13 C Dynamic Nuclear Polarization Using a Trimeric Gd 3+ Complex as an Additive.

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 20 000-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 T ₁ 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 T ₁ 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.