identifier, suggesting the possibility, in combination with recent results in diamonds harbouring shallow NV(-) implants, that the spectra of spins external to the diamond can be similarly detected.

Liquid–liquid phase separation in micropores

Phase separation of binary liquids with an upper critical temperature in porous materials is studied by H-1 NMR cryoporometry and cross-relaxation spectroscopy and by N-15 NMR spectroscopy. The fir ...

Use of 1H cross-relaxation nuclear magnetic resonance spectroscopy to probe the changes in bread and its components during aging

1H nuclear magnetic cross-relaxation spectroscopy was used to probe the molecular mobility/rigidity in bread and its components during storage. The Z-spectra lineshapes, attributed to the solid-like polymer fractions of the samples, differed for the bread, gelatinized waxy starch (GX), gelatinized wheat starch (GW), heated flour (HF), and heated gluten (HG). Upon storage, no change was observed in the Z-spectrum of the bread sample, while the Z-spectra for GX, GW, and HG increased in the width at half height of the decomposed broad component (increased rigidity). These trends in the Z-spectra detected by NMR were contradictory to the DSC results that showed an increase in amylopectin retrogradation enthalpy for all samples containing starch, including bread. These trends in the Z-spectra detected by NMR were not reflected by the DSC results that showed an increase in amylopectin retrogradation enthalpy for all samples, including bread. The differences in molecular mobility could not be therefore, due to recrystallized amylopectin and may be attributed to the role of gluten and/or redistribution of water in the amorphous regions of the samples.

Diffusion parameters of B in Cu determined by β-radiation-detected NMR

Using \ensuremath{\beta}-radiation-detected nuclear magnetic resonance (\ensuremath{\beta}-NMR) in the special form of cross-relaxation spectroscopy we studied the diffusional behavior of implanted ${}^{12}\mathrm{B}$ nuclei in Cu single crystals at temperatures $T=115--750\mathrm{K}.$ An extended and completely revised dynamic theory of nuclear cross relaxation is developed to analyze the experimental data. Especially the incorporation of (hitherto neglected) spin-lattice relaxation effects turned out to be an important improvement. Applying this formalism we obtain activation energies of ${E}_{a}=0.57(5)\mathrm{eV}$ for the migration of interstitial B, the dominating fraction for $T\ensuremath{\lesssim}400\mathrm{K},$ and of ${E}_{a}=1.15(10)\mathrm{eV}$ for substitutional B formed at higher temperatures. The previously found direct-exchange mechanism for the diffusion of substitutional B in Cu is confirmed by our analysis.

Factors Affecting the Collection and Fitting of Nuclear Magnetic Cross-Relaxation Spectroscopy Data with Application to Waxy Corn Starch

An examination of the methods for nuclear magnetic cross-relaxation spectroscopy (CRS) data collection and analysis was conducted using water and an aqueous waxy corn starch suspension to better perform and interpret the results obtained using CRS. The CRS data collection properties evaluated were the time to achieve steady state saturation, the direct saturation of liquid protons, generation of transverse magnetization, and dependence of the offset frequency and radio frequency (RF) field strength of longitudinal relaxation in the presence of RF saturation. Effects were evaluated for variations of input values of RF saturation field strength, apparent cross-relaxation rate, and solid longitudinal relaxation rate on the results for solid content and solid internal mobility from fitting NMR data to modified theoretical expressions. Discrepancies between fitted and stoichiometric values for the solid to liquid proton ratio were investigated. The fitting procedure used a Gaussian line shape for RF saturation of the solid-like spin system and a Lorentzian line shape for RF saturation of the liquid-spin system. Conditions under which acceptable results can be obtained with limited data sets are discussed.

