Direct Dynamic Nuclear Polarization as a Tool or Selective NMR Spectroscopy of Biomolecules

Dynamic nuclear polarization (DNP) under magic angle spinning (MAS) has become an important technique in the structural investigation of large biomolecules. Following the hyperfine-mediated direct transfer from the polarizing agent to the surrounding nuclear spins, fast dispersion of polarization to the analyte by proton-proton spin diffusion enables high sensitivity with minimum detriment from the paramagnet; as a result, DNP is most often performed by indirect DNP experiments where a cross-polarization transfer is performed from hyperpolarized 1 H to the nuclear species of interest. However, the rapid distribution of polarization hampers the utilization of site-specific and selective structural information potentially obtainable by DNP. In contrast, direct polarization readout of (dilute) nuclei with small gyromagnetic ratio may yield selective structural information due to the attenuation of spin diffusion. This article provides an overview of recent developments and applications of direct-DNP-based experiments at high field under MAS in biomolecules. We present topics including labeling of proteins with polarizing agent tags for selective hyperpolarization, the use of paramagnetic metal ions for (endogenous) direct DNP, and the combination of direct and indirect transfer using specific cross-relaxation enhancement by active motions under DNP (SCREAM-DNP) as a filter for site-specific information. Finally, we consider advantages and limits of direct polarization transfer methods and provide an outlook about the applicability of direct polarization methods in modern DNP.