title: Paramagnet induced signal quenching in MAS-DNP experiments in frozen homogeneous solutions.
abstract: The effects of nuclear signal quenching induced by the presence of a paramagnetic polarizing agent are documented for conditions used in magic angle spinning (MAS)-dynamic nuclear polarization (DNP) experiments on homogeneous solutions. In particular, we present a detailed analysis of three time constants: (1) the longitudinal build-up time constant TB for (1)H; (2) the rotating frame relaxation time constant T1ρ for (1)H and (13)C and (3) T2 of (13)C, the transverse relaxation time constant in the laboratory frame. These relaxation times were measured during microwave irradiation at a magnetic field of 5 T (140 GHz) as a function of the concentration of four polarizing agents: TOTAPOL, 4-amino-TEMPO, trityl (OX063), and Gd-DOTA and are compared to those obtained for a sample lacking paramagnetic doping. We also report the EPR relaxation time constants T1S and T2S, the DNP enhancements, ε, and the parameter E, defined below, which measures the sensitivity enhancement for the four polarizing agents as a function of the electron concentration. We observe substantial intensity losses (paramagnetic quenching) with all of the polarizing agents due to broadening mechanisms and cross relaxation during MAS. In particular, the monoradical trityl and biradical TOTAPOL induce ∼40% and 50% loss of signal intensity. In contrast there is little suppression of signal intensity in static samples containing these paramagnetic species. Despite the losses due to quenching, we find that all of the polarizing agents provide substantial gains in signal intensity with DNP, and in particular that the net enhancement is optimal for biradicals that operate with the cross effect. We discuss the possibility that much of this polarization loss can be regained with the development of instrumentation and methods to perform electron decoupling.
authors.affiliation: Francis Bitter Magnet Laboratory, Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
authors.forename: Björn
authors.initials: B
authors.lastname: Corzilius
authors.affiliation: Francis Bitter Magnet Laboratory, Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
authors.forename: Loren B
authors.initials: LB
authors.lastname: Andreas
authors.affiliation: Francis Bitter Magnet Laboratory, Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
authors.forename: Albert A
authors.initials: AA
authors.lastname: Smith
authors.affiliation: Francis Bitter Magnet Laboratory, Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
authors.forename: Qing Zhe
authors.initials: QZ
authors.lastname: Ni
authors.affiliation: Francis Bitter Magnet Laboratory, Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Electronic address: rgg@mit.edu.
authors.forename: Robert G
authors.initials: RG
authors.lastname: Griffin
mesh_terms: D000465:Algorithms; D016880:Anisotropy; D002247:Carbon Isotopes; D003497:Cyclic N-Oxides / Q000737:chemistry; D003903:Deuterium; D004574:Electromagnetic Fields; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005609:Free Radicals; D006571:Heterocyclic Compounds / Q000737:chemistry; D006575:Heterocyclic Compounds, 3-Ring / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation / Q000379:methods*; D008872:Microwaves; D009942:Organometallic Compounds / Q000737:chemistry; D020005:Propanols / Q000737:chemistry; D012996:Solutions
chemical_list: C521769:1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol; D002247:Carbon Isotopes; D003497:Cyclic N-Oxides; D005609:Free Radicals; D006571:Heterocyclic Compounds; D006575:Heterocyclic Compounds, 3-Ring; D009942:Organometallic Compounds; C495909:Oxo63; D020005:Propanols; D012996:Solutions; C015527:4-amino-TEMPO; C050823:gadolinium 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetate; D003903:Deuterium
keywords: Dynamic nuclear polarization; Electron paramagnetic resonance; Electron spin resonance; Magic angle spinning NMR; Nuclear magnetic resonance; Paramagnetic relaxation; Polarizing agents