Off-resonance NOVEL

Dynamic nuclear polarization (DNP) is theoretically able to enhance the signal in nuclear magnetic resonance (NMR) experiments by a factor gamma(e)/gamma(n), where gamma's are the gyromagnetic ratios of an electron and a nuclear spin. However, DNP enhancements currently achieved in high-field, high-resolution biomolecular magic-angle spinningNMRare well below this limit because the continuous-wave DNP mechanisms employed in these experiments scale as omega(-n)(0) where n similar to 1-2. In pulsed DNP methods, such as nuclear orientation via electron spin-locking (NOVEL), the DNP efficiency is independent of the strength of the main magnetic field. Hence, these methods represent a viable alternative approach for enhancing nuclear signals. At 0.35 T, the NOVEL scheme was demonstrated to be efficient in samples doped with stable radicals, generating H-1 NMR enhancements of similar to 430. However, an impediment in the implementation of NOVEL at high fields is the requirement of sufficient microwave power to fulfill the on-resonance matching condition, omega(0I) = omega(1S), where omega(0I) and omega(1S) are the nuclear Larmor and electron Rabi frequencies, respectively. Here, we exploit a generalized matching condition, which states that the effective Rabi frequency, omega(eff)(1S), matches omega(0I). By using this generalized off-resonance matching condition, we generate H-1 NMR signal enhancement factors of 266 (similar to 70% of the onresonanceNOVEL enhancement) with omega(1S)/2 pi = 5 MHz. We investigate experimentally the conditions for optimal transfer of polarization from electrons to H-1 both for the NOVEL mechanism and the solid-effect mechanism and provide a unified theoretical description for these two historically distinct forms of DNP. Published by AIP Publishing.