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

Abstract

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.

Influence of Dy3+ and Tb3+ doping on 13C dynamic nuclear polarization

“…[54][55][56] The change in DNP spectrum seen with lanthanide doping may be understood equally well by this model. The shortening of electronic T 1 induced by the lanthanide ion changes the level of contribution from the SE and CE, in this case accentuating the contributions from both mechanisms.…”

“…24,[50][51][52][53] Other DNP data at similar conditions suggested that the mechanism could be a combination of the solid effect (SE) and cross effect (CE). [54][55][56] Unlike the solid effect (SE) 57 and the cross effect (CE), 54,58 TM is a many-spin model for polarization that treats spin interactions as thermodynamic reservoirs. 24,59 Namely, these reservoirs are the nuclear Zeeman system (NZS) and electron Zeeman system (EZS), and the electron dipolar system (EDS).…”

“…37,60,68 Under the thermal mixing model, this may be seen in Equation (1) in which η is reduced by a shortening of electronic T 1 , leading to greater achievable polarization. It should be noted that the improvement of 13 C polarization may also be described using a combination of the cross and solid effects as presented by Ravera et al 55 In either case, it is clear that there are a number of factors in addition to electronic T 1 that must contribute given that EPR shows that the trityl electron relaxation rate is similarly affected by each of the lanthanides studied. One possible explanation is the effect of a given lanthanide on the nuclear relaxation rate as discussed previously.…”

“…The shortening of electronic T 1 induced by the lanthanide ion changes the level of contribution from the SE and CE, in this case accentuating the contributions from both mechanisms. 55 It is important to note that full quantum mechanical models have been developed to explain this experimental DNP effect using these 2-spin and 3-spin processes. [54][55][56] Once the polarization peaks were determined by way of microwave frequency sweeps, polarization build-up curves were taken by irradiating samples at the positive polarization peak P(+) while monitoring polarization at 3 min intervals.…”

“…55 It is important to note that full quantum mechanical models have been developed to explain this experimental DNP effect using these 2-spin and 3-spin processes. [54][55][56] Once the polarization peaks were determined by way of microwave frequency sweeps, polarization build-up curves were taken by irradiating samples at the positive polarization peak P(+) while monitoring polarization at 3 min intervals. Initially, concentrations of 0 mM, 1 mM, 1.5 mM, 2 mM, 3 mM, and 4 mM of both Dy 3+ and Tb 3+ were studied.…”