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

We demonstrate the feasibility of one-dimensional and two-dimensional H-1-C-13 double resonance NMR experiments with dynamic nuclear polarization (DNP) at 9.4T and temperatures below 20 K, including both H-1-C-13 cross-polarization and H-1 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 H-1-C-13 cross-polarization and high-power proton decoupling. Experiments at 8 K and 16 K reveal a significant T-2 relaxation of C-13, 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 C-13-labeled biomolecules was demonstrated with a two-dimensional C-13-C-13 exchange spectrum of selectively C-13-labeled beta-amyloid fibrils. (C) 2012 Elsevier Inc. All rights reserved.