Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery

Abstract

Dynamic nuclear polarization (DNP) increases NMR sensitivity by transferring polarization from electron to nuclear spins.Herein, we demonstrate that electron decoupling with chirped microwave pulses enables improved observation of DNP-enhanced 13 Cs pins in direct dipolar contact with electron spins,t herebyl eading to an optimal delay between transients largely governed by relatively fast electron relaxation. We report the first measurement of electron longitudinal relaxation time (T 1e )d uring magic angle spinning (MAS) NMR by observation of DNP-enhanced NMR signals (T 1e = 40 AE 6ms, 40 mm trityl, 4.0 kHz MAS,4 .3 K). With a5ms DNP period, electron decoupling results in a195 %increase in signal intensity.MAS at 4.3 K, DNP,electron decoupling,and short recycle delays improve the sensitivity of 13 Cinthe vicinity of the polarizing agent. This is the first demonstration of recovery times between MAS-NMR transients being governed by short electron T 1 and fast DNP transfer.Magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy reveals structure and dynamics on ar ange of molecular architectures,i ncluding membrane proteins, [1][2][3][4][5][6] fibrils, [7][8][9][10] and interfaces. [11][12][13][14] Mechanical rotation of the sample about the magic angle of 54.78 8 with respect to the magnetic field partially averages anisotropic interactions of the magnetic resonance Hamiltonian, improving spectral resolution and sensitivity. [15][16][17][18] Often, sensitivity is also increased through the transfer of polarization from spins with stronger Zeeman interactions by radiofrequency [16,19] or microwave irradiation. In the case of dynamic nuclear polarization (DNP), hyperfine interactions are required to transfer spin polarization from electron to nuclear spins. [20][21][22][23][24][25] DNP transfer mechanisms require polarizing agents exhibiting electron paramagnetic resonance (EPR). [26] Here, we use as table organic monoradical, trityl (Finland radical), which has ar elatively narrow EPR lineshape. [27] Then arrow electron spin resonance and high-power microwave chirps generated by ac ustom-built frequency-agile gyrotron enable time-domain manipulation of electronic and hyperfine interactions. [28,29] We have shown that chirped microwave pulses can partially decouple the electron-nuclear spin interaction after DNP in am ethod known as electron decoupling,w ith amore pronounced effect on observed nuclear spins in direct proximity to electron spins. [30,31] DNP experiments typically require seconds to hours to spread enhanced polarization via nuclear spin diffusion to chemical sites of interest. [32][33][34] This slow spin diffusion significantly limits experiments.D NP is faster if nuclear spins proximal to the electron are polarized directly through hyperfine interactions.H owever,s uch direct DNP requires electron decoupling after the polarization transfer period to mitigate the detrimental effects of hyperfine interactions on NMR signatures. [31,35] Theo ptimal repetition time for direct DNP is li...