Category Archives: Gd

(13)C Dynamic Nuclear Polarization Using a Trimeric Gd(3+) Complex as an Additive #DNPNMR

Niedbalski, P., et al., (13)C Dynamic Nuclear Polarization Using a Trimeric Gd(3+) Complex as an Additive. J. Phys. Chem. A, 2017. 121(27): p. 5127-5135.

https://www.ncbi.nlm.nih.gov/pubmed/28631929

Dissolution dynamic nuclear polarization (DNP) is one of the most successful techniques that resolves the insensitivity problem in liquid-state nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) by amplifying the signal by several thousand-fold. One way to further improve the DNP signal is the inclusion of trace amounts of lanthanides in DNP samples doped with trityl OX063 free radical as the polarizing agent. In practice, stable monomeric gadolinium complexes such as Gd-DOTA or Gd-HP-DO3A are used as beneficial additives in DNP samples, further boosting the DNP-enhanced solid-state (13)C polarization by a factor of 2 or 3. Herein, we report on the use of a trimeric gadolinium complex as a dopant in (13)C DNP samples to improve the (13)C DNP signals in the solid-state at 3.35 T and 1.2 K and consequently, in the liquid-state at 9.4 T and 298 K after dissolution. Our results have shown that doping the (13)C DNP sample with a complex which holds three Gd(3+) ions led to an improvement of DNP-enhanced (13)C polarization by a factor of 3.4 in the solid-state, on par with those achieved using monomeric Gd(3+) complexes but only requires about one-fifth of the concentration. Upon dissolution, liquid-state (13)C NMR signal enhancements close to 20000-fold, approximately 3-fold the enhancement of the control samples, were recorded in the nearby 9.4 T high resolution NMR magnet at room temperature. Comparable reduction of (13)C spin-lattice T1 relaxation time was observed in the liquid-state after dissolution for both the monomeric and trimeric Gd(3+) complexes. Moreover, W-band electron paramagnetic resonance (EPR) data have revealed that 3-Gd doping significantly reduces the electron T1 of the trityl OX063 free radical, but produces negligible changes in the EPR spectrum, reminiscent of the results with monomeric Gd(3+)-complex doping. Our data suggest that the trimeric Gd(3+) complex is a highly beneficial additive in (13)C DNP samples and that its effect on DNP efficiency can be described in the context of the thermal mixing mechanism.

The effect of Gd on trityl-based dynamic nuclear polarisation in solids

Ravera, E., et al., The effect of Gd on trityl-based dynamic nuclear polarisation in solids. Phys. Chem. Chem. Phys., 2015. 17(40): p. 26969-78.

https://www.ncbi.nlm.nih.gov/pubmed/26403358

In dynamic nuclear polarisation (DNP) experiments performed under static conditions at 1.4 K we show that the presence of 1 mM Gd(iii)-DOTAREM increases the (13)C polarisation and decreases the (13)C polarisation buildup time of (13)C-urea dissolved in samples containing water/DMSO mixtures with trityl radical (OX063) concentrations of 10 mM or higher. To account for these observations further measurements were carried out at 6.5 K, using a combined EPR and NMR spectrometer. At this temperature, frequency swept DNP spectra of samples with 5 or 10 mM OX063 were measured, with and without 1 mM Gd-DOTA, and again a (13)C enhancement gain was observed due to the presence of Gd-DOTA. These measurements were complemented by electron-electron double resonance (ELDOR) measurements to quantitate the effect of electron spectral diffusion (eSD) on the DNP enhancements and lineshapes. Simulations of the ELDOR spectra were done using the following parameters: (i) a parameter defining the rate of the eSD process, (ii) an “effective electron-proton anisotropic hyperfine interaction parameter”, and (iii) the transverse electron spin relaxation time of OX063. These parameters, together with the longitudinal electron spin relaxation time, measured by EPR, were used to calculate the frequency profile of electron polarisation. This, in turn, was used to calculate two basic solid effect (SE) and indirect cross effect (iCE) DNP spectra. A properly weighted combination of these two normalized DNP spectra provided a very good fit of the experimental DNP spectra. The best fit simulation parameters reveal that the addition of Gd(iii)-DOTA causes an increase in both the SE and the iCE contributions by similar amounts, and that the increase in the overall DNP enhancements is a result of narrowing of the ELDOR spectra (increased electron polarisation gradient across the EPR line). These changes in the electron depolarisation profile are a combined result of shortening of the longitudinal and transverse electron spin relaxation times, as well as an increase in the eSD rate and in the effective electron-proton anisotropic hyperfine interaction parameter.

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.

The effect of Gd on trityl-based dynamic nuclear polarisation in solids

Ravera, E., et al., The effect of Gd on trityl-based dynamic nuclear polarisation in solids. Phys Chem Chem Phys, 2015. 17(40): p. 26969-78.

http://www.ncbi.nlm.nih.gov/pubmed/26403358

In dynamic nuclear polarisation (DNP) experiments performed under static conditions at 1.4 K we show that the presence of 1 mM Gd(iii)-DOTAREM increases the (13)C polarisation and decreases the (13)C polarisation buildup time of (13)C-urea dissolved in samples containing water/DMSO mixtures with trityl radical (OX063) concentrations of 10 mM or higher. To account for these observations further measurements were carried out at 6.5 K, using a combined EPR and NMR spectrometer. At this temperature, frequency swept DNP spectra of samples with 5 or 10 mM OX063 were measured, with and without 1 mM Gd-DOTA, and again a (13)C enhancement gain was observed due to the presence of Gd-DOTA. These measurements were complemented by electron-electron double resonance (ELDOR) measurements to quantitate the effect of electron spectral diffusion (eSD) on the DNP enhancements and lineshapes. Simulations of the ELDOR spectra were done using the following parameters: (i) a parameter defining the rate of the eSD process, (ii) an “effective electron-proton anisotropic hyperfine interaction parameter”, and (iii) the transverse electron spin relaxation time of OX063. These parameters, together with the longitudinal electron spin relaxation time, measured by EPR, were used to calculate the frequency profile of electron polarisation. This, in turn, was used to calculate two basic solid effect (SE) and indirect cross effect (iCE) DNP spectra. A properly weighted combination of these two normalized DNP spectra provided a very good fit of the experimental DNP spectra. The best fit simulation parameters reveal that the addition of Gd(iii)-DOTA causes an increase in both the SE and the iCE contributions by similar amounts, and that the increase in the overall DNP enhancements is a result of narrowing of the ELDOR spectra (increased electron polarisation gradient across the EPR line). These changes in the electron depolarisation profile are a combined result of shortening of the longitudinal and transverse electron spin relaxation times, as well as an increase in the eSD rate and in the effective electron-proton anisotropic hyperfine interaction parameter.

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