Dynamic nuclear polarization with a water-soluble rigid biradical

Abstract

A new biradical polarizing agent, bTbtk-py, for dynamic nuclear polarization (DNP) experiments in aqueous media is reported. The synthesis is discussed in light of the requirements of the optimum, theoretical, biradical system. To date, the DNP NMR signal enhancement resulting from bTbtk-py is the largest of any biradical in the ideal glycerol/water solvent matrix, ε = 230. EPR and X-ray crystallography are used to characterize the molecule and suggest approaches for further optimizing the biradical distance and relative orientation.

Figure 1

DNP enhanced polarization was measured under microwave irradiation using a gyrotron beam current of 34 mA, which corresponds to ~12 W microwave power. (Upper) ¹³C-NMR (CPMAS) of 1M urea-¹³C in 60/30/10 glycerol-d₈/D₂O/H₂O (v/v/v) with 10 mM of the appropriate biradical acquired with and without microwave irradiation. (Lower) Build-up curves were recorded at the respective field yielding the maximum enhancement for each radical (i.e. 4.982 T for bTbtk-py, 4.980 T for TOTAPOL; see Figure 3) and subsequently scaled in order to normalize the off signal intensities at t = ∞. Proton longitudinal relaxation time constants were determined as T_1I = 3.9 s for bTbtk-py and T_1I = 4.5 s for TOTAPOL. Note that aside from the radical used and the magnetic field strength, the experimental conditions are the same for each trace.

Figure 2

Power dependence of the steady-state DNP enhancement ε_∞ for bTbtk-py and TOTAPOL given by: 1/ε = 1/ε_∞(1+1/aP). The experiments with the two polarizing agents were carried out at the field position giving the optimal enhancement for each. Otherwise, the experimental conditions were identical (see supporting information).

Figure 3

(upper) EPR spectrum of bTbtk-py measured by echo detection at 140 GHz and T=80 K and (lower) field enhancement profiles of bTbtk-py and TOTAPOL (see supporting information). The enhancement profile was recorded by directly observing the ¹H polarization under ~8 W of microwave power using a Bloch decay and scaling the obtained data with respect to the maximum enhancement determined by cross-polarization to ¹³C. TOTAPOL field profile adapted from Hu, et al.

Figure 4

Two views of the X-ray crystal structure of 3.

Scheme 1

Synthesis of bTbtk-py.