Category Archives: Metal Ions

Paramagnetic Metal Ions for Dynamic Nuclear Polarization #DNPNMR

Corzilius, Björn. “Paramagnetic Metal Ions for Dynamic Nuclear Polarization,” 7:16, 2018.

https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470034590.emrstm1593

Paramagnetic metal ions have been utilized as polarizing agents already in the early days of dynamic nuclear polarization (DNP) and have been more recently introduced for magic-angle spinning (MAS) DNP at high magnetic field. In this article, a comprehensive overview is given about the concepts relevant to DNP with high-spin metal ions. The theoretical basis covering the peculiar electron spin dynamics including spin-orbit coupling and zero-field splitting is reviewed, and prerequisites for efficient DNP are introduced. Subsequently, special considerations about the relevant DNP mechanisms (i.e., solid effect and cross effect) are derived. The practical aspects particular to high-spin metal ion DNP are discussed, focusing on differences with respect to conventional (i.e., radical) polarizing agents. In the final section, several demonstrations of MAS DNP on model systems as well as samples relevant to structural biology and materials research are presented. At last, an outlook is given about the prospects of metal ion DNP in light of recent and future advances in modern DNP.

Paramagnetic metal ion dopants as polarization agents for DNP NMR spectroscopy in inorganic solids #DNPNMR

Chakrabrty Tanmoy, Goldin Nir, Feintuch Akiva, Houben Lothar, and Leskes Michal. “Paramagnetic Metal Ion Dopants as Polarization Agents for DNP NMR Spectroscopy in Inorganic Solids.” ChemPhysChem 0, no. ja (May 17, 2018).

https://doi.org/10.1002/cphc.201800462.

Dynamic nuclear polarization (DNP), a technique in which the high electron spin polarization is transferred to surrounding nuclei via microwaves irradiation, equips solid state NMR spectroscopy with unprecedented sensitivity. The most commonly used polarization agents for DNP are nitroxide radicals. However, their applicability to inorganic materials is mostly limited to surface detection. Paramagnetic metal ions were recently introduced as alternatives for nitroxides. Doping inorganic solids with paramagnetic ions can be used to tune material properties and introduces endogenous DNP agents that can potentially provide sensitivity in the particles’ bulk and surface. Here we demonstrate the approach by doping Li4Ti5O12 (LTO), an anode material for lithium ion batteries, with paramagnetic ions. By incorporating Gd(III) and Mn(II) in LTO we gain up to 14 fold increase in signal intensity in static 7Li DNP?NMR experiments. These results suggest that doping with paramagnetic ions provides an efficient route for sensitivity enhancement in the bulk of micron size particles.

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.

Gd(iii) and Mn(ii) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins #DNPNMR

Kaushik, M., et al., Gd(iii) and Mn(ii) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins. Phys Chem Chem Phys, 2016. 18(39): p. 27205-27218.

https://www.ncbi.nlm.nih.gov/pubmed/27545112

We investigate complexes of two paramagnetic metal ions Gd3+ and Mn2+ to serve as polarizing agents for solid-state dynamic nuclear polarization (DNP) of 1H, 13C, and 15N at magnetic fields of 5, 9.4, and 14.1 T. Both ions are half-integer high-spin systems with a zero-field splitting and therefore exhibit a broadening of the mS = -1/2 <–> +1/2 central transition which scales inversely with the external field strength. We investigate experimentally the influence of the chelator molecule, strong hyperfine coupling to the metal nucleus, and deuteration of the bulk matrix on DNP properties. At small Gd-DOTA concentrations the narrow central transition allows us to polarize nuclei with small gyromagnetic ratio such as 13C and even 15N via the solid effect. We demonstrate that enhancements observed are limited by the available microwave power and that large enhancement factors of >100 (for 1H) and on the order of 1000 (for 13C) can be achieved in the saturation limit even at 80 K. At larger Gd(iii) concentrations (>/=10 mM) where dipolar couplings between two neighboring Gd3+ complexes become substantial a transition towards cross effect as dominating DNP mechanism is observed. Furthermore, the slow spin-diffusion between 13C and 15N, respectively, allows for temporally resolved observation of enhanced polarization spreading from nuclei close to the paramagnetic ion towards nuclei further removed. Subsequently, we present preliminary DNP experiments on ubiquitin by site-directed spin-labeling with Gd3+ chelator tags. The results hold promise towards applications of such paramagnetically labeled proteins for DNP applications in biophysical chemistry and/or structural biology.

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