Kaushik, M., et al., Bis-Gadolinium Complexes for Solid Effect and Cross Effect Dynamic Nuclear Polarization. Angew Chem Int Ed Engl, 2017. 56(15): p. 4295-4299.
High-spin complexes act as polarizing agents (PAs) for dynamic nuclear polarization (DNP) in solid-state NMR spectroscopy and feature promising aspects towards biomolecular DNP. We present a study on bis(Gd-chelate)s which enable cross effect (CE) DNP owing to spatial confinement of two dipolar-coupled electron spins. Their well-defined GdGd distances in the range of 1.2-3.4 nm allowed us to elucidate the GdGd distance dependence of the DNP mechanism and NMR signal enhancement. We found that GdGd distances above 2.1 nm result in solid effect DNP while distances between 1.2 and 2.1 nm enable CE for 1 H, 13 C, and 15 N nuclear spins. We compare 263 GHz electron paramagnetic resonance (EPR) spectra with the obtained DNP field profiles and discuss possible CE matching conditions within the high-spin system and the influence of dipolar broadening of the EPR signal. Our findings foster the understanding of the CE mechanism and the design of high-spin PAs for specific applications of DNP.