In situ high-field dynamic nuclear polarization--direct and indirect polarization of 13C nuclei.

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. In a dynamic nuclear polarization (DNP) experiment, the large Boltzmann polarization of a paramagnetic polarizing agent is transferred to surrounding nuclei by microwave irradiation at the electron paramagnetic resonance (EPR) frequency.[1] The initial DNP experiments were performed in the early 50's [2], but until recently the approach was of limited applicability because it was constrained to low magnetic fields due to a dearth of high frequency microwave sources. However, high-frequency (130-660 GHz), high-power microwave sources are now available and improving, and as a result there have been substantial efforts to move DNP to magnetic fields used in contemporary NMR experiments (200–1000 MHz for 1 H). Thus, in the last decade high-frequency DNP has emerged as a valuable method to overcome the intrinsic low sensitivity of liquid and solid-state NMR experiments.[3–5] In particular, with its increased signal intensities, DNP is of significant interest in applications ranging from particle physics [6] to structural biology [4,5] and clinical imaging.[7] Recent DNP experiments have been directed at enhancing signal intensities in both solid and liquid state NMR spectra. Due to their superior dispersion and resolution, as well as for technical reasons discussed below, both sets of experiments have relied primarily on observation of low-γ spectra such as 13 C or 15 N. These nuclear species can be polarized either directly (e − → 13 C/ 15 N) or indirectly via 1 H (e − → 1 H → 13 C/ 15 N), the indirect method relying on a Hartmann-Hahn transfer for the 1 H → 13 C/ 15 step. The purpose of this communication is twofold: (1) to compare the efficiency of the direct and indirect polarization transfers, and (2) to delineate the conditions under which the direct 13 C polarization process is most efficient. Our results indicate that direct 13 C polarization is optimal under different conditions when compared to the indirect process. For direct polarization, the maximum enhancement occurs on the low-field, rather than the high-field side of the DNP enhancement profile. This is likely because of the lower Larmor frequency of 13 C and the distribution of spectral intensity in the EPR lineshape. In addition, we find that the time constant for the polarization buildup, τ B , for the direct 13 C polarization process is a factor of ≥ 20 longer …