DNP-NMR Spectroscopy Research Articles

Background signal suppression by opposite polarity subtraction for targeted DNP NMR spectroscopy on mixture samples.

A novel method for background signal suppression is introduced to improve the selectivity of dynamic nuclear polarization (DNP) NMR spectroscopy in the study of target molecules within complex mixtures. The method uses subtraction between positively and negatively enhanced DNP spectra, leading to an improved contrast factor, which is the ratio between the target and background signal intensities. The proposed approach was experimentally validated using a reverse-micelle system that confines the target molecules together with the polarizing agent, OX063 trityl. A substantial increase in the contrast factor was observed, and the contrast factor was optimized through careful selection of the DNP build-up time. A simulation study based on the experimental results provides insights into a strategy for choosing the appropriate DNP build-up time and the corresponding selectivity of the method. Further analysis revealed a broad applicability of the technique, encompassing studies from large biomolecules to surface-modified polymers, depending on the nuclear spin diffusion rate with a range of gyromagnetic ratios.

End-Groups of Poly(p-phenylene sulfide) Characterized by DNP NMR Spectroscopy

End-Groups of Poly(<i>p</i>-phenylene sulfide) Characterized by DNP NMR Spectroscopy

DNP NMR spectroscopy enabled direct characterization of polystyrene-supported catalyst species for synthesis of glycidyl esters by transesterification.

Polymer-supported catalysts have been of great interest in organic syntheses, but have suffered from the difficulty in obtaining direct structural information regarding the catalyst species embedded in the polymer due to the limitations of most analytical methods. Here, we show that dynamic nuclear polarization (DNP)-enhanced solid-state NMR is ideally positioned to characterize the ubiquitous cross-linked polystyrene (PS)-supported catalysts, thus enabling molecular-level understanding and rational development. Ammonium-based catalysts, which show excellent catalytic activity and reusability for the transesterification of methyl esters with glycidol, giving glycidyl esters in high yields, were successfully characterized by DNP 15N NMR spectroscopy at 15N natural abundance. DNP 15N NMR shows in particular that the decomposition of quaternary alkylammonium moieties to tertiary amines was completely suppressed during the catalytic reaction. Furthermore, the dilute ring-opened product derived from glycidol and NO3− was directly characterized by DNP 15N CPMAS and 1H–15N and 1H–13C HETCOR NMR using a 15N enriched (NO3) sample, supporting the view that the transesterification mechanism involves an alkoxide anion derived from an epoxide and NO3−. In addition, the detailed analysis of a used catalyst indicated that the adsorption of products on the cationic center is the major deactivation step in this catalysis.

DNP NMR spectroscopy of cross-linked organic polymers: rational guidelines towards optimal sample preparation.

Cross-linked polystyrenes (PS) are an important class of polymers, whose properties are strongly dependent on incorporated functionalities, for which detailed understanding of their structure remains a challenge. Here, we develop a rational guideline for dynamic nuclear polarization (DNP) sample formulation for cross-linked PS to interrogate their structure. We show that the DNP enhancement on a series of cross-linked PS bearing alkylammonium groups as prototypical organic polymers correlates with the polymer swelling properties in both apolar and polar formulations (TEKPol/1,1,2,2-tetrachloroethane and AMUPol/dimethyl sulfoxide). This work provides guidelines to easily optimize DNP formulation using a simple swelling test and enables natural abundance 15N NMR to be recorded on a series of PS-supported quaternary alkylammonium salts, allowing a detailed structural analysis.

Design Modifications in RF Interaction Cavity of a 140 GHZ Gyrotron to Achieve Wide Tunable Bandwidth for DNP NMR Applications

The tapered RF interaction cavity of 140 GHz tunable continuous wave (CW) gyrotron operating in TE0,3,q mode has been tailored with the aim of providing RF power over a tunable bandwidth for 212 MHz DNP NMR spectroscopy applications. Gyrotron device RF interaction cavity design, its beam absent RF characteristics as well as electron beam and RF wave interaction behavior, both analytical and Particle-in-Cell (PIC) simulation studies have been presented. Using linear analysis, the start oscillation currents and the RF field profiles for the various axial operating modes indices q = 1, 2, 3 have been determined. Suitable modifications in the interaction cavity have been incorporated for the enhancement of device tunable bandwidth by operating the gyrotron in the high order axial indices via magnetic tuning. Gyrotron beam-wave interaction behaviour explored using time dependent non-linear multi-mode analysis for various beam currents and magnetic fields and more than &gt;15W of RF power over a tunable bandwidth of 400MHz has been achieved through magnetic field tuning. This tunable bandwidth gyrotron design will of immense use for enhancement of sensitivity of the DNP NMR spectroscopy.