Protein Mediated Magnetic Coupling between Lactate and Water Protons

The magnetic coupling between methyl lactate protons and water protons in samples of cross-linked bovine serum albumin (BSA) is studied. Cross-relaxation spectroscopy shows efficient magnetization transfer from immobilized BSA to both water and methyl lactate protons. Transient and steady-state NOE experiments reveal a negative intermolecular NOE between methyl lactate and water protons. Lactate is indirectly detected by selectively saturating the methyl lactate protons and measuring the decrease in water proton magnetization. Indirect detection of methyl lactate protons is an order of magnitude more sensitive than direct detection in these model systems. Lactate was indirectly imaged, via the water proton resonance, with 1.1-μl voxels in 2 min. Immobilized BSA reduces the intermolecular correlation time between water and lactate protons into the spin-diffusion limit where the NOE is negative. Possible molecular mechanisms for this coupling and applications toin vivospectroscopy are discussed.

Temperature dependence and phase transition of proton relaxation of hydrated collagen in intact beef tendon specimens via cross-relaxation spectroscopy.

The role of water of hydration in proton relaxation in tissues as exemplified by hydrated collagen in beef tendon was studied as a function of temperature from -40 degrees to 37 degrees C by using cross-relaxation spectroscopy. Experimental data were fitted to a simple binary spin-bath model. The outcome of this procedure allows the construction of a semi-quantitative depiction of proton relaxation in a heterogeneous system and its change as one of the water fractions freezes at about -10 to -20 degrees C, a transition observed by NMR and confirmed independently by differential scanning calorimetry. Such physical depiction provides a crude but insightful interpretation of the role "bound" water plays in proton relaxation. This may be important in shedding light on the mechanism of tissue relaxation and its role in MRI diagnosis, particularly for those diseases such as liver cirrhosis where the water-macromolecular interaction plays a prominent role.

Diffusion of Boron in Copper by Direct-Exchange Mechanism.

The behavior of B impurities implanted into Cu single crystals has been investigated by means of b radiation detected nuclear magnetic resonance and cross-relaxation spectroscopy. Diffusion of substitutional B (Bs) in Cu is observed in the temperature range of T ­ 600 750 K. By combining information from new and formerly published data it is shown that this diffusion is not mediated by any other defect; it rather takes place by a direct site exchange between Bs and neighboring Cu atoms. To our knowledge Bs in Cu is the first system for which this long discussed diffusion mechanism has been established experimentally. [S0031-9007(96)01798-X]

Double NQR and cross-relaxation spectroscopy of cocaine hydrochloride

Double NQR and cross-relaxation spectroscopy of cocaine hydrochloride

The dynamics of water-protein interactions.

The magnetic field and temperature dependence of the water proton nuclear spin-lattice relaxation rate requires that the motion timescale for water molecules in contact with proteins is close to that for pure water at room temperature. Nevertheless, there are a few water molecules, which may be detected by high-resolution, cross-relaxation spectroscopy, that must have relatively long protein-bound lifetimes and that carry the bulk of the relaxation coupling between the protein and the water. The water-protein magnetic coupling affects the interpretation of water relaxation rates in heterogeneous protein systems, such as tissues, and provides new ways to extract useful information about the immobilized components through the effects on the water NMR spectrum. The discussion shows that the conclusions concerning the rapid water molecule motions at the interface are not in conflict with the observations of many water oxygen atom positions in protein crystal structures.

P-H4-02 Temperature dependence of interactions in hydro-crystallin system in intact rat lens by magnetic cross-relaxation spectroscopy: §Tzou D. L., §Lee S. M., ‡Hur, Y., ‡Huang, F. Y., and §Yeung, H. N. § Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan. ‡ Dept. of Chemistry, National Cheng Kung Univ., Tainan, Taiwan

P-H4-02 Temperature dependence of interactions in hydro-crystallin system in intact rat lens by magnetic cross-relaxation spectroscopy: §Tzou D. L., §Lee S. M., ‡Hur, Y., ‡Huang, F. Y., and §Yeung, H. N. § Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan. ‡ Dept. of Chemistry, National Cheng Kung Univ., Tainan, Taiwan

Elucidation of the structure of constrained bicyclopeptides in solution by two-dimensional cross-relaxation spectroscopy: amatoxin analogues.

