Category Archives: Cross-Effect

Dynamic nuclear polarization via the cross effect and thermal mixing: B. Energy transport #DNPNMR

Wenckebach, W.Th. “Dynamic Nuclear Polarization via the Cross Effect and Thermal Mixing: B. Energy Transport.” Journal of Magnetic Resonance 299 (February 2019): 151–67.

https://doi.org/10.1016/j.jmr.2018.12.020

The fundamental process of dynamic nuclear polarization (DNP) via the cross effect (CE) and thermal mixing (TM) is a triple spin flip, in which two interacting electron spins and a nuclear spin interacting with one of these electron spins flip together. In the previous article (Wenckebach, 2018) these triple spin flips were treated by first determining the eigenstates of the two interacting electron spins exactly and next investigating transitions involving these exact eigenstates and the nuclear spin states. It was found that two previously developed approaches—the scrambled states approach and the fluctuating field approach—are just two distinct limiting cases of this more general approach. It was also shown that triple spin flips constitute a single process causing two flows of energy: a flow originating in the electron Zeeman energy and a flow originating in the mutual interactions between the electron spins. In order to render their definitions more precise, the former flow was denoted as the CE and the latter as TM.

High resolution observed in 800 MHz DNP spectra of extremely rigid type III secretion needles #DNPNMR

Fricke, P., et al., High resolution observed in 800 MHz DNP spectra of extremely rigid type III secretion needles. J Biomol NMR, 2016. 65(3-4): p. 121-6.

https://www.ncbi.nlm.nih.gov/pubmed/27351550

The cryogenic temperatures at which dynamic nuclear polarization (DNP) solid-state NMR experiments need to be carried out cause line-broadening, an effect that is especially detrimental for crowded protein spectra. By increasing the magnetic field strength from 600 to 800 MHz, the resolution of DNP spectra of type III secretion needles (T3SS) could be improved by 22 %, indicating that inhomogeneous broadening is not the dominant effect that limits the resolution of T3SS needles under DNP conditions. The outstanding spectral resolution of this system under DNP conditions can be attributed to its low overall flexibility.

Temperature dependence of cross-effect dynamic nuclear polarization in rotating solids: advantages of elevated temperatures #DNPNMR

Geiger, M.A., et al., Temperature dependence of cross-effect dynamic nuclear polarization in rotating solids: advantages of elevated temperatures. Phys. Chem. Chem. Phys., 2016. 18(44): p. 30696-30704.

https://www.ncbi.nlm.nih.gov/pubmed/27791210

Dynamic nuclear polarization exploits electron spin polarization to boost signal-to-noise in magic-angle-spinning (MAS) NMR, creating new opportunities in materials science, structural biology, and metabolomics studies. Since protein NMR spectra recorded under DNP conditions can show improved spectral resolution at 180-200 K compared to 110 K, we investigate the effects of AMUPol and various deuterated TOTAPOL isotopologues on sensitivity and spectral resolution at these temperatures, using proline and reproducibly prepared SH3 domain samples. The TOTAPOL deuteration pattern is optimized for protein DNP MAS NMR, and signal-to-noise per unit time measurements demonstrate the high value of TOTAPOL isotopologues for Protein DNP MAS NMR at 180-200 K. The combined effects of enhancement, depolarization, and proton longitudinal relaxation are surprisingly sample-specific. At 200 K, DNP on SH3 domain standard samples yields a 15-fold increase in signal-to-noise over a sample without radicals. 2D and 3D NCACX/NCOCX spectra were recorded at 200 K within 1 and 13 hours, respectively. Decreasing enhancements with increasing 2H-content at the CH2 sites of the TEMPO rings in CD3-TOTAPOL highlight the importance of protons in a sphere of 4-6 A around the nitroxyl group, presumably for polarization pickup from electron spins.

Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization #DNPNMR

Kubicki, D.J., et al., Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization. Chem. Sci., 2016. 7(1): p. 550-558.

http://dx.doi.org/10.1039/C5SC02921J

A series of 37 dinitroxide biradicals have been prepared and their performance studied as polarizing agents in cross-effect DNP NMR experiments at 9.4 T and 100 K in 1,1,2,2-tetrachloroethane (TCE). We observe that in this regime the DNP performance is strongly correlated with the substituents on the polarizing agents, and electron and nuclear spin relaxation times, with longer relaxation times leading to better enhancements. We also observe that deuteration of the radicals generally leads to better DNP enhancement but with longer build-up time. One of the new radicals introduced here provides the best performance obtained so far under these conditions.

