Abstract
The efficacy in 1H Overhauser dynamic nuclear polarization in liquids at ultralow magnetic field (ULF, B0 = 92 ± 0.8 µT) and polarization field (Bp = 1-10 mT) was studied for a broad variety of 26 different spin probes. Among others, piperidine, pyrrolidine, and pyrroline radicals specifically synthesized for this study, along with some well-established commercially available nitroxides, were investigated. Isotope-substituted variants, some sterically shielded reduction-resistant nitroxides, and some biradicals were included in the measurements. The maximal achievable enhancement, Emax, and the radio frequency power, P1/2, needed for reaching Emax/2 were measured. Physico-chemical features such as molecular weight, spectral linewidth, heterocyclic structure, different types of substituents, deuteration, and 15N-labeling as well as the difference between monoradicals and biradicals were investigated. For the unmodified nitroxide radicals, the Emax values correlate with the molecular weight. The P1/2 values correlate with the spectral linewidth and are additionally influenced by the type of substituents neighboring the nitroxide group. The nitroxide biradicals with high intramolecular spin-spin coupling show low performance. Nitroxides enriched with 15N and/or 2H afford significantly higher |Emax| and require lower power to do so, compared to their unmodified counterparts containing at natural abundance predominantly 14N and 1H. The results allow for a correlation of chemical features with physical hyperpolarization-related properties and indicate that small nitroxides with narrow spectral lines have clear advantages for the use in Overhauser dynamic nuclear polarization experiments. Perdeuteration and 15N-labeling can be used to additionally boost the spin probe performance.
Highlights
Introduction to the Theory andPractice of nuclear magnetic resonance (NMR) and electron spin resonance (ESR) in Biological Systems (Wiley-Interscience, London, 1976). 59M
Besides commercially available tetramethyl-substituted nitroxides (M4, M6–M8), which are well established in many Overhauser dynamic nuclear polarization (ODNP) related publications[12,43,46,47] and provide a point of reference for other compounds, we tested some bioreduction resistant, tetraethyl-substituted nitroxides (E1–E4)
The goal of this work is to better understand the various factors that influence the physical properties of free radical spin probes
Summary
Introduction to the Theory andPractice of NMR and ESR in Biological Systems (Wiley-Interscience, London, 1976). 59M. In a nitroxide radical molecule, hyperfine splitting of the electron energy levels occurs due to interactions with the scitation.org/journal/jcp nitrogen spin.
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