Time domain DNP with the NOVEL sequence

Abstract

We present results of a pulsed dynamic nuclear polarization (DNP) study at 0.35 T (9.7 GHz/ 14.7 MHz for electron/ 1 H Larmor frequency) using a lab frame-rotating frame cross polarization experiment that employs electron spin locking fields that match the 1 H nuclear Larmor frequency, the so called NOVEL (nuclear orientation via electron spin locking) condition. We apply the method to a series of DNP samples including a single crystal of diphenyl nitroxide (DPNO) doped benzophenone (BzP), 1,3-bisdiphenylene-2-phenylallyl (BDPA) doped polystyrene (PS), and sulfonated-BDPA (SA-BDPA) doped glycerol/water glassy matrices. The optimal Hartman-Hahn matching condition is achieved when the nutation frequency of the electron matches the Larmor frequency of the proton, ω 1S = ω 0I , together with possible higher order matching conditions at lower efficiencies. The magnetization transfer from electron to protons occurs on the time scale of ∼100 ns, consistent with the electron-proton couplings on the order of 1-10 MHz in these samples. In a fully protonated single crystal DPNO/BzP, at 270 K, we obtained a maximum signal enhancement of ε = 165 and the corresponding gain in sensitivity of ε(T 1 /T B ) 1/2 = 230 due to the reduction in the buildup time under DNP. In a sample of partially deuterated PS doped with BDPA, we obtained an enhancement of 323 which is a factor of ∼3.2 higher compared to the protonated version of the same sample and accounts for 49% of the theoretical limit. For the SA-BDPA doped glycerol/water glassy matrix at 80 K, the sample condition used in most applications of DNP in nuclear magnetic resonance, we also observed a significant enhancement. Our findings demonstrate that pulsed DNP via the NOVEL sequence is highly efficient and can potentially surpass continuous wave DNP mechanisms such as the solid effect and cross effect which scale unfavorably with increasing magnetic field. Furthermore, pulsed DNP is also a promising avenue for DNP at high temperature. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4927087] INTRODUCTIONDynamic nuclear polarization (DNP) is a process whereby the large polarization present in an electron spin reservoir of a paramagnetic polarizing agent is transferred via microwave irradiation to nuclei, thereby enhancing the nuclear spin polarization. The initial mechanism supporting the DNP process, the Overhauser effect (OE), was proposed in 1953 1 and confirmed experimentally 2,3 in samples with mobile electrons (i.e., metals, solutions, and 1D conductors). In contrast, in insulating solids, such as glycerol/water glasses and biological samples, the OE was thought to be forbidden, but we have recently observed Overhauser enhancements using polarizing agents with narrow EPR spectra that exhibit strong 1 H−e − hyperfine couplings. 4 Furthermore, in these sorts of samples, DNP processes can also be mediated by three other mechanisms -the solid effect (SE), 5,6 the cross effect, 7-11 and/or thermal mixing. 12 Initially, the primary application of DNP was preparation of p...

New NMR tools for protein structure and function: Spin tags for dynamic nuclear polarization solid state NMR

“…Room temperature DNP has been demonstrated in very encouraging experiments [173,174], and further successes in DNP at room temperature would allow experiments that are compatible with biologically relevant conditions and studies of conformational dynamics. Combining DNP with proton detection, another promising emerging solid-state NMR technology, can lead to even higher sensitivity.…”

Ramped-amplitude NOVEL

“…[51][52][53] Recently, we demonstrated high DNP efficiency using the NOVEL pulse sequence under the sample conditions used in DNP/NMR experiments. 54,55 Inspired by the advantage of RA-CP (vide supra), we report herein the performance of the ramped amplitude (RA)-NOVEL sequence utilizing the newly available arbitrary waveform generator (AWG) function on an X-band EPR spectrometer. The AWG allows precise and convenient manipulations of microwave pulses and was used to ramp the amplitude of the spin locking pulse.…”

Frequency swept microwaves for hyperfine decoupling and time domain dynamic nuclear polarization

“…Time domain DNP also yields much higher enhancements for non-cryogenic NMR. The integrated solid effect (ISE) and nuclear orientation via electron spin locking (NOVEL) are pulsed DNP experiments that yield high signal enhancements at room temperature [24–29]. However, most time domain DNP experiments have only been demonstrated at lower frequencies (~9 GHz) where intense microwave fields are readily available.…”