Dynamic nuclear polarization-enhanced 1H–13C double resonance NMR in static samples below 20 K

Abstract

We demonstrate the feasibility of one-dimensional and two-dimensional 1H–13C double resonance NMR experiments with dynamic nuclear polarization (DNP) at 9.4 T and temperatures below 20 K, including both 1H–13C cross-polarization and 1H decoupling, and discuss the effects of polarizing agent type, polarizing agent concentration, temperature, and solvent deuteration. We describe a two-channel low-temperature DNP/NMR probe, capable of carrying the radio-frequency power load required for 1H–13C cross-polarization and high-power proton decoupling. Experiments at 8 K and 16 K reveal a significant T2 relaxation of 13C, induced by electron spin flips. Carr–Purcell experiments and numerical simulations of Carr–Purcell dephasing curves allow us to determine the effective correlation time of electron flips under our experimental conditions. The dependence of the DNP signal enhancement on electron spin concentration shows a maximum near 80 mM. Although no significant difference in the absolute DNP enhancements for triradical (DOTOPA-TEMPO) and biradical (TOTAPOL) dopants was found, the triradical produced greater DNP build-up rates, which are advantageous for DNP experiments. Additionally the feasibility of structural measurements on 13C-labeled biomolecules was demonstrated with a two-dimensional 13C–13C exchange spectrum of selectively 13C-labeled β-amyloid fibrils.

Helium-cooling and -spinning dynamic nuclear polarization for sensitivity-enhanced solid-state NMR at 14T and 30K

“…1 By this definition, the DNP enhancement factor can be obtained independently of specific polarization buildup time assuming the time constants for the buildup are similar with and without microwave irradiation [25,41]. A background 1 H spectrum recorded for a rotor without sample was subtracted at each temperature from both microwave-on and -off spectra before calculating the enhancement factor.…”

“…Lowering sample temperature is an effective approach for improving DNP efficiency at high fields [28,[40][41][42] as slower spin relaxation at lower temperature generally favors electronto-nuclear polarization transfer. In addition, both electron and nuclear spin polarizations at thermal equilibrium are larger at lower temperature.…”

Modern solid state NMR techniques and concepts in structural studies of synthetic polymers

“…13). [34,35] Recently, two-themed issues devoted to DNP were published. [36,37] In cited papers, more retrospective and perspective articles, covering many problems related with development of both the equipment and methodology in DNP SSNMR, were reviewed.…”

“…4.7%) of chemical shift evolution in the t1 and t2 dimensions. All sequences employ independent (0, 180) phase cycles of the first 1 H π/2 pulse and the last π pulse on 13 C. Taken from Reference [35] with permission. 29 Si, conventional NMR techniques have a limited scope of applications, especially in the investigation of the surface interactions.…”

NMR at Low and Ultralow Temperatures

“…43 In our hands, the absolute DNP-enhanced 1 H NMR sensitivity at 20 K with 30 mW microwave power was comparable to the absolute sensitivity expected hypothetically at 80 K with ε DNP ≈ 130 and T DNP ≈ 1.4 s. Similar enhancements of cross-polarized 13 C NMR sensitivity were observed in double-resonance DNP experiments at ultra-low temperatures. 44 These results encouraged us to construct a version of the ultra-low-temperature MAS probe that includes provisions for microwave irradiation for DNP.…”