title: High-frequency dynamic nuclear polarization in the nuclear rotating frame.
abstract: A proton dynamic nuclear polarization (DNP) NMR signal enhancement (epsilon) close to thermal equilibrium, epsilon = 0.89, has been obtained at high field (B(0) = 5 T, nu(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(1rho)), 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(1rho) 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 epsilon(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.
authors.affiliation: Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: C T
authors.initials: CT
authors.lastname: Farrar
authors.forename: D A
authors.initials: DA
authors.lastname: Hall
authors.forename: G J
authors.initials: GJ
authors.lastname: Gerfen
authors.forename: M
authors.initials: M
authors.lastname: Rosay
authors.forename: J H
authors.initials: JH
authors.lastname: Ardenkjaer-Larsen
authors.forename: R G
authors.initials: RG
authors.lastname: Griffin
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D015394:Molecular Structure; D014320:Triphenylmethyl Compounds / Q000737:chemistry*
chemical_list: D014320:Triphenylmethyl Compounds