The evaluation of peptide structures in solution is made feasible by the combined use of two-dimensional NMR in the laboratory (NOESY) and rotating frames (ROESY), and by the use of molecular dynamics calculations. The present paper describes how both the NMR method and molecular dynamics calculations were applied to very rigid synthetic bicycle peptides that are analogues of natural amatoxins. The NMR theory, which allows the estimate of interatomic distances between interacting nuclei, is briefly discussed. The experimental data were compared with those of known solid-state structures. Three amatoxin analogues have been examined. Of these, one is biologically active (S-deoxo gamma[R] OH-Ile3-amaninamide) and its structure in the solid state has recently been worked out. The second and third analogues (S-dexo-Ile3-Ala5-amaninamide and S-deoxo-D-Ile3-amaninamide, respectively) are inactive and their solid-state structures are unknown. The data presented confirm the authors previous hypothesis that lack of biological activity of S-deoxo-Ile3-Ala5-amaninamide is due to the masking of the tryptophan ring by the methyl group of L-Ala and not to massive conformational changes of the analogue.

Full-Matrix Analysis of the Error Propagation in Two-Dimensional Chemical-Exchange and Cross-Relaxation Spectroscopy

Propagation of errors in a multispin system, from the experimental peak volumes into magnetization-exchange (cross relaxation or chemical exchange) rates, was calculated analytically. An explicit expression was derived for the error propagator. The error for each magnetization-exchange rate constant has a minimum, with the position dependent on the structure of the exchange network. Error analysis allows optimization of the mixing time for arbitrarily complex exchange systems. In cross-relaxation spectroscopy, the error analysis provides the upper and the lower boundaries of calculated interproton distances for all spin pairs. These limits differ throughout the molecule for the same distance. The differences are a consequence of the spin diffusion, which the present analysis fully takes into account.

Broadband excitation and detection of cross-relaxation NMR spectra

Nuclear magnetic resonance cross-relaxation spectroscopy maps the proton solidstate NMR spectrum onto the proton water NMR signal of aqueous heterogeneous materials ( Z-3). Application of a relatively long, low-intensity RF pulse, off resonance from the water Larmor frequency but within the linewidth of the solid, partially saturates the nuclear magnetism of the solid protons. Magnetic cross relaxation between the immobile and mobile protons transfers the saturation of the solid protons to the water protons, decreasing the magnitude of the water NMR signal. A cross-relaxation spectrum is obtained by saturating the immobile protons at many discrete frequencies and plotting the ratio of the water signal with and without RF saturation as a function of the saturation frequency. The intensity and the width ofthe cross-relaxation spectrum are a function of the longitudinal, transverse, and cross-relaxation rates of the liquid and solid components, the mole fraction of liquid to solid protons, and the magnitude and duration of the saturating RF pulse. This Communication describes a method for obtaining the entire cross-relaxation spectrum of a spatially homogeneous material with two acquisitions: RF saturation on and RF saturation off. The pulse sequence is demomtrated schematically in Fig. I. A magnetic field gradient and RF pulse are applied to the sample concurrently. The gradient creates a continuum of frequencies across the !;ample and establishes at once all conditions of off-resonance saturation for the RF pulse. The decrease in water magnetization by cross relaxation as a function of off-resonance frequency is stored in the sample as a function of the position in the sample and detected by recording a onedimensional image with the readout gradient applied along the same axis as the frequency-dispersal gradient. The cross-relaxation spectrum is created by dividing the image obtained with RF saturation turned on by one obtained with RF saturation turned off. The pulse sequence shown in Fig. 1 represents the simplest implementation of broadband cross-relaxation spectroscopy. The sequence may be modified to include slice selection, spin-echo detection, and imaging by phase encoding in either or both of the two spatial dimensions which are not needed to record the cross-relaxation spectrum. In addition the magnitude of the readout gradient may be decreased to lower the bandwidths of the receiver and increase the signal-to-noise of the crossrelaxation spectrum.

