Abstract
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.
Highlights
Over the last two decades high field dynamic nuclear polarization (DNP) has emerged as a prominent field of research and has impacted approaches to solution NMR, solid-state or magic-angle spinning (MAS) NMR, electron paramagnetic resonance (EPR), and magnetic resonance imaging (MRI)
While SA-BDPA shows no significant variation in linewidth between 140 GHz (5 T) and 275 GHz (9.8 T) due to negligible g-anisotropy and inhomogeneous broadening dominated by unresolved hyperfine couplings to 1H, the axially symmetric g-anisotropy of trityl leads to an increase of spectral
A detailed description of protein expression, purification, and labeling is given in the Electronic supplementary information (ESI).† 4-Mercaptomethyl dipicolinic acid (4MMDPA) has been synthesized following the procedure published by Potapov et al.;63 1,4,7,10-tetraazacyclododecane1,4,7-tris-acetic acid-10-maleimidoethylacetamide (DOTA-M) was
Summary
Over the last two decades high field dynamic nuclear polarization (DNP) has emerged as a prominent field of research and has impacted approaches to solution NMR, solid-state or magic-angle spinning (MAS) NMR, electron paramagnetic resonance (EPR), and magnetic resonance imaging (MRI). The development of the instrumentation for MAS DNP at high magnetic fields[1,2,3,4] has catalyzed fundamental studies of quantum mechanical properties of spin-systems,[5,6,7,8,9,10,11,12,13,14] applications in materials science[15,16,17,18,19,20] and structural biology.[21,22,23,24,25,26,27,28,29,30]. Even though the enhancement factors cannot compete quantitatively with the highly efficient bis-nitroxides, the natural occurrence in metalloproteins—in the case of Mn2+—or the possibility to replace diamagnetic metal ions such as Mg2+ or Ca2+ make these high-spin metal ions interesting targets for further research
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