A tailored multi-frequency EPR approach to accurately determine the magnetic resonance parameters of dynamic nuclear polarization agents: application to AMUPol

Abstract

To understand the dynamic nuclear polarization (DNP) enhancements of biradical polarizing agents, the magnetic resonance parameters need to be known. We describe a tailored EPR approach to accurately determine electron spin-spin coupling parameters using a combination of standard (9 GHz), high (95 GHz) and ultra-high (275 GHz) frequency EPR. Comparing liquid- and frozen-solution continuous-wave EPR spectra provides accurate anisotropic dipolar interaction D and isotropic exchange interaction J parameters of the DNP biradical AMUPol. We found that D was larger by as much as 30% compared to earlier estimates, and that J is 43 MHz, whereas before it was considered to be negligible. With the refined data, quantum mechanical calculations confirm that an increase in dipolar electron-electron couplings leads to higher cross-effect DNP efficiencies. Moreover, the DNP calculations qualitatively reproduce the difference of TOTAPOL and AMUPol DNP efficiencies found experimentally and suggest that AMUPol is particularly effective in improving the DNP efficiency at magnetic fields higher than 500 MHz. The multi-frequency EPR approach will aid in predicting the optimal structures for future DNP agents.

In Silico Design of DNP Polarizing Agents: Can Current Dinitroxides Be Improved?

“…Generally,inagreement with previoust heoretical work [39,46] as well as the experimental work of Mathiesa nd co-workers for TEMTriPols, [32] the highestD NP performance is achieved when the exchange cou-pling is in the range of 40-100 MHz. These data are tabulated in Ta ble 2.…”

“…The previously reported electron relaxation times [41] as well as exchange (43 MHz) and dipolar (53 MHz) coupling strengths [45,46] from AMUPolw ere used for the calculation. The previously reported electron relaxation times [41] as well as exchange (43 MHz) and dipolar (53 MHz) coupling strengths [45,46] from AMUPolw ere used for the calculation.…”

“…The previously reported electron relaxation times [41] as well as exchange (43 MHz) and dipolar (53 MHz) coupling strengths [45,46] from AMUPolw ere used for the calculation. The gt ensor was set to g xx = 2.006, g yy = 2.0021, and g zz = 2.0092, corresponding to an itroxide spin, [46,47] and the MAS frequency was fixed at 8kHz;o ther experimental parameters used in the simulations are described in Computational Details below.P owdera veraging was performedo ver all three Euler angles using as et of 343 orientationso ft he gt ensor with respect to the laboratory frame.I nt he calculations, the relative orientation of the two g tensors of the nitroxides is specified by the Euler angles (a, b, g)u sing the ZYZ convention. [48] Namely,r otationsw ere first performed along g zz by a,f ollowed by subsequent rotations along the new g yy and g zz directions by b and g,r espectively.…”

Efficiency of Water‐Soluble Nitroxide Biradicals for Dynamic Nuclear Polarization in Rotating Solids at 9.4 T: bcTol‐M and cyolyl‐TOTAPOL as New Polarizing Agents

“…In addition, a resolved intramolecular, effective electron–electron coupling is visible as splitting in g zz , and g yy peaks for bcTol , AMUPol , and bcTol‐M . This coupling is caused by the relatively close distance of nitroxide moieties (dipolar contribution) imposed by the short urea bridging element as well as by an exchange interaction J ex mediated through the molecular orbital system …”

“…In addition, a resolved intramolecular,e ffective electron-electron coupling is visible as splitting in g zz ,a nd g yy peaks for bcTol, AMUPol,a nd bcTol-M.T his coupling is caused by the relatively close distance of nitroxide moieties( dipolar contribution)i mposed by the short urea bridging element as well as by an exchange interaction J ex mediated through the molecular orbital system. [18] Electron spin relaxation at 263 GHz Figure 3s hows T 1e (saturation) and T 2e (echo)d ecay curves obtained at the fieldp ositionclosetot he MW irradiation position under typical DNP conditions (field position Ci nF igure S14). The relaxation time constantsw ere determined from the decay curves and are comparedi nT able1,t ogether with the oftendiscussed saturation factor T 1e T 2e .Adetailed description of how the decay curvesw ere calculated and how the time con-stants were extracted are shown in the Supporting Information.…”