Lineshape of magnetization transfer via cross relaxation

Magnetization transfer, a well-known technique in NMR ( 1) , has recently become of interest in magnetic resonance imaging. This is largely due to the discovery by Wolf and Balaban that a new form of tissue-specific contrast, which they refer to as magnetization-transfer contrast (MTC), can be generated with the technique (2). While the detailed mechanism for MTC is an area of active research, current consensus is that the contrast is generated through cross relaxation between protons in the mobile water and immobile protons in macromolecules (3). Magnetization transfer is usually observed by RF irradiation at an offset frequency which results in the partial saturation of immobile protons in macromolecules and minimal energy absorption by the mobile water. Subsequently, the cross relaxation between the mobile and immobile protons results in a reduction of the longitudinal magnetization 44, on the mobile water protons. Although For&n and Hofhnan (I ) and Mann (4) derived formulas relating the mobile water M, reduction to the crossrelaxation rate constant in the 1960s and 1970s respectively, their formulas do not describe the functional dependence of magnetization transfer on offset frequency and RF irradiation power. Experimental results indicate that the magnitude of the magnetization-transfer effect can be controlled by these factors (2). A theoretical understanding of the problem is of fundamental importance to applications of the technique and should be especially useful in MRI applications. Recently, Grad and Bryant derived an expression for the cross-relaxation spectroscopy lineshape, from which one can determine the effect of offset frequency and RF irradiation power on magnetization transfer (5). However, they employed a truncated set of the coupled Bloch equations, and their results are valid only if the transverse magnetization of mobile water spins is negligible. This assumption may not be met in MRI experiments where the offset frequency is relatively small or where the RF irradiation power is large. In these cases Grad and Bryant’s lineshape expression will overestimate the degree of magnetization transfer. In the present work, a complete set of coupled Bloch equations is used to obtain the cross-relaxation spectroscopy lineshape. The result is more general than Grad and Bryant’s and reduces to their expression when the transverse magnetization of mobile water spins is negligible. The coupled Bloch equations for two cross-relaxed spins (e.g., via the dipole-dipole coupling) are

Conformational Analysis of the Tachykinins in Solution: Substance P and Physalaemin

A determination of the solution conformational behavior of two tachykinins, substance P and physalaemin, is described. Two-dimensional homonuclear Hartmann-Hahn (HOHAHA) and rotating-frame cross relaxation spectroscopy (ROESY) are used to obtain complete proton resonance assignments. Interproton distance restraints obtained from ROESY spectroscopy are used to characterize the conformational behavior. These data show that in solution both substance P and physalaemin exist in a mixture of conformational states, rather than as a single three-dimensional structure. In water both peptides prefer to be in an extended chain structure. In methanol, their behavior is described as a mixture of beta-turn conformations in dynamic equilibrium. Solvent titration data and chemical shift temperature coefficients complement the NMR estimate of interproton distances by locating hydrogen bonds and serving to identify predominant conformational states. The C-terminal tetrapeptide segment has the same conformational behavior for both substance P and physalaemin. In physalaemin, the midsegment of the peptide may also be constrained by formation of a salt bridge. The conformational behavior of substance P and physalaemin is discussed in relation to potency and receptor binding properties.

The structure of gramicidin A in dimethylsulfoxide/acetone

It has been demonstrated by two-dimensional NMR cross-relaxation spectroscopy that gramicidin A exists in dimethylsulfoxide/acetone solution in random coil form. This contradicts earlier conclusions by Hawkes et al. [Hawkes, G. E., Lian, L. Y., Randall, E. W., Sales, K. D. & Curzon, E. H. (1987) Eur. J. Biochem. 166, 437-445] that were based on the interpretation of vicinal proton coupling constants.