¹H Thermal Mixing Dynamic Nuclear Polarization with BDPA as Polarizing Agents

Abstract

Dynamic Nuclear Polarization (DNP) is a sensitivity enhancing technique for Nuclear Magnetic Resonance. A recent discovery of Overhauser Effect (OE) DNP in insulating systems under cryogenic conditions using 1,3-bisdiphenylene-2-phenylallyl (BDPA) as the polarizing agent (PA) has caught attention due to its promising DNP performance at a high magnetic field and under fast magic angle spinning conditions. However, the mechanism of OE in insulating-solids/BDPA is unclear. We present an alternative explanation that the dominant underlying DNP mechanism of BDPA is Thermal Mixing (TM). This is ascertained with the discovery that TM effect is enhanced by multi-electron spin coupling, which is corroborated by an asymmetric electron paramagnetic resonance line shape signifying the coexistence of clustered and isolated BDPA species, and by hyperpolarized electron spin populations giving rise to an electron spin polarization gradient which are characteristic signatures of TM DNP. Finally, quantum mechanical simulations using spatially asymmetrically coupled three electron spins and a nuclear spin demonstrate that triple-flip DNP, with hyperfine fluctuations turned off, can yield the 1 H DNP profile as observed with BDPA. Clarifying the DNP mechanism is critical to develop design principles for optimizing the PA for achieving optimal DNP efficiency.

Solid-like Dynamic Nuclear Polarization Observed in the Fluid Phase of Lipid Bilayers at 9.4 T

“…In contrast, our observed SE DNP on the lipid protons starts by a direct polarization transfer step from the BDPA electron spin to the lipid proton followed by a dispersion of the polarization mediated by lateral diffusion of the lipids. Our observation of DNP enhancements with the field profile of TM could also indicate that indeed such effects could contribute to the DNP enhancements observed at low temperatures in solid samples, as proposed recently …”

“…Our observation of DNP enhancements with the field profile of TM could also indicate that indeed such effects could contribute to the DNP enhancements observed at low temperatures in solid samples, as proposed recently. 34 In conclusion, in this work we described the first high field DNP experiments in the liquid phase of DMPC at ambient temperatures, with field profiles indicative of the SE and TM. The essential steps to achieve this were (i) the use of stripline/ Fabry−Peŕot double resonance DNP structure, allowing us to perform the measurements in the liquid phase with a very high microwave B 1 field, (ii) the use of the polarizing agent BDPA with a narrow EPR line, and (iii) the use of DMPC lipid bilayers as an environment having much slower molecular motions in the aqueous medium compared to the BDPA radical alone.…”

“…Therefore, this enhancement pattern could be due to small BDPA clusters in the lipid, as visualized in Figure . Such clusters have been observed in low temperature DNP experiments of BDPA in an ortho -terphenyl matrix . Different from the DNP enhancement observed at ω S /γ e ± ω H /γ e , attributed to the SE, this enhancement varied from sample to sample considerably (see SI).…”

“…Such clusters have been observed in low temperature DNP experiments of BDPA in an ortho-terphenyl matrix. 34 Different from the DNP enhancement observed at ω S /γ e ± ω H /γ e , attributed to the SE, this enhancement varied from sample to sample considerably (see SI). Previous EPR and UV−vis spectroscopy studies revealed that BDPA radicals, and especially BDPA clusters, have a limited thermal stability.…”

Observation of a four-spin solid effect

“…These terms can be safely ignored because they have negligible influence if the condition A k ≪ ω 0I , ω 1S is satisfied. 15 Following that, we obtained the matching conditions for the two quantum (2Q) and four quantum (4Q) 4SSE transitions, and the corresponding 4Q and 2Q transition amplitudes upon microwave excitation with Rabi field, ω 1S , are where are the eigenstates in . Next, we apply Fermi’s golden rule, which states that the transition rate between the energy eigenstates (in the case of weak perturbation) is proportional to the square of the matrix element, 22 Hence, the results show that the 4SSE DNP transfer efficiency has a dependence compared to the relation exhibited by the conventional 2SSE.…”