Compact radiation module for THz spectroscopy using 300 GHz continuous-wave clinotron.

The results of the development of compact radiation module based on a 300 GHz continuous-wave (CW) clinotron are presented. The clinotron oscillator is proposed as a part of the module designated for high-field dynamic nuclear polarization (DNP) systems for applications in nuclear magnetic resonance (NMR) spectroscopy. The simulation results of clinotron radiation spectra considering the influence of accelerating voltage pulsations are compared with the requirements for THz radiation linewidth for efficient NMR signal enhancement. Based on the simulations, the 300 GHz CW clinotron oscillator was developed and tested together with the high-voltage (HV) power supply, providing the output voltage stability better than 20 ppm. The frequency stability of 33 ppm was observed during the clinotron operation within several hours. The spectral linewidth is better than 8 MHz at 300 GHz that satisfies the requirements for DNP-NMR spectroscopy.

Investigations on an improved transmission line for THz DNP-NMR spectroscopy

Investigations on an improved transmission line for THz DNP-NMR spectroscopy

A novel THz-band double-beam gyrotron for high-field DNP-NMR spectroscopy.

We present the first experimental results of the study on a novel second harmonic THz-band double-beam gyrotron. The tube has demonstrated a stable single-mode operation with output parameters that are appropriate for the next-generation 1.2 GHz dynamic nuclear polarization-nuclear magnetic resonance spectroscopy. Besides the design mode (TE8,5), a series of other fundamental and second harmonic modes have been excited. This makes the new gyrotron a versatile radiation source, which can be used also in other applications of the high-power science and technologies.

A gyrating electron beam source for frequency tunable, 200 GHz gyrotron

Magnetron injection gun (MIG) is an important and critical part of any gyrotron. The overall performance of gyrotron deeply depends on the quality of gyrating electron beam. In this article, the design of triode type magnetron injection gun for 200 GHz frequency tunable gyrotron is discussed. It is planned to design and develop a 200 GHz gyrotron for 300 MHz DNP-NMR spectroscopy. The triode MIG is designed in such a way that it can be used in 200 GHz gyrotron of higher RF power for other applications also such as electron spin resonance, imaging. First, geometrical and electrical parameters of MIG are calculated using trade-off and scaling equations. Based on the calculated parameters, further, electron beam trajectory simulations are performed in EGUN. The geometry of electrodes are optimized through numerous simulation iterations to achieve the required beam parameters, such as, minimum spread in velocity and guiding center radius, an optimum value of velocity ratio (1.3–1.5), guiding center radius as per the maximum beam-wave coupling strength. The magnet system is designed to achieve the smooth Gaussian type of magnetic field profile with peak value of 7.35 T at cavity center. Further detail parametric analyses are performed which would be helpful in the frequency tuning and long-time stable operation of gyrotron device.

Real‐Time Analysis of Folding upon Binding of a Disordered Protein by Using Dissolution DNP NMR Spectroscopy

AbstractThe kinase inhibitory domain of the cell cycle regulatory protein p27Kip1 (p27) was nuclear spin hyperpolarized using dissolution dynamic nuclear polarization (D‐DNP). While intrinsically disordered in isolation, p27 adopts secondary structural motifs, including an α‐helical structure, upon binding to cyclin‐dependent kinase 2 (Cdk2)/cyclin A. The sensitivity gains obtained with hyperpolarization enable the real‐time observation of 13C NMR signals during p27 folding upon binding to Cdk2/cyclin A on a time scale of several seconds. Time‐dependent intensity changes are dependent on the extent of folding and binding, as manifested in differential spin relaxation. The analysis of signal decay rates suggests the existence of a partially folded p27 intermediate during the timescale of the D‐DNP NMR experiment.

Real-Time Analysis of Folding upon Binding of a Disordered Protein by Using Dissolution DNP NMR Spectroscopy.

The kinase inhibitory domain of the cell cycle regulatory protein p27Kip1 (p27) was nuclear spin hyperpolarized using dissolution dynamic nuclear polarization (D-DNP). While intrinsically disordered in isolation, p27 adopts secondary structural motifs, including an α-helical structure, upon binding to cyclin-dependent kinase 2 (Cdk2)/cyclin A. The sensitivity gains obtained with hyperpolarization enable the real-time observation of 13 C NMR signals during p27 folding upon binding to Cdk2/cyclin A on a time scale of several seconds. Time-dependent intensity changes are dependent on the extent of folding and binding, as manifested in differential spin relaxation. The analysis of signal decay rates suggests the existence of a partially folded p27 intermediate during the timescale of the D-DNP NMR experiment.

Chiral Recognition by Dissolution DNP NMR Spectroscopy of 13C-Labeled dl-Methionine.

A method based on d-DNP NMR spectroscopy to study chiral recognition is described for the first time. The enantiodifferentiation of a racemic metabolite in a millimolar aqueous solution using a chiral solvating agent was performed. Hyperpolarized 13C-labeled dl-methionine enantiomers were differently observed with a single-scan 13C NMR experiment, while the chiral auxiliary at thermal equilibrium remained unobserved. The method developed entails a step forward in the chiral recognition of small molecules by NMR spectroscopy, opening new possibilities in situations where the sensitivity is limited, for example, when a low concentration of analyte is available or when the measurement of an insensitive nucleus, like 13C, is required. The advantages and current limitations of the method, as well as future perspectives, are discussed.

Correlating Synthetic Methods, Morphology, Atomic-Level Structure, and Catalytic Activity of Sn-β Catalysts

Sn-β zeolites prepared using different recipes feature very different catalytic activities for aqueous phase glucose isomerization, suggesting the presence of different active sites. A systematic study of the morphology and atomic-level structure of the materials using DNP NMR spectroscopy in combination with first-principles calculations allows for the discrimination between potential sites and leads to a proposal of specific structural features that are important for high activity. The results indicate that the materials showing the highest activity possess a highly hydrophobic, defect-free zeolite framework. Those materials show so-called closed and associated partially hydrolyzed Sn(IV) sites in the T6 and T5/T7 lattice positions. On the other hand, postsynthetically synthesized Sn-β samples prepared in two steps via dealumination and subsequent solid-state ion exchange from Al-β show significantly lower activity, which is associated with a hydrophilic framework and/or a lower accessibility and differ...

Identifying low-coverage surface species on supported noble metal nanoparticle catalysts by DNP-NMR.

DNP-NMR spectroscopy has been applied to enhance the signal for organic molecules adsorbed on γ-Al2O3-supported Pd nanoparticle catalysts. By offering >2500-fold time savings, the technique enabled the observation of (13)C-(13)C cross-peaks for low coverage species, which were assigned to products from oxidative degradation of methionine adsorbed on the nanoparticle surface.

High-Speed Frequency Modulation of a 460-GHz Gyrotron for Enhancement of 700-MHz DNP-NMR Spectroscopy

The high-speed frequency modulation of a 460-GHz Gyrotron FU CW GVI (the official name in Osaka University is Gyrotron FU CW GOI) was achieved by modulation of acceleration voltage of beam electrons. The modulation speed f m can be increased up to 10 kHz without decreasing the modulation amplitude δf of frequency. The amplitude δf was increased almost linearly with the modulation amplitude of acceleration voltage ΔV a. At the ΔV a = 1 kV, frequency spectrum width df was 50 MHz in the case of f m < 10 kHz. The frequency modulation was observed as both the variation of the IF frequency in the heterodyne detection system measured by a high-speed oscilloscope and the widths of frequency spectra df measured on a frequency spectrum analyzer. Both results well agree reasonably. When f m exceeds 10 kHz, the amplitude δf is decreased gradually with increasing f m because of the degradation of the used amplifier in response for high-speed modulation. The experiment was performed successfully for both a sinusoidal wave and triangle wave modulations. We can use the high-speed frequency modulation for increasing the enhancement factor of the dynamic nuclear polarization (DNP)-enhanced nuclear magnetic resonance (NMR) spectroscopy, which is one of effective and attractive methods for the high-frequency DNP-NMR spectroscopy, for example, at 700 MHz. Because the sensitivity of NMR is inversely proportional to the frequency, high-speed frequency modulation can compensate the decreasing the enhancement factor in the high-frequency DNP-NMR spectroscopy and keep the factor at high value. In addition, the high-speed frequency modulation is useful for frequency stabilization by a PID control of an acceleration voltage by feeding back of the fluctuation of frequency. The frequency stabilization in long time is also useful for application of a DNP-NMR spectroscopy to the analysis of complicated protein molecules.

The Development of 460 GHz gyrotrons for 700 MHz DNP-NMR spectroscopy

Two demountable gyrotrons with internal mode converters were developded as sub-THz radiation sources for 700 MHz DNP (Dynamic Nuclear Polarization) enhanced NMR spectroscopy. Experimental study on the DNP-NMR spectroscopy will be carried out in Osaka University, Institute for Protein Research, as a collaboration with FIR UF. Both gyrotrons operate near 460 GHz and the output CW power measured at the end of transmission system made by circular waveguides is typically 20 to 30 watts. One of them named Gyrotron FU CW GVI (we are using “Gyrotron FU CW GO-1” as an official name in Osaka University) is designed to have a special function of high speed frequency modulation δf within 100 MHz band. This will expand excitable band width of ESR and increase the number of electron spins contributing to DNP. The other gyrotron, Gyrotron FU CW GVIA (“Gyrotron FU CW GO-II”) has a function of frequency tunability Δf in the range of wider than 1.5 GHz, which is achieved in steady state by changing magnetic field intensity. This function should be used for adjusting the output frequency at the optimal value to achieve the highest enhancement factor of DNP.

Design of 132 GHz gyrotron with 3 GHz tunability for 200 MHz DNP/NMR spectrometer

A complete design of 132GHz gyrotron for 200MHz DNP–NMR spectroscopy application is presented in this article. The design is performed considering a frequency tunability range of 3GHz and output power around 100W. A smooth frequency tunability is achieved via the excitation of TE03p modes (p=1–6) through the variation of cavity magnetic field. The start oscillation current calculation is performed to estimate the required magnetic field for each TE03p mode. Cold cavity analysis and beam–wave interaction computation are carried out for the estimation of quality factor, resonant frequency corresponding to each TE03p mode, axial electric field profile and output power. Other important components of gyrotron such as magnetron injection gun, non-linear taper and RF window are also designed considering the smooth frequency tunability a main design parameter.

Broadband Continuously Frequency Tunable Gyrotron for 600 MHz DNP-NMR Spectroscopy

A broadband continuously frequency tunable gyrotron with a triode-type magnetron injection gun was developed as power source for analysis of protein structures. The TE7,3 oscillation mode was selected to avoid mode competitions in the high magnetic field side. Axial modes of the TE7,3,−10 were sequentially excited by changing the cavity magnetic field, and frequency tuning of about 4 GHz around 395 GHz was observed with output power greater than 50 W. The frequency also varied about 1 GHz as the anode–cathode voltage varied. Thus, the broadest tuning bandwidth in the 400 GHz band gyrotrons was achieved.

Dissolution DNP-NMR spectroscopy using galvinoxyl as a polarizing agent

The goal of this work was to test feasibility of using galvinoxyl (2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy) as a polarizing agent for dissolution dynamic nuclear polarization (DNP) NMR spectroscopy. We have found that galvinoxyl is reasonably soluble in ethyl acetate, chloroform, or acetone and the solutions formed good glasses when mixed together or with other solvents such as dimethyl sulfoxide. W-band electron spin resonance (ESR) measurements revealed that galvinoxyl has an ESR linewidth D intermediate between that of carbon-centered free radical trityl OX063 and the nitroxide-based 4-oxo-TEMPO, thus the DNP with galvinoxyl for nuclei with low gyromagnetic ratio γ such as 13C and 15N is expected to proceed predominantly via the thermal mixing process. The optimum radical concentration that would afford the highest 13C nuclear polarization (approximately 6% for [1-13C]ethyl acetate) at 3.35T and 1.4K was found to be around 40mM. After dissolution, large liquid-state NMR enhancements were achieved for a number of 13C and 15N compounds with long spin–lattice relaxation time T1. In addition, the hydrophobic galvinoxyl free radical can be easily filtered out from the dissolution liquid when water is used as the solvent. These results indicate that galvinoxyl can be considered as an easily available free radical polarizing agent for routine dissolution DNP-NMR spectroscopy.

Continuously Tunable 250 GHz Gyrotron with a Double Disk Window for DNP-NMR Spectroscopy.

In this paper, we describe the design and experimental results from the rebuild of a 250 GHz gyrotron used for Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance spectroscopy on a 380 MHz spectrometer. Tuning bandwidth of approximately 2 GHz is easily achieved at a fixed magnetic field of 9.24 T and a beam current of 95 mA producing an average output power of >10 W over the entire tuning band. This tube incorporates a double disk output sapphire window in order to maximize the transmission at 250.58 GHz. DNP Signal enhancement of >125 is achieved on a 13C-Urea sample using this gyrotron.