Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing

Abstract

The available theoretical approaches aiming at describing Dynamic Nuclear spin Polarization (DNP) in solutions containing molecules of biomedical interest and paramagnetic centers are not able to model the behaviour observed upon varying the concentration of trityl radicals or the polarization enhancement caused by moderate addition of gadolinium complexes. In this manuscript, we first show experimentally that the nuclear steady state polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is substantially independent from the average internuclear distance. This evidences a leading role of electron (over nuclear) spin relaxation processes in determining the ultimate performances of DNP. Accordingly, we have devised a variant of the Thermal Mixing model for inhomogenously broadened electron resonance lines which includes a relaxation term describing the exchange of magnetic anisotropy energy of the electron spin system with the lattice. Thanks to this additional term, the dependence of the nuclear polarization on the electron concentration can be properly accounted for. Moreover, the model predicts a strong increase of the final polarization on shortening the electron spin-lattice relaxation time, providing a possible explanation for the effect of gadolinium doping.

On the present and future of dissolution-DNP

“…The EPR line of the trityl was already effectively homogeneous (fast spectral diffusion). However, the electron spin T1 of the radical was dramatically shortened by Gd from about 1 s to 0.1 s. The data suggests that the ratio of nuclear to electron T1 is a critical parameter [28,29]. The addition of Gd or another paramagnetic transition metal to enhance the solid state DNP efficiency, is to eliminate or minimize the relaxation effect in the liquid state post dissolution.…”

Transition Metal Doping Reveals Link between Electron T1 Reduction and 13C Dynamic Nuclear Polarization Efficiency

“…40–42 The common perception is that the reduction of electron T 1 of the polarizing agent brought about by the addition of paramagnetic agent is responsible for the increase in polarization. 33,41 This is corroborated by measurements of the electron T 1 for samples doped with the various paramagnetic agents that have been shown to enhance 13 C DNP. 32,35,36,38 In each case, the electron T 1 was shortened by about one order of magnitude.…”

“…While there are other possible mechanisms that could be responsible for polarization, there is a wealth of evidence that suggests thermal mixing is dominant under the conditions used herein. 9,22,41,56,57 In the most current models of thermal mixing, the shape of the EPR spectrum as well as the electron T 1 of the radical used largely determine both the shape of the DNP spectrum and the maximum achievable polarization. 56,58,59 Though there is an only minimal change in trityl’s EPR spectra with the addition of transition metal (Figure 4a), the electron T 1 measurement is more demonstrative.…”

Hyperpolarized NMR Spectroscopy: d‐DNP, PHIP, and SABRE Techniques

“…The efficiency of a given mechanism often depends on the magnetic field strength and temperature. Recent approaches to gain a better understanding of this dependence were reported by Wenckebach, Vega and co‐workers, and Rosso and co‐workers . Two families of PAs have been intensively used in d ‐DNP, namely nitroxide and triarylmethyl (trityl) radicals, with the former group being especially effective for the polarization of nuclei with large gyromagnetic ratios (γ; e.g.…”