Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Sheetal Kumar Jain, Chung Jui Yu, Christopher Blake Wilson, Tarnuma Tabassum, Danna E. Freedman*, Songi Han

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Dynamic nuclear polarization (DNP) harnesses the large polarization of electron spins to dramatically increase nuclear magnetic resonance (NMR) sensitivity. This study expands the scope of DNP beyond its traditional focus on hyperpolarizing the solvent network using exogenous polarizing agents (PAs). We introduce 1H DNP with endogenous V4+ centers positioned in a set of vanadyl complexes with tunable V4+-1H distances. We traced the polarization transfer from V4+ to 1H spins, specifically differentiating between direct V4+-1Hs polarization transfer and the 1H spin-diffusion-mediated bulk solvent 1H polarization buildup and illuminated the effect of the V4+-1H distance on these processes. These results deepen our understanding of polarization pathways and expand the catalog of PAs to broad-line transition metals. This study establishes crucial first steps toward employing strategically positioned endogenous paramagnetic metal centers for DNP and the conceptual framework of hyperfine DNP spectroscopy that merges both spatial and chemical diagnosis of target nuclear spins. Dynamic nuclear polarization (DNP) offers a strategy to amplify the inherently insensitive nuclear magnetic resonance (NMR) signal by orders of magnitude, dramatically enhancing the NMR studies of molecules and materials. To date, most DNP studies have relied on exogenous polarizing agents (PAs) with narrow electron paramagnetic resonance (EPR) lines. Expanding the catalog of PAs to paramagnetic centers at the core of active sites in enzymes and functional materials will offer the opportunity to illuminate the local structure around these metal centers during chemical transformations. This study is the first demonstration of DNP using V4+ centers with broad EPR lines as PAs to enhance and detect the NMR signal of 1H nuclei at discrete distances from the V4+ center. These results pave the way toward endogenous DNP studies of active molecules and materials in which paramagnetic metals play key functional roles. We demonstrate dynamic nuclear polarization (DNP) with a set of complexes with controlled V4+-1H distances and gain mechanistic insight into DNP. DNP from the paramagnetic V4+center directly polarized 1H nuclei located 12.6 Å from the electronic spin. Critically, we find the buildup rate of polarization is dependent on the separation of the 1H from the V4+center. These results lay the foundation for endogenous DNP studies of systems in which intrinsic paramagnetic metals serve as the polarizing agent.

Original languageEnglish (US)
Pages (from-to)421-435
Number of pages15
JournalChem
Volume7
Issue number2
DOIs
StatePublished - Feb 11 2021

Keywords

  • SDG3: Good health and well-being
  • coordination chemistry
  • direct DNP
  • endogenous DNP
  • polarizing agent
  • spin decoherence
  • spin diffusion
  • spin diffusion barrier
  • spin dynamics
  • transition metal EPR
  • vanadium DNP

ASJC Scopus subject areas

  • Chemistry(all)
  • Biochemistry
  • Environmental Chemistry
  • Chemical Engineering(all)
  • Biochemistry, medical
  • Materials Chemistry

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