High-frequency dynamic nuclear polarization using biradicals: A multifrequency EPR lineshape analysis

Abstract

To date, the cross effect (CE) and thermal mixing (TM) mechanisms have consistently provided the largest enhancements in dynamic nuclear polarization (DNP) experiments performed at high magnetic fields. Both involve a three-spin electron-electron-nucleus process whose efficiency depends primarily on two electron-electron interactions—the interelectron distance R and the correct electron paramagnetic resonance (EPR) frequency separation that matches the nuclear Larmor frequency, ∣ωe2−ωe1∣=ωn. Biradicals, for example, two 2,2,6,6-tetramethyl-piperidine-1-oxyls (TEMPOs) tethered with a molecular linker, can in principle constrain both the distance and relative g-tensor orientation between two unpaired electrons, allowing these two spectral parameters to be optimized for the CE and TM. To verify this hypothesis, we synthesized a series of biradicals—bis-TEMPO tethered by n ethylene glycol units (a.k.a. BTnE)—that show an increasing DNP enhancement with a decreasing tether length. Specifically at 90K and 5T, the enhancement grew from ∼40 observed with 10mM monomeric TEMPO, where the average R∼56Å corresponding to electron-electron dipolar coupling constant ωd∕2π=0.3MHz, to ∼175 with 5mM BT2E (10mM electrons) which has R∼13Å with ωd∕2π=24MHz. In addition, we compared these DNP enhancements with those from three biradicals having shorter and more rigid tethers—bis-TEMPO tethered by oxalyl amide, bis-TEMPO tethered by the urea structure, and 1-(TEMPO-4-oxyl)-3-(TEMPO-4-amino)-propan-2-ol (TOTAPOL) TOTAPOL is of particular interest since it is soluble in aqueous media and compatible with DNP experiments on biological systems such as membrane and amyloid proteins. The interelectron distances and relative g-tensor orientations of all of these biradicals were characterized with an analysis of their 9 and 140GHz continuous-wave EPR lineshapes. The results show that the largest DNP enhancements are observed with BT2E and TOTAPOL that have shorter tethers and the two TEMPO moieties are oriented so as to satisfy the matching condition for the CE.