High-Frequency Dynamic Nuclear Polarization in the Nuclear Rotating Frame

A proton dynamic nuclear polarization (DNP) NMR signal enhancement (e) close to thermal equilibrium, ∈ = 0.89, has been obtained at high field (B₀ = 5 T, ν_epr = 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spin-lock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T_1p), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T_1n). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T_1p and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T_1n). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time^1/2 of ∈_t = 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for high-field DNP/NMR studies of biological systems,