Dynamic nuclear polarization enhancement of protons and vanadium-51 in the presence of pH-dependent vanadyl radicals.

Angel J Perez Linde,Geoffrey Bodenhausen,Andrzej Sienkiewicz,Pascal Miéville,Diego Carnevale

doi:10.1002/mrc.4138

Abstract

We report applications of dynamic nuclear polarization to enhance proton and vanadium-51 polarization of vanadyl sulfate samples doped with TOTAPOL under magic angle spinning conditions. The electron paramagnetic resonance response stemming from the paramagnetic (51)V species was monitored as a function of pH, which can be adjusted to improve the enhancement of the proton polarization. By means of cross-polarization from the proton bath, (51)V spins could be hyperpolarized. Enhancement factors, build-up times, and longitudinal relaxation times T1((1)H) and T1((51)V) were investigated as a function of pH.

Highlights

Vanadium possesses two Nuclear magnetic resonance (NMR) active isotopes, 50V and 51V; the latter has a spin S(51V) = 7/2, a 99.8% natural abundance, and a favorable gyromagnetic ratio since γ(51V)/γ(13C) ≈ 1.05
Vanadium exists in several forms; one of the most stable is the paramagnetic vanadyl ion (VIVO2+, with the electronic configuration for the metal VIV d1),[1] which can be found in rare minerals[2] and food supplements
Paramagnetic agents with very short longitudinal relaxation times T1e or with much greater line widths than TOTAPOL may be detrimental for Dynamic nuclear polarization (DNP) enhancements, especially at high concentrations when they may increase the leakage of polarization.[23,26,28]

Summary

Introduction

Dynamic nuclear polarization enhancement of protons and vanadium-51 electron spins and one proton spin The efficiency of this polarization transfer mechanism can be affected by other EPR transitions.[15,22,23,24,25,26,27,28,29,30] Careful selection of two radicals with different EPR line shapes and different T1e can lead to improved DNP enhancements.[25,26,27,31] paramagnetic agents with very short longitudinal relaxation times T1e or with much greater line widths than TOTAPOL may be detrimental for DNP enhancements, especially at high concentrations when they may increase the leakage of polarization.[23,26,28] DNP with 51V in solution can be achieved using the Overhauser effect.[32] In solids, DNP can be achieved using high-spin metal ions[33] or other radicals.[34,35]. The enhancements εDNP(1H) were measured by comparing signal intensities observed with rotor-synchronized spin echoes with and without microwave irradiation

Enhancement Enhancement

Proton Polarization

Conclusions