Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves

Abstract

We report solid state 13 C and 1 H nuclear magnetic resonance (NMR) experiments with magic-angle spinning (MAS) on frozen solutions containing nitroxide-based paramagnetic dopants that indicate significant perturbations of nuclear spin polarizations without microwave irradiation. At temperatures near 25 K, 1 H and cross-polarized 13 C NMR signals from 15 N, 13 C-labeled L-alanine in trinitroxidedoped glycerol/water are reduced by factors as large as six compared to signals from samples without nitroxide doping. Without MAS or at temperatures near 100 K, differences between signals with and without nitroxide doping are much smaller. We attribute most of the reduction of NMR signals under MAS near 25 K to nuclear spin depolarization through the cross-effect dynamic nuclear polarization mechanism, in which three-spin flips drive nuclear polarizations toward equilibrium with spin polarization differences between electron pairs. When T 1e is sufficiently long relative to the MAS rotation period, the distribution of electron spin polarization across the nitroxide electron paramagnetic resonance lineshape can be very different from the corresponding distribution in a static sample at thermal equilibrium, leading to the observed effects. We describe three-spin and 3000-spin calculations that qualitatively reproduce the experimental observations. [http://dx

Theoretical aspects of Magic Angle Spinning - Dynamic Nuclear Polarization

“…Recently it has been shown [46,36,47] that the signal intensities of these rotating samples without the presence of MW are smaller than the equilibrium signals obtained from the same samples when they are not rotating. This depolarization effect enhances the experimental enhancement factors.…”

“…Although the spectral diffusion mechanism is also present during sample rotation, it seems that this mechanism is less important for the enhancement process during MAS-DNP than during static DNP. The MW and D rotor-events are experienced by almost all of the electrons in the sample, and as long as the longitudinal electron relaxation times are short enough, these events should dominate the electron polarizations [36].…”

“…Recent theoretical work performed by Thurber and Tycko [34][35][36] as well as in our group [37,33] has revealed the differences between the static DNP and MAS-DNP mechanism and its underlying complexity. They emphasized that the polarization transfer between the electrons and the nuclei in rotating samples takes place during crossings of the energy levels of the rotating samples.…”

Ultra-Low Temperature Nuclear Magnetic Resonance

“…a polarization state lower than the Boltzmann equilibrium, leading to an effective polarization difference between electron spins and protons much greater than the expected factor of 658. It has been shown that more than half the proton polarization can be depleted at ~110 K [28] and up to ~80 % at 25 K [29]. Therefore, εon/off can also dramatically over exaggerate the gain from DNP.…”

“…The experimental value of 1Hεon/off ~680 obtained at 55 K thus clearly demonstrates that εon/off does not directly relate to the theoretical enhancement. The reason for this has been thoroughly described in a recent publication [28], which followed the initial inspired work by Thurber and Tycko [29]. The ratio εon/off can only then be related to the theoretical maximum when both the electron and nuclear thermal steady-state polarizations are at their respective Boltzmann equilibriums in the absence of irradiation.…”