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

• Published in: The Journal of chemical physics 2008-02, Vol.128 (5), p.052302-052302-17
• Main Authors: Hu, Kan-Nian, Song, Changsik, Yu, Hsiao-hua, Swager, Timothy M., Griffin, Robert G.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2816783

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, ∣ ω e 2 − ω e 1 ∣ = ω n . Biradicals, for example, two 2,2,6,6-tetra methyl-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 90 K and 5 T , the enhancement grew from ∼ 40 observed with 10 mM monomeric TEMPO, where the average R ∼ 56 Å corresponding to electron-electron dipolar coupling constant ω d ∕ 2 π = 0.3 MHz , to ∼ 175 with 5 mM BT2E ( 10 mM electrons) which has R ∼ 13 Å with ω d ∕ 2 π = 24 MHz . 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 140 GHz 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.