Dynamic Nuclear Polarization with a Rigid Biradical

Dynamic nuclear polarization (DNP) is an approach that can enhance NMR signal intensities of solids and liquids by two to three orders of magnitude. During a DNP experiment, the large Boltzmann polarization of an exogenous or endogenous paramagnetic species, such as a stable free radical, is transferred to the nuclei of interest by microwave (mw) irradiation of the sample at the electron paramagnetic resonance (EPR) frequency. The maximum theoretical enhancement achievable is given by the ratio γs/γI, where γs and γI are the gyromagnetic ratios of the electron and the nucleus, respectively. The enhanced nuclear polarization is of considerable interest in a variety of applications ranging from particle physics [1, 2] to structural biology [3, 4] and clinical imaging.[5] The mechanism that dominates the electron-nuclear polarization transfer depends on the relative sizes of the homogeneous linewidth, δ, and the inhomogeneous breadth, Δ of the EPR spectrum of the paramagnetic polarizing agent compared to the nuclear Larmor frequency, ω0I. When δ,Δ ω0I>δ the cross-effect (CE) is operative.[3] In general, the largest signal enhancements observed at high magnetic fields (≥ 5 T) are in experiments where the CE controls the polarization transfer.[6, 7] The underlying mechanism of the CE is a two-step process that involving two electrons with Larmor frequencies ω0S1 and ω0S2 and a nucleus with a frequency ω0I. Initially, the EPR transition of one electron is irradiated and then nuclear polarization is generated in a subsequent three-spin flip-flop process through transitions such as |α1Sβ2SβI> ↔ |β1Sα2SαI> or |β1Sα2SβI> ↔ |α1Sβ2SαI>.[8–10] Therefore, the CE is optimized when there is a sufficiently strong dipolar coupling between the two electrons, and the difference between the electron Larmor frequencies approximates the nuclear Larmor frequency (ω0S1 – ω0S2 ≈ ± ω0I). These requirements can be more easily fulfilled within a biradical than among dispersed monoradicals,[11] and we have demonstrated that the optimal polarizing agent for experiments in glycerol/water is currently the TEMPO based biradical 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL) [12] (Figure 1, bottom). However, at high magnetic fields the effective electron resonance frequency can depend strongly on the molecular orientation with respect to the external magnetic field, and in a biradical the matching condition is controlled by the relative orientations of the electron g-tensors. In particular, since the propan-2-ol tether in TOTAPOL is relatively flexible,[6] the relative orientation of the two TEMPO moieties is not well constrained and many orientations do not lead to the correct frequency separation. Therefore, a more rigid tether that locks the two TEMPO moieties at a desirable relative orientation should further increase the enhancement obtained from the polarizing agent.