Correction: Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids #DNPNMR

Corzilius, B., Correction: Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids. Phys. Chem. Chem. Phys., 2016. 18(42): p. 29643-29643.

http://dx.doi.org/10.1039/C6CP90249A

Correction for ‘Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids’ by Bjorn Corzilius et al., Phys. Chem. Chem. Phys., 2016, DOI: 10.1039/c6cp04621e.

Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids #DNPNMR

Corzilius, B., Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids. Phys. Chem. Chem. Phys., 2016. 18(39): p. 27190-27204.

http://dx.doi.org/10.1039/C6CP04621E

Dynamic nuclear polarization (DNP) is a powerful method to enhance sensitivity especially of solid-state magic-angle spinning (MAS) NMR by up to several orders of magnitude. The increased interest both from a practical as well as theoretical viewpoint has spawned several fields of active research such as the development of new polarizing agents with improved or unique properties and description of the underlying DNP mechanisms such as solid effect (SE) and cross effect (CE). Even though a novel class of unique polarizing agents based on high-spin metal ions such as Gd(iii) and Mn(ii) has already been utilized for MAS DNP a theoretical description of the involved DNP mechanism is still incomplete. Here, we review several aspects of DNP-relevant electron-paramagnetic resonance (EPR) properties of the general class of these half-integer high-spin metal ions with isotropic Zeeman interaction but significant zero-field splitting (ZFS). While the SE can be relatively easily described similar to that of a S = 1/2 system and is assumed to be effective only for polarizing agents featuring a narrow central EPR transitions (i.e., mS = -1/2 [rightward arrow] +1/2) with respect to the nuclear Larmor frequency, the CE between two high-spin ions requires a more detailed theoretical investigation due to a multitude of possible transitions and matching conditions. This is especially interesting in light of recent understanding of CE being induced by MAS-driven level anti-crossings (LACs) between dipolar-coupled electron spins. We discuss the requirements of such CE-enabling LACs to occur due to anisotropy of ZFS, the expected adiabaticity, and the resulting possibilities of high-spin metal ion pairs to act as polarizing agents for DNP. This theoretical description serves as a framework for a detailed experimental study published directly following this work.

Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature

This is an incredible article. It shows the temperature dependence of the DNP enhancement over a wide temperature regime. Most importantly it shows that at room temperature still an enhancement of 15-20 can be achieved.
Just a few years ago the common believe was that solid-state MAS-DNP experiments have to be performed at 90 K or below. This article clearly demonstrates that there is still so much room for improvements of DNP. I think the most exciting moments in DNP are still ahead of us and the method has the potential to become an integral part of the DNP toolbox.

Lelli, M., et al., Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature. J Am Chem Soc, 2015. 137(46): p. 14558-61.

http://www.ncbi.nlm.nih.gov/pubmed/26555676

Efficient dynamic nuclear polarization (DNP) in solids, which enables very high sensitivity NMR experiments, is currently limited to temperatures of around 100 K and below. Here we show how by choosing an adequate solvent, (1)H cross effect DNP enhancements of over 80 can be obtained at 240 K. To achieve this we use the biradical TEKPol dissolved in a glassy phase of ortho-terphenyl (OTP). We study the solvent DNP enhancement of both TEKPol and BDPA in OTP in the range from 100 to 300 K at 9.4 and 18.8 T. Surprisingly, we find that the DNP enhancement decreases only relatively slowly for temperatures below the glass transition of OTP (Tg = 243 K), and (1)H enhancements around 15-20 at ambient temperature can be observed. We use this to monitor molecular dynamic transitions in the pharmaceutically relevant solids Ambroxol and Ibuprofen.

Theoretical Aspects of Magic Angle Spinning – Dynamic Nuclear Polarization

Mentink-Vigier, F., et al., Theoretical Aspects of Magic Angle Spinning – Dynamic Nuclear Polarization. J. Magn. Reson., 2015.

http://www.sciencedirect.com/science/article/pii/S1090780715001500

Magic Angle Spinning combined with Dynamic Nuclear Polarization has been proven in recent years to be a very powerful method for increasing solid state NMR signals. Since the advent of biradicals such as TOTAPOL to increase the nuclear polarization new classes of radicals, with larger molecular weight and/or different spin properties have been developed. These have led to unprecedented signal gain, with varying results for different experimental parameters, in particular the microwave irradiation strength, the static field, and the spinning frequency. Recently it has been shown that spinning of the sample imposes DNP enhancement processes that differ from the DNP processes enhancing the nuclear polarizations in static samples. During the sample spinning the DNP enhancements are the results of energy level anticrossings occurring periodically during each rotor cycle. In this work we present experimental results of the MAS spinning frequency DNP enhancement profiles of four nitroxide based radicals at two different sets of temperature 110 and 160 K. These results emphasize the reduction of these enhancements for increasing spinning frequencies. The simulation code calculating MAS-DNP powder enhancements of small model spin systems has been improved to extend our studies of the influence of variations in the interaction and relaxation parameters on powder enhancements. These studies provide a better understanding of the impact of changes in these parameters on the MAS-DNP mechanism. To accomplish this we simulated the spin dynamics of a single three-spin system { e a – e b – n } during its steady state rotor periods and used the Landau-Zener formula to characterize the influence of the different anti-crossings on the polarizations of the system and their necessary action for reaching steady state conditions together with spin relaxation processes. Based on these model calculations we demonstrate that the maximal steady state nuclear polarization cannot become larger than the maximal polarization difference between the two electrons during the steady state rotor cycle. This study also shows the complexity of the MAS-DNP process and therefore the necessity to rely on numerical simulations for understanding parametric dependences of the enhancements. Finally an extension of the three-spin system allowed us to probe the first steps of the transfer of polarization from the nuclei coupled to the electrons to further away nuclei, demonstrating a decrease in the spin-diffusion barrier under MAS conditions.

Simultaneous DNP enhancements of (1)H and (13)C nuclei: theory and experiments

Shimon D, Hovav Y, Kaminker I, Feintuch A, Goldfarb D, Vega S. Simultaneous DNP enhancements of (1)H and (13)C nuclei: theory and experiments. Phys Chem Chem Phys. 2015;17(17):11868-83.

http://www.ncbi.nlm.nih.gov/pubmed/25869779

DNP on heteronuclear spin systems often results in interesting phenomena such as the polarization enhancement of one nucleus during MW irradiation at the “forbidden” transition frequencies of another nucleus or the polarization transfer between the nuclei without MW irradiation. In this work we discuss the spin dynamics in a four-spin model system of the form {ea-eb-((1)H,(13)C)}, with the Larmor frequencies omegaa, omegab, omegaH and omegaC, by performing Liouville space simulations. This spin system exhibits the common (1)H solid effect (SE), (13)C cross effect (CE) and in addition high order CE-DNP enhancements. Here we show, in particular, the “proton shifted (13)C-CE” mechanism that results in (13)C polarization when the model system, at one of its (13)C-CE conditions, is excited by a MW field at the zero quantum or double quantum electron-proton transitions omegaMW = omegaa +/- omegaH and omegaMW = omegab +/- omegaH. Furthermore, we introduce the “heteronuclear” CE mechanism that becomes efficient when the system is at one of its combined CE conditions |omegaa – omegab| = |omegaH +/- omegaC|. At these conditions, simulations of the four-spin system show polarization transfer processes between the nuclei, during and without MW irradiation, resembling the polarization exchange effects often discussed in the literature. To link the “microscopic” four-spin simulations to the experimental results we use DNP lineshape simulations based on “macroscopic” rate equations describing the electron and nuclear polarization dynamics in large spin systems. This approach is applied based on electron-electron double resonance (ELDOR) measurements that show strong (1)H-SE features outside the EPR frequency range. Simulated ELDOR spectra combined with the indirect (13)C-CE (iCE) mechanism, result in additional “proton shifted (13)C-CE” features that are similar to the experimental ones. These features are also observed experimentally in (13)C-DNP spectra of a sample containing 15 mM of trityl in a glass forming solution of (13)C-glycerol/H2O and are analyzed by calculating the basic (13)C-SE and (13)C-iCE shapes using simulated ELDOR spectra that were fitted to the experimental ones.

Mechanisms of dynamic nuclear polarization in insulating solids

Can, T.V., Q.Z. Ni, and R.G. Griffin, Mechanisms of dynamic nuclear polarization in insulating solids. J Magn Reson, 2015. 253(0): p. 23-35.

http://www.ncbi.nlm.nih.gov/pubmed/25797002

Dynamic nuclear polarization (DNP) is a technique used to enhance signal intensities in NMR experiments by transferring the high polarization of electrons to their surrounding nuclei. The past decade has witnessed a renaissance in the development of DNP, especially at high magnetic fields, and its application in several areas including biophysics, chemistry, structural biology and materials science. Recent technical and theoretical advances have expanded our understanding of established experiments: for example, the cross effect DNP in samples spinning at the magic angle. Furthermore, new experiments suggest that our understanding of the Overhauser effect and its applicability to insulating solids needs to be re-examined. In this article, we summarize important results of the past few years and provide quantum mechanical explanations underlying these results. We also discuss future directions of DNP and current limitations, including the problem of resolution in protein spectra recorded at 80-100K.

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