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 Agents1. Issue 51 (13th August 2018)

• Record Type: Journal Article
• Authors: Geiger, Michel‐Andreas; Jagtap, Anil P.; Kaushik, Monu; Sun, Han; Stöppler, Daniel; Sigurdsson, Snorri T.; Corzilius, Björn; Oschkinat, Hartmut
• Is Part Of: Chemistry. Volume 24:Issue 51(2018)
• Page Start: 13485
• Page End: 13494
• Publication Date: 2018-08-13
• Subjects: dynamic nuclear polarization -- magic angle spinning -- NMR spectroscopy -- radicals -- water-soluble; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201801251
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: Nitroxide biradicals are very efficient polarizing agents in magic angle spinning (MAS) cross effect (CE) dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR). Many recently synthesized, new radicals show superior DNP‐efficiency in organic solvents but suffer from insufficient solubility in water or glycerol/water for biological applications. We report DNP efficiencies for two new radicals, the water‐solublebcTol‐M andcyolyl‐TOTAPOL, and include a comparison with three known biradicals, TOTAPOL, bcTol, andAMUPol . They differ by linker groups, featuring either a 3‐aminopropane‐1, 2‐diol or a urea tether, or by the structure of the alkyl substituents that flank the nitroxide groups. For evaluating their performances, we measured both signal enhancements ϵ and DNP‐enhanced sensitivity κ, and compared the results to electron spin relaxation data recorded at the same magnetic field strength (9.4 T). In our study, differences in DNP efficiency correlate with changes in the nuclear polarization dynamics rather than electron relaxation. The ratios of their individual ϵ and κ differ by up to 20 %, which is explained by starkly different nuclear polarization build‐up rates. For the radicals compared here empirically, using proline standard solutions, the new radicalbcTol‐M performs best while being most soluble in water/glycerol mixtures. Abstract : The syntheses of two new nitroxide biradicals are reported and their evaluation as polarizing agents in DNP MAS NMR, in comparison to three known biradicals. Specifically, their enhancement factors ϵ and sensitivity κ were compared at 100 K and 9.4 T using standard proline samples. This work shows that the efficiency of differently alkylated urea‐based biradicals correlates with nuclear relaxation and size of effective electron–electron